T53B-0479 1340h
Seismotectonics of the Caucasus and surrounding regions: source parameters and rupture histories of the recent destructive earthquakes
The complexity of plate interactions and associated crustal deformation in the eastern Mediterranean region is marked by the occurence of many destructive earthquakes throughout the recorded history. The wide range of deformational processes occuring in this region means that eastern Mediterranean provides a unique opportunity to improve our understanding of the complex kinematics of continental collision, including strike-slip faulting and crustal extension, as well as associated seismicity and volcanism. This presentation is concerned with the seismotectonics of Caucasus and surrounding regions that are prominent features in the collision zone between the African and Eurasian plates, and provides us with a complex pattern of continental deformation of the region. We have studied the tectonic characteristics of a comprehensive seismicity occurred in the region during the last 20 years, based on newly retrieved and obtained source mechanisms and rupture histories of $\sim$40 earthquakes, Mw$>$5.3, by using waveform inversion of teleseismic long-period P-, SH-, and broad-band P-waveforms recorded in the distance range of 30-90$\deg$. They all exhibit the characteristics and structural complexities associated with strike-slip, thrust and normal faulting as a result of ongoing crustal deformation. Earthquakes in the greater Caucasus mainly exhibit NW-SE thrust faulting mechanisms in general. The western part of the mountain belt has only one moderate size event and verifies that northward dipping shallow thrust fault is active beneath the southern border of the western greater Caucasus. In contrary, the Racha earthquake, and its four moderate size aftershocks, and earthquake of Barisakho indicate that most of the stress accumulation occurs within the central part of the Caucasus mountain belt. A series of earthquakes shows that northern boundary thrust faults of the eastern greater Caucasus dips southwards with a very shallow angle ($\sim$5-10$\deg$). Furthermore, the Lesser Caucasus is not capable of generating destructive earthquakes as the Greater Caucasus. However, Spitak earthquake indicates that right-lateral motion with thrust component active on the Pampak-Sevan Fault (PSF). In contrast to the field observations, preliminary reported parameters, and slip distribution of the Spitak earthquake exhibits that the rupture is not too complex. Furthermore, the energy release and rupture propagation also agree well with the field observations and aftershock distributions reported. The existence and active seismicity of the North-East Anatolian Fault (NEAF), and the Borjomi-Kazbeg Fault (BKF) that divides the Greater Caucasus has significant implications on the present study. The main active faults strike to the southwest end of the NEAFZ in the direction of $\sim$N45$\deg$E. On the other hand, right-lateral strike-slip faulting mechanism has not been clearly observed beyond Georgia-Turkey-Armenia border. This is the northeast end of the right-lateral strike-slip of NEAF zone, and the major events strike approximately in the direction of N60$\deg$E. The difference in fault strike angles ($\sim$15$\deg$) and slip vector orientations ($\sim$20$\deg$) between the both ends of the NEAF indicates that is bounded by block rotations clockwise along the strike: SW to NE. There is no further seismological evidence for a left-lateral strike-slip faulting, except the northerly dipping thrust faulting of Racha sequences in the region to the north of the Greater Caucasus. Although the large earthquakes in the Caucasus generally have uniform slip distribution, there are earthquakes with complex rupture mechanisms along the southern Caspian to NW Iran region.
T53B-0480 1340h
High Resolution Velocity Structure in Eastern Turkey
We investigate the crust and upper mantle structure of eastern Turkey where the Anatolian, Arabian and Eurasian Plates meet, forming a complex tectonic regime. The Bitlis suture is a continental collision zone between the Anatolian plateau and the Arabian plate. Broadband data available through the Eastern Turkey Seismic Experiment (ETSE) provide a unique opportunity for studying the high resolution velocity structure of the region. Zor et al. (2003) found an average 46 km thick crust in the Anatolian plateau using a six-layered grid search inversion of the ETSE receiver functions. Receiver functions are sensitive to the velocity contrast of interfaces and the relative travel time of converted and reverberated waves between those interfaces. The interpretation of receiver functions alone, however, may result in an apparent depth-velocity trade-off [Ammon et al., 1990]. In order to improve upon this velocity model, we have combined the receiver functions with surface wave data using the joint inversion method of Julia et al. (2000). In this technique, the two sets of observations are combined into a single algebraic equation and each data set is weighted by an estimate of the uncertainty in the observations. The receiver functions are calculated using an iterative time-domain deconvolution technique. We also consider azimuthal changes in the receiver functions and have stacked them into different groups accordingly. We are improving our surface wave model by making Love and Rayleigh dispersion measurements at the ETSE stations and incorporating them into a regional group velocity model for periods between 10 and 100 seconds. Preliminary results indicate a strong trend in the long period group velocities toward the northeast, indicating slow upper mantle velocities in the area consistent with Pn, Sn and receiver function results. Starting models used for the joint inversions include both a 1-D model from a 12-ton dam shot recorded by ETSE [Gurbuz et al., 2004] and the models from the original receiver function inversions. We observe that the joint inversion results are independent of the starting model and converge to the same final model, with some differences compared to the original profiles. While we don't observe significant changes in the first order discontinuities of the model, such as Moho depth, we are better able to resolve features in the crust.
T53B-0481 1340h
Seismotectonics of Western Turkey: A Synthesis of Source Parameters and Rupture Histories of Recent Earthquakes
The Aegean region, including western Turkey and Greece, is indeed one of the most seismically active and rapidly deforming continental domains in the Earth. The wide range of deformational processes occurring in this region means that the eastern Mediterranean provides a unique opportunity to improve our understanding of the complex kinematics of continental collision, including strike-slip faulting and crustal extension, as well as associated seismicity and volcanism. The tectonic evolution of the Eastern Mediterranean region is dominated by effects of subduction along the Hellenic (Aegean) arc and of continental collision in eastern Anatolia and the Caucasus. Northward subduction of the African plate beneath western Anatolia and the Aegean region is causing crustal extension in the overlying Aegean province. The interplay between dynamic effects of the relative motions of adjoining plates thus controls large-scale crustal deformation and the associated earthquake activity in Turkey. The Aegean region has been subject to extension since Miocene time, and this extension has left a pronounced expression in the present-day topography. It is further widely accepted that the rapid extension observed in western Turkey is mainly accommodated by large active normal faults that control the geomorphology which is dominated by a series of E-W trending normal-fault-bounded horst and graben structures; the N-S extension inferred from these structures is consistent with regional earthquake focal mechanisms. The E-W trending Menderes graben, the NE-SW trending Burdur, Acigol and Baklan, and NW-SE trending Dinar and Sultandag-Aksehir basins all bounded by large faults form a system of half-graben whose orientation is evident in both the topography and the tilting of Neogene sediments adjacent to them. We have studied source mechanisms and rupture histories of $\sim$20 earthquakes using body-waveform modelling, and have compared the shapes and amplitudes of teleseismic long-period P-, SH-, and broadband P-waveforms recorded by GDSN stations in the distance range of 30$\deg$-90$\deg$. The final solutions were also constrained by P-wave first motion polarities of near-field stations. They all exhibit the characteristics and structural complexities associated with strike-slip and normal faulting as a result of ongoing crustal deformation. We found strike, dip, rake, centroid depth, seismic moment, and source time functions and rupture history and slip distributions.
T53B-0482 1340h
Tectonic Motion Monitoring at the Altimeter Calibration Facility on Gavdos, Crete, Greece.
The intense tectonic activity of Eastern Mediterranean is of great interest for many decades. Recently, sea-level monitoring and climate change studies generated great interest as well as for its regional oceanography. A plethora of observations has convincingly demonstrated the importance of the area for regional meteorological and climatologic changes affecting Eurasia and North Africa. GPS monitors tectonics, while tide gauges record the variations in Mean Sea Level (MSL). Continuous monitoring of tide gauge locations with GPS removes the uncertainties introduced by local tectonics, that contaminate the observed sea level variations. Such a global tide gauge network with long historical records is already used to calibrate satellite altimeters (e.g. on TOPEX/POSEIDON, GFO, JASON-1, ENVISAT, etc.), at present, a common IOC-GLOSS-IGS effort --TIGA. Crete hosts two of the oldest tide gauges in the regional network, at Souda Bay and Heraklion. A third site, state-of-the-art MSL monitoring facility in southwestern Crete was established, on the isle of Gavdos, the southernmost European parcel of land, under a joint effort of the European Union, NASA, and the Swiss Federal Government. The site at Souda Bay is only 5 km away from the continuously operating GPS site at TUC, Chania, with a nearly seven year record of operation. The Gavdos facility is an ideal altimeter calibration site if the tectonic motions are monitored precisely and continuously. This presentation focuses on this aspect of the project, the local and regional tectonic motions relative to the "stable" part of the Eurasian plate. The facility hosts in addition to two tide gauges, multiple GPS receivers, a DORIS beacon for positioning and orbit control, and a transponder for direct calibration. During 2003, the French Transportable Laser Ranging System (FTLRS) completed a co-location campaign at Chania, Crete, for improved orbit control over the site, and to ensure the best possible and most reliable results in connecting the regional sites to ITRF2000.
<a href='http://www.gavdos.tuc.gr/' >http://www.gavdos.tuc.gr/
T53B-0483 1340h
Seismicity Properties as a Marker of the Active Plate Convergence in the western Hellenic Arc
The western Hellenic Arc is one of the most seismically active regions in the western Eurasia. Large, shallow and intermediate depth earthquakes take place there with maximum magnitudes up to 7.5 occurring around Kythira isl. between Peloponnese to the north and Crete isl. to the south. This is also supported by historical seismicity data which indicate that some of the large shocks were tsunamigenic, that is very possibly they were associated with dip-slip faulting. In the north section of the western Hellenic Arc, particularly between Zakynthos isl. and Peloponnese, the upper bound of earthquake magnitude appears to be relatively low not exceeding 6. This is attributed to the high degree of crustal heterogeneity as seismological, sedimentary and tectonic data imply. However, the highly heterogeneous structure of the seismogenic layer favours the incidence of foreshock activity. The lateral distribution of large earthquake rupture zones, the spatial distribution of moderate and low magnitude earthquakes as well as focal mechanisms are used to improve the delineation of the active plate margin in the region. The result is independently examined from bathymetry and field observations regarding evidence of active faulting on land. Additional evidence comes from the propagation field of strong historical tsunamis. Volcanism is not existant in the western Hellenic Arc in contrast to the central and eastern segments of the Arc. This is possibly interpreted by the relatively more shallow position of the foci of large intermediate depth earthquakes which does not favour melting of the upper mantle material.
T53B-0484 1340h
Investigation of the Influence of Different Velocity Models on the Focal Mechanism Solutions Derived from Inversion of Regional Broadband Waveforms for Earthquakes in Greece
The routine use of regional broadband data for the determination of moment tensors for even very small (Mw $\sim$ 3.5) events, has considerably enhanced our understanding of tectonic processes in many active regions around the world. However, this advance comes at the expense of having to know an adequate regional one-dimensional model that can successfully predict the source-to-receiver path effects, thus minimizing the differences between observed and synthetic data during the source inversion process. This is especially true for the data recorded by the Greek national network, due to the limited bandwidth of its stations that record periods of up to 20 s, increasing the dependency of the source inversions on the velocity model used. The network is operated by the Institute of Geodynamics of the National Observatory of Athens and consists of 22 digital, three-component seismometers that cover most parts of the country. In order to investigate this dependency, we selected five moderate to large (Mw $\sim$ 5.0-6.4) earthquakes based on the following two criteria: (1) the available moment tensor solutions for these events reported by the Harvard CMT group, the USGS and/or the Swiss Seismological Service showed good agreement with each other, (2) the events should occur in geographically different parts of the Greek region. As a first step before the inversion we gathered all the published velocity models for the Greek region resulting from teleseismic/local earthquake/surface-wave tomography, created average one-dimensional models and did forward modelling of the waveforms of each event. If the calculated misfit of the forward-synthetic and observed waveform was small, then this velocity model was further used in the source inversion in the frequency band 0.05-0.08 Hz. The results from the inversions showed that in all five cases the focal mechanism was exhibiting only minor differences when compared to the other available solutions. However, the amount of CLVD was high ($\sim$ 70%) for events in the central and northern Aegean Sea, while the misfit versus depth curve showed no clear minimum for a range of depth values. The situation was better for events occuring in western and southern Greece with lower CLVD values ($\sim$ 2-34%) and clear minima in the corresponding misfit versus depth curves. These observations suggest that most published models for the Greek region are probably insufficient for matching all path effects for the frequency bandwidth used in our inversions, but at the same time seem adequate for the recovery of a reliable focal mechanism. The results of this study have great importance in view of future efforts to routinely invert regional waveforms for the determination of focal mechanism solutions for earthquakes in Greece.
T53B-0485 1340h
Macrobrecciation at a Plate Boundary: The Iskenderun Block
The area of intersection of the East Anatolian and the Dead Sea fault zones in southeast Turkey and northwest Syria represents the intersection of three tectonic Plates: African,Anatolian, and Arabian. Widespread deformation occurs in the area of the plate triple junction including the presence of multiple faults bounding plate fragments along the plate tectonic boundaries. Earthquake fault plane solutions and epicenter clustering in the area of the intersectiong fault zones has been used to identify plate fragments. This study delineates a plate boundary near the mouth of the Gulf of Iskenderun based on linear trends in epicenters and the orientation of fault plane solution nodal planes. The boundary located near the mouth of the Gulf of Iskenderun is proposed to be the southern boundary of a plate fragment roughly the size of the state of Delaware, herein named the Iskenderun block. Plate motions and moho depths suggest that the Iskenderun block is a promontory that was originally joined to the African plate and has been caught up in the deformation at the Anatolian-Arabian-African triple junction and has been subsequently torn from the African plate.
T53B-0486 1340h
Niklas - a Hitherto Unknown Deep Magmatic Massif in the Eastern Mediterranean
A Niklas massif was discovered recompiling the gravity and magnetic maps and interpreting in 3-D mode all the available data in the area around Eratosthenes Seamount (ESM). The updated datasets clearly show two partially superimposed magnetic dipoles, which also correspond well to disturbances in the gravity field. The pronounced Eratosthenes magnetic anomaly (EMA) is only the positive part of the southeastern dipole. There is no large gravity anomaly here, however the specific gravity pattern corresponds to the magnetic body. The northwestern `magnetic dipole coincides with a prominent (about of 100mGal) gravity high that was recently delineated by Russian geophysicists. Such grav/mag combination allowed us to interpret the anomalies as being caused by a hitherto unknown dense and magnetic body which we have named the Niklas massif. The parameters and depth of causative bodies were calculated by inverse programs and forward modeling using the seismic refraction and reflection constraints. The reliability of the final model was verified using forward modeling. The magnetic data were interpreted by assuming an induced magnetization as the main magnetizing factor. The final model consists of two large compact features oriented NE-SW and located south of the Cyprian arc,as the Eratosthenes and Niklas bodies. The gravity and magnetic pattern of the Niklas is typical for the ophiolite massifs of the Eastern Mediterranean and Southern Turkey (Troodos, Hatay, Antalya). Based on this likeness we assume the Niklas composed by dense and magnetic ophyolites. This large (~100*75km) deep-seated (~7km) thick (~7km) massif is located ~95km southwest of Cyprus. We consider the Niklas as the south-most fragment of the large allochthonous ophiolite thrust slab including the Troodos massif. The tectonic situations of the Niklas area and the central segment of the Cyprian Arc are similar to that of the Eastern Taurus, Bayer-Bassit and Hatay areas. Interaction of the large Late Cretaceous overthrusts, younger Miocene reverse faults inclined mainly to north and Miocene- Pliocene normal faults and strike-slip faults revealed to the existing tectonic pattern. A system of reverse faults of the Cyprian arc separates the Niklas massif from the Troodos. This system is the main reason of the considerably lower level of the Niklas massif relative to the Troodos one. A pronounce tectonic boundary separates probably Niklas area from the Eratosthenes Block. Upper Cretaceous section of this block is sterile of ophiolites suggesting that during the Mesozoic the paleo-Eratosthenes area was located far from present-day position.
T53B-0487 1340h
Geophysical Investigations of the Troodos Ophiolite, Cyprus
The IANGASS 95 geophysical survey involved the acquisition of a wide-angle reflection/refraction seismic and gravity profile across the Troodos Ophiolite, extending from the centre of the ophiolite complex eastwards into the circum-Troodos sedimentary succession. The seismic data has recently been modelled using 2-D tomographic and ray tracing methods followed by modelling of the Bouguer gravity anomaly based upon the seismic model through simple velocity-density conversion. Both velocity and density models indicate a 5 layered structure. A thin, intermittent, low density layer with velocity 2.9km/s is interpreted as a combination of sediments and the upper pillow lava sequence. This is underlain by a 0.5km thick layer with average velocity of 3.56km/s and density 2.3g/cm$^3$ that is consistent with the lower pillow lava sequence consisting of pillows and massive flows. A 1 km thick layer of velocity 4.6-5.0km/s outcropping at the western end of the profile and c. 2km depth in the east corresponds with the sheeted dyke complex and has a modelled density of 2.7g/cm$^3$. The boundary between the lavas and sheeted dykes is defined primarily by seismic diving waves and does not preclude the presence of a gradational `basal layer'. The layer beneath the sheeted dykes is c. 3-6km thick with velocity 6 - 6.7km/s and density 2.9g/cm$^3$ and is interpreted as comprising gabbroic rocks, possibly including ultramafic plutonics. Several large (up to ~500m) offsets modelled in these upper layers, some extending into the gabbroic layer, are interpreted as faults, interestingly apparently downthrown to the west, and presumably associated with spreading related extension in the ophiolite. The base of the gabbroic rocks at ~4-8km depth from west to east is likely to be a relic petrological Moho and is underlain by a very thick (c. 5km) layer of $>$ 7km/s. We interpret this to be serpentinized harzburgite having a density of 3.0g/cm$^3$. This is thought to be underlain by normal mantle although velocity control at this depth is limited. A deep reflector is imaged at c. 60km depth but its cause is unknown. One possibility is that it may result from the present-day downgoing slab in the subduction zone to the south of Cyprus.
T53B-0488 1340h
Miocene to Quaternary Folding and Thrusting Offshore Lebanon From SHALIMAR Seismic Profiles
The dense grid of 6-channel seismic profiles, complemented by 25 high-resolution profiles, shot offshore Lebanon during the SHALIMAR cruise provides exceptional insight into the stratigraphy of the top 3.5 km of sediments, and into the evolution of Miocene to Quaternary tectonic deformation. The topmost sediments are Plio-Quaternary turbidites, reaching a maximum thickness of 1000-1300m near shore between Beyrouth and Batroun, from an average of 400-600m north of Tripoli, south of Saida, and farther west in the Levantine abyssal plain. Two strong reflectors mark the top and base of the seismically transparent Messinian evaporite layer, which thins landwards from a maximum thickness of 1500-1900m. Deeper down, regularly bedded horizons likely represent Miocene carbonates, and stronger reflectors below, Eocene and Late Cretaceous limestone. There is no trace of reactivation of the passive, Mesozoic Levantine basin margin north of Tripoli. The whole sediment sequence is essentially undeformed, with the evaporite layer pinching out at the base of the continental slope. A rough erosion surface related to the Messinian emersion event, unconformably draped by the Plio-Quaternary sequence, reaches down to 1700m depth. By contrast, strong shortening affects Messinian and younger sediments between Tripoli and Saida. Steeply east-dipping thrust faults mark the base of the steep continental slope. Offshore Jounieh, folding of the turbidites and underlying sediments extends as far as 30 km from the coast. Up to 4 rows of large west-vergent anticlines, 4-7km wide, underlain by mostly blind thrust ramps are observed; extensional faulting affects the turbidites above the hinges of these fault-bend folds. The Plio-Quaternary growth of the anticlines, synchronous with offshore sedimentation, has dammed onlapping units of turbidites into broad and deep piggy-back synclines. The base of the Messinian evaporites is offset by east-dipping thrust ramps, and diapirs have risen in the pinched cores of several anticlines. South of a NNW-striking lateral ramp system, the submarine region between Saida and Sour shows much less evidence of Neogene shortening. The 90km-long, thin-skinned, fold and thrust belt observed between Saida and Tripoli therefore represents the Miocene-Quaternary foreland thrust-wedge linked with the growth of Mt Lebanon. The offshore decollements and thrust ramps likely root into a steep crustal ramp plunging beneath the coastal flexure. Overall, the amount of shortening in the last 15 Ma may be on order of a few tens of kilometers.
T53B-0489 1340h
Continental or Oceanic Crust? A new Study on the Crustal Structure of the Levantine Basin
The Levantine Basin in the Eastern Mediterranean Sea is a key area for understanding the closing of the Neo-Tethys. To unravel the origin and nature of the underlying crust Meteor cruise M52-2 was carried out in spring 2002. Two refraction seismic profiles were recorded, along with a net of multichannel seismic, gravity and magnetic lines. Two of the gravity and multichannel seimic lines coincide with the refraction profiles. Forward modelling of these profiles was carried out and the models show a Moho depth of 20 - 23km with a total crustal thickness of roughly 8km. The velocities of the upper crust are 6.0 - 6.3km/s, those of the lower crust 6.5 - 6.9km/s. Above lies a layer with 1 - 3km thickness and a velocity of 4.5km/s, which is covered by 5 - 7km of pre-Messinian sediments with an average velocity of 3.8km/s. On top of these sediments is an evaporite layer with a velocity of 4.3 - 4.4km/s, covered by young sediments of about 1km thickness and a water column of up to 1.5km. The gravity models support this structure and show additionally a steep decrease of the Moho depth down to almost 30km near the coast, without a change of velocity in the two crustal layers. Based on these velocity and density models and as a result of subsidence analysis we suggest the presence of continental crust beneath the Levantine Basin. The depth migrated nultichannel sections reveal recently active faults that cut through the entire evaporitic and post-Messinian sediment layer. This supports the thesis of active strike slip faults running subparallel to the DST.
T53B-0490 1340h
Neogene and active shortening offshore the reactivated Levant margin in Lebanon: results of the SHALIMAR cruise
The objectives of the SHALIMAR cruise were to study recent deformation of the Mediterranean seafloor west of Mt Lebanon. We collected multibeam bathymetry and back-scatter images, reflection seismic profiles - surface and deep-towed, 3.5 kHz echo-sounder data, gravity and magnetic data over an 80 km-wide zone offshore the entire Lebanese coast. The bathymetry reveals a very steep slope between Beyrouth and Batroun, with a water depth of 1500 m only 5 km offshore. Between Saida and Tripoli (33.5N to 34.5N), both the bathymetry and seismic lines show a series of ramp anticlines affecting Plio-Quaternary deposits and a seismically transparent layer with variable thickness corresponding to the Messinian evaporites (5.6 Ma). This fold-and-thrust belt is the offshore expression of shortening related to the formation of Mt Lebanon. It is limited westwards by a prominent fold front 30 km from shore. Some ramps appear to be submarine continuations of faults documented ashore (e.g., Aabde and Tripoli thrusts). The strikes of fold axes are consistent with WNW-ESE shortening and slip-partitioning along the 30$^°$ Lebanese bend of the Levant fault. North of Tripoli and south of Saida, the continental margin displays a wider shelf (20 km) and gentler slope. In the south, at 1200-1500 m depth small, closely spaced, NE-trending scarps attest to young, distributed dip-slip faulting, although deformation is much less than north of Beyrouth. At the southern extremity of our survey, NW-trending normal fault scarps roughly aligned with the Mt Carmel-Haifa fault vanish 50 km offshore in the Levant basin. Bouguer gravity anomalies, estimated by removing from free-air gravity data the effect of bathymetry, display a very steep gradient between Beyrouth and Tripoli, marking the passage from thickened Mt Lebanon crust to thin crust in the Levant basin. Gravity anomalies also outline large, NE-SW trending steps between the basin crust and thinned continental crust near and south of Beyrouth. The particularly large negative anomalies in the north likely reflect flexure of the lithosphere due to subduction under Cyprus and underthrusting beneath Lebanon. The SHALIMAR data suggest that local compression induced by the bend in the Dead Sea Transform reactivated faults of the Mesozoic Levant margin into lateral ramps and thrusts whose evolution may lead to subduction. It is possible that the shorter distance between the transform and margin in central Lebanon (35 km) than in Syria (65 km) is one measure of continental shortening. That the oceanic lithosphere is strong may explain why the Carmel and Roum faults did not extend into the Levant basin.
T53B-0491 1340h
GPS Evidence for Northward Motion of the Sinai Block: Implications for E. Mediterranean Tectonics
We present GPS yearly survey-mode observations for the period 1997 to 2003 at 12 sites within and adjacent to the Sinai Peninsula to constrain motion of the Sinai Block. Sites in the Sinai Peninsula show northerly motion relative to Africa (Nubian plate) at an average rate of 1.5 +/- 1 mm/yr north and -0.8 +/- 1 mm/yr west (1 sigma). While there is some variability, 11 of 12 sites show northerly motion strongly suggesting that the Sinai as a whole is moving northward relative to Nubia. Continuous IGS GPS sites in Israel, west of the Dead Sea fault (greater than 45 km from the fault trace and outside the main zone of elastic strain accumulation) show a similar northerly sense of motion relative to Africa (~ 2.7 +/- 1mm/yr north and -0.9 +/- 1 mm/yr west), suggesting that the entire Sinai Block south of Lebanon is characterized by northward translation relative to Africa. Using an elastic block model we estimate slip rates for the Gulf of Aqaba - southern Dead Sea fault system of 5.1 +/- 0.7 mm/yr, left lateral, and along the Gulf of Suez of 3.0 +/- 0.5 mm/yr left lateral. Present-day opening of the Gulf of Suez is less than 1 mm/yr. This block model provides a quantitative, kinematic explanation for the first order, present-day tectonic character of the easternmost Mediterranean, including the Dead Sea fault system, the Gulf of Suez, and the Cyprus Arc.
T53B-0492 1340h
Active Faulting in the French Western Pyrenees: Paleoseismic and Macroseismic Evidence
We have identified a 50-km-long active fault scarp between Lourdes and Arette in the French Pyrenees. This region was affected by large and moderate earthquakes in 1660 (Io = VIII-IX), in 1750 (Io = VIII, Lourdes) and in 1967 (M=5.7, Io = VIII, Arette). Historical and instrumental catalogues that extend back to the 14th century suggest an East-West elongated active zone parallel to the Lourdes fault scarp. Most earthquakes in this area are shallow but available focal mechanism solutions do not indicate a coherent pattern. We have compared the felt areas of the main earthquakes in order to estimate their macroseismic magnitude. In addition we re-calculate the instrumental magnitude of the 1967 earthquake (the largest instrumental earthquake recorded in the Pyrenees) and determine a new focal mechanism solution according to WWSSN network data. Paleoseismic studies conducted in this area indicate a fault scarp characterised by an East-West Trending 50-m-high geomorphologic structure, composed of 3 continuous and linear segments. To the north, the fault controls Quaternary basins and shows uplifted and tilted alluvial terraces along strike. Evidence for active faulting is indicated by stream channels which are deviated and abandoned likely due to the successive uplift of the northern block of the fault. In Capbis and Arcizac, geomorphic studies, georadar prospecting and trenching along the fault scarp illustrate the successive fault movements during the late Holocene. Trenches exhibit shear contacts with flexural slip, and thrust ruptures with deformed alluvial units in buried channels. Geomorphologic and paleoseismic investigations, and magnitude-length scaling laws (Wells and Coppersmith, 1994) reveal that the Lourdes fault has the potential of producing an earthquake with Mw 6.1 to 6.9. The fault would thus correspond to a major seismic source in the western Pyrenees with possible magnitude larger than the 1660 main historical event. \hspace \hline Reference : Wells, D.L., and Coppersmith, K.J., 1994, Bull. Seism. Soc. Am., 84/4, 974-1002.
<a href='http://eost.u-strasbg.fr/recherche/Equipe7.html' >http://eost.u-strasbg.fr/recherche/Equipe7.html
T53B-0493 1340h
Rheological constrains for the seismicity in a diffuse plate boundary: the Gibraltar Arc in the Westernmost Mediterranean
The Alboran Sea (Westernmost Mediterranean) is located in the inner part of an arcuate Alpine orogenic belt comprised by the Betic and Rif, connected through the Gibraltar Arc. The boundary between the African and Eurasian plates in the Betic-Rif-Alboran region is a large zone of diffuse deformation and strain partitioning with simultaneous extension and lithospheric attenuation. In fact, the distribution of local seismicity is quite variable, with scattered seismic swarms which spread out over a broad area of plate convergence deformation. Focal depth distribution of earthquakes shows that most of seismicity is located in the crust (86%), and preferentially in the upper, brittle crust (90% of the crustal seismicity at $<$15 km). A three dimensional rheological model has been developed in the region to characterize the brittle-ductile transition (BDT); calculating a multiple set of regularly-spaced strength profiles based on a synthetic 3D lithospheric structure that gathers most of the available geological and geophysical data. Additionally, data from geodetic measurements of crustal deformations and earthquakes focal mechanisms have been used to validate the boundary conditions that govern the rheological model. Predicted rheological domains in the crust, either brittle or ductile, agree with focal depth distribution of crustal earthquakes in the westernmost Mediterranean; moreover, the scarce micro-seismicity located in the lithosphere mantle ($<$14%) is also in agreement with these results. Most of the seismicity tends to nucleate in the brittle domains, up to the BDT, resulting in a gap between uppermost crustal and lithosphere mantle intermediate seismicity. In spite of seismic swarms that seem to depict active faults, it is needed to apply relative location methods to constrain the prolongation in depth of active faults. This sort of comparison between the rheological modelling results and the distribution of seismicity in the Westernmost Mediterranean provides important clues for any attempt to constrain the strain velocity field related to the Africa and Eurasia plate convergence. This type of study in the Betic-Rif-Alboran crustal domain deserves to integrate the geometry and timing of active faults, to unravel the link between these structures and active mountain uplift (and coeval subsidence) processes.
T53B-0494 1340h
The CESIS project: a new satellite seismic and CGPS network in Southern Italy to study plate boundary deformation in the Central Mediterranean
The collision between Africa and Eurasia is associated with a complex pattern of deformation within the plate boundary zone, with subduction of oceanic fragments, crustal extension along formerly contracting orogenic belts and back-arc spreading in Tertiary basins. First-order scientific problems regarding the existence of rigid blocks within the plate boundary, the present-day activity of the Calabrian slab and the regional crust and upper mantle structures are still awaiting for a better understanding. Established in 2002 by the INGV (Istituto Nazionale di Geofisica e Vulcanologia) the CESIS project is deploying a permanent, integrated and real time monitoring system of 60 co-located permanent GPS and broad band seismometers in Southern Italy. The network is connected to the acquisition centre (located in Rome and duplicated in Grottaminarda) by a satellite system (VSAT). 50 seismic stations will be equipped with broad-band (0.033-40 Hz) seismometers and accelerometers, while 10 will be equipped with very broad-band seismometers. All the sites will be equipped with 1 Hz CGPS receivers. Average station distance is planned to be about 30 km, with closer spacing near active faults. The research activity resulting from the data coming from the CESIS network will thus explore the full range of temporal and spatial frequencies that characterize plate boundary deformation, allowing a large range of scientific problems, ranging from earthquake source studies to regional plate kinematics, to be tackled. Some of the most intriguing targets concern (a) the study of present activity of the Calabrian slab and its associated crustal deformation, (b) the southern boundary of the Adriatic block (a rigid microplate whose existence have been proposed on the basis of seismicity distribution, earthquake slip-vectors, and space geodesy), and (c) the monitoring of strain build-up along seismogenic faults. We present (a) the technical description of seismic and geodetic data acquisition, (b) the GPS and seismic stations monumentation, (c) the planned and existing site distribution, (d) the flow and archiving of seismic and geodetic data, and (e) first results of data analysis.
T53B-0495 1340h
Vertical Tectonics in the Calabria-Apennine Arc-Continent Collision Orogen from Geomorphic Features (CATSCAN Project)
Calabria is an exotic continental fragment that forms a ridge parallel to the Aeolian volcanic arc and separates accretion and rollback-subduction of Ionian lithosphere from forearc extension on the Tyrrhenian side. Along-strike, the Calabria ridge merges into the Apennines and the Maghrebides, which formed in the progressive collision and emplacement of the arc onto Apulian and African passive margins. Flights of marine terraces in Calabria and adjoining parts of Sicily and southern Italy indicate rapid late-Pleistocene uplift along both collisional and rollback portions of the chain. The pattern of uplift appears to vary along- and across-strike as well as through time during the late evolution of the orogen, suggesting multiple causes and a time-transgressive pattern. We seek further constraints from river and ridge profiles and slope distribution using the SRTM 90m-DEM. The following results are preliminary: 1. The Calabrian ridge is a broad antiform. A bowed erosional paleosurface is preserved in crestal regions where chemical weathering and denudation may be relatively uniform. In this region slopes are gentle and drainage valleys are wide and low grade. A sharp boundary separates this region from the flanks of the antiform, which are gouged by energetic rivers into upwardly concave slopes and deep canyons. The western flank is also cut by longitudinal basin-forming dip-slip faults. Western rivers profiles exhibit sharp concavities suggesting fault-controlled uplift. Eastern river profiles are systematically smoother, as expected in response to seaward tilting. Regarding the pattern of uplift, morphology and structure are generally consistent, suggesting a steady-state regime. 2. The foredeep between the southern Apennines and the Apulian foreland is characterized by a set of longitudinal rivers, draining along the Apennine mountain front into the Gulf of Taranto. These rivers have narrow closely spaced drainage basins with little concavity and seem remarkably immature. This suggests rapid uplift of the foredeep in the Gulf of Taranto, where the Apennine collision is still incomplete. About 100km north of the gulf, drainage shifts to the NE and crosses the foredeep into the Adriatic Sea. This transverse drainage is more deeply incised and tends to capture the longitudinal drainage, thus moving the divide between them to the SE. This suggests that the transition from longitudinal to transverse drainage is tracking the collision to the SE.
T53B-0496 1340h
Dynamic Development of the Europe/Adria Plate Boundary During the Transition from Oceanic Subduction to Continental Collision in the Northern Apennines: a Case History for Subduction Erosion
In the Northern Apennines the dynamic of the margin frontal wedge during the Oligo-Miocene transition from oceanic subduction to continental collision is preserved in a chaotic complex called the Sestola Vidiciatico Unit. There, oceanic sedimentary rocks accreted and deformed within the Late Cretaceous to Late Eocene accretionary prism have been successively involved in the plate boundary through underthrusting in a process of rock recycling. The Sestola Vidiciato Unit is a less than 1 km thick rock unit containing both gravitational and tectonic components: the first being massive debris flows accumulated at the frontal part of the prism and the second slivers of the inactive accretionary prism usually disrupted and occurring as broken formations. The boundary between the debris flows and the broken formations are always tectonic. Also, the debris flows are typically in stratigraphic contact with the foredeep turbidites representing the incoming, continental plate, indicating that the decollement is not located at the frontal prism/incoming sediment boundary. The older deformation phase recorded in this complex unit is of widespread extension: the same structures are present at the top of the foredeep turbidites indicating how the Sestola Vidiciatico Unit has been deformed together with the rocks of the incoming plate and both have been involved in the footwall of the megathrust fault representing the plate boundary. As deeper parts of the system are approached the deformation changes from widespread extension to compression. At the same time of the Late Cretaceous to Late Eocene accretionary prism development and Oligo-Miocene formation of the Sestola Vidiciatico Unit, sediments had been accumulated on the upper plate slope. These sequences testify a general, stable deep water environment during the accretionary prism development. In the Early Miocene the slope basins are characterised by a regional unconformity, after which the sedimentation starts again with deepening upwards sequences (i.e. the Bismantova Group) testifying the occurrence of long term subsidence. The long term subsidence of the margin and the concomitant entrance of progressively thicker continental crust in the subduction zone may be compared to modern subduction zones where the entrance of aseismic ridges, for example, caused the switch from tectonic accretion to tectonic erosion. So, we interpret the Sestola-Vidiciatico Unit as the best candidate to represent the first case history of a fossil subduction channel formed in a tectonically erosive convergent margin.
T53B-0497 1340h
Block modeling of Crustal Velocity in Italy and Surrounding Regions
We use GPS measurements and block modeling to investigate the present-day deformation of the Italian peninsula and surrounding regions. The central Mediterranean displays an assemblage of lithospheric blocks with different structural and kinematics features and a variety of geodynamic processes, including subduction, back-arc spreading, rifting, thrusting, normal and strike-slip faulting, trapped between the relatively rigid African and Eurasian plates, for which global plate motion models predict a NW-SE convergence at about 7 mm/yr. The block model incorporates secular velocity and fault geometry estimates, as well as elastic strain accumulation. With this model we can assess whether different hypotheses are compatible with geodetic data, estimates of fault slip rates and locking depths, areas of rigid block rotation, and regions of anomalous strain accumulation. We present a geodetic velocity solution for Italy and surrounding areas, obtained from the analysis of continuous and survey-mode Global Positioning System observations collected between 1991 and 2002. The velocities are relative to a stable Eurasian frame. The block model shows extension in the Apennines, shortening in the central Alps, Dinarides and Ionian Island (Epiro coast), and right-lateral slip along the Kefallinia fault zone. The predicted faults slip rates are in good agreement with geodetic and geologic observations. The deformation pattern observed in the Adriatic domain suggests that the Adriatic is a microplate (Adria) and that the southern boundary with the Nubia plate and the Aegean domain may be located along the Apulia Escarpment and the Kefallinia fault.
T53B-0498 1340h
Topographic Signature of a Subduction and Collision Zone: Uplift Pattern from Marine Terraces and Delta Sequences along the Calabrian Ridge, South Italy (CAT SCAN Project)
Ancient coastlines, in the form of abrasion platforms or coastal deposits, provide markers in the landscape that can be correlated and dated to determine patterns and rates of uplift. We present new data on an excellently exposed flight of Pleistocene coastlines to constrain patterns of uplift associated with rollback subduction and arc-continent collision in the Calabrian arc and the southern Apennines. While many aspects of this type orogen of Mediterranean tectonics are coming into focus, major issues are still unresolved. A more complete and detailed resolution of vertical tectonics is expected to discern between competing hypotheses: for instance, whether or not subduction is still ongoing. We present results from a preliminary database of marine terraces mapped on a regional scale with an SRTM-3arcsec DEM and field observations. Supplementary to the geometric correlation of terraces, we are currently processing samples for cosmogenic isotope analysis to determine age relationships. Four prominent, seaward dipping terrace levels were identified on the west coast at heights of 100m, 170m, 400m and 700m, which we call T1, T2, T3 and T4 respectively. The upper terrace levels (T2, T3 and T4) comprise both bedrock-carved platforms and fossiliferous coastal deposits, with rounded paleocliff edges that form clear scarps in the DEM. The lower terrace level (T1) consists of shallow marine deposits with fossiliferous horizons containing in-situ coral. The geographic distribution of the T4 terrace level is particularly extensive, forming a continuous ridge along the backbone of Calabria, in the footwall of a major N-S trending extensional fault bounding the M\'{e}sima Valley. Similar terraces with well-defined sea-cliffs and thick accumulations of shallow marine deposits were identified on the eastern flank of Calabria. We believe this is the first documentation of terraces at this level on the east coast, and is a particularly interesting find, as it suggests that the earliest uplift affected both sides of Calabria. In the M\'{e}sima Valley, fossiliferous shallow marine sediments, with abundant oysters, dip at 10$\deg$E, which we interpret as hangingwall tilt toward the eastern-bounding fault of the M\'{e}sima Valley. These results demonstrate that ancient marine shorelines provide a reliable datum for calibration of (1) regional vertical motion since the formation of the earliest-formed terrace, and (2) local tectonic deformation following terrace formation.
T53B-0499 1340h
Toroidal mantle flow around the calabrian slab (italy) from shear-wave splitting
Shear-wave birefringence is likely caused by ongoing and/or fossil mantle flow. Splitting parameters in the Southern Tyrrhenian subduction zone define a quite complex pattern of fast directions with large delay times up to 2.7s. Fast directions are prevalently trench parallel below the slab and rotate to trench normal at the western edge of the slab depicting a ring around it. We locate the different contribution of sub and supraslab anisotropy using SKS phases and local S from deep earthquakes. We interpret these anisotropic pattern as toroidal mantle flow induced by rollback subduction as observed in laboratory experiment.
T53B-0500 1340h
Resolving Mantle Flow Beneath Italy: The Scientific Goals of the RETREAT Seismological Deployment, Northern Apennines, Italy
The Apennines mountains in Italy are associated with subduction by many researchers, motivated by uplift beside thick accretions of sediments in the Po River valley and the Adriatic Sea; deep earthquakes and volcanism in southern Italy, and a long tabular high-wavespeed feature that is observed in mantle tomography from the base of the Apennines to the transition zone. The objective of the RETREAT seismic array is to resolve the pattern of mantle flow associated with the inferred rollback of the Adriatic slab. Because both sides of the active orogen are continental, the Apennines differ from typical oceanic subduction zones. The descent of lithosphere has not, in the historical record, been accompanied by great thrust earthquakes. GPS estimates of convergence are small, no more than a few mm/year. It is not known how much of the crust of the downgoing plate descends with the mantle lithosphere, and how much accretes to the upper crust of the overriding plate. Our seismic results will allow us to resolve the Moho beneath the Apennines and the transition into the actively flowing asthenospheric mantle. Many researchers have argued that subduction of the Adriatic slab has induced a corner flow in the asthenosphere above the slab. Geodynamic modelling suggests that a complex double-cell corner flow is necessary to generate the observed extension of the overriding plate. RETREAT includes broadband seismometers in both 2-D- and linear-arrays that straddle the Apennines and its mantle high-velocity features. In several subduction zones (Kamchatka, Cascadia, Alaska) receiver functions detect P-to-S converted waves from both top and bottom of the subducted oceanic crust. Anisotropy near the top of the slab enhances the P-to-S conversion and suggests the presence of hydrous minerals. If subduction of the full lithosphere is occurring beneath Italy, we expect to observe P-to-S converted phases, with an anisotropic signature, from crust within the subducted lithosphere. Weak or absent P-to-S slab conversions will favor a scenario in which the upper crust does not subduct. The latter scenario would bolster the Rudnick model for continental composition, i.e., that detached lower crust recycles back into the mantle.
T53B-0501 1340h
Mantle Anisotropy Below the Northern Apennines From RETREAT Seismic Data
Progressive collision between the Eurasian and African plates is associated with a steady growth of the Apennine mountain range. Simultaneous extension in the convergent margin of the Adriatic and Tyrrhenian domains results in a retreat of the subducting Adriatic plate from the orogenic front caused by sub-lithospheric mantle processes. A study of seismic anisotropy in the mantle around the contact of the two lithospheric domains by the multidisciplinary project RETREAT aims to develop a self-consistent dynamic model of syn-convergent extension in the Northern Apennines. The distribution of seismic anisotropy in the upper mantle, the crust and lithosphere thicknesses, and the location and geometry of the Adriatic slab are specific objectives of the project. We model the large-scale seismic anisotropy due to preferred orientation of olivine by a joint analysis/inversion of body-wave data (P residuals, shear-wave splitting) recorded during the RETREAT field measurement as well as data of permanent INGV observatories. Initial analysis of splitting of core shear waves (SKS) has detected both distinct lateral variations of anisotropic parameters (splitting time delay and the fast shear-wave orientation) and their dependence on the direction of propagation (back azimuth, incidence angle) within the anisotropic upper mantle. Similar to continental regions without an active subduction zone, a significant part of the observed anisotropic signal should be generated by a frozen-in fabric of different lithospheric domains (e.g., terranes or microplates). At domain boundaries, the effective anisotropy should vanish or suffer large spatial variation (e.g., at the Alps-N. Apennines; Adriatic-Tyrrhenian), while within a domain itself it remains consistent. To develop a 3D self-consistent anisotropic model of the upper mantle structure we have to consider both the frozen (or deformed) part of anisotropy within the mantle lithosphere and that part of anisotropy which reflects a sub-lithospheric mantle flow, related to the more viscous high-velocity heterogeneity represented by the subducting plate.
T53B-0502 1340h
Preliminary results from the CAT/SCAN seismic experiment: Structure and kinematics in a slab-rollback setting
The Calabrian-Apennine-Tyrrhenian/Subduction-Collision-Accretion Network (CAT/SCAN) is a multi-disciplinary study of the Calabrian Arc in both its subduction-rollback and collisional settings. In the first phase of this international experiment, we have deployed 39 broad-band seismometers in a two-dimensional array extending from the southern Apennines into Calabria. 12 ocean bottom seismometers will extend the coverage into the Ionian and Tyrrhenian seas with two cross-strike profiles, after early Fall 2004. The data from these and other stations in southern Italy are combined with pre-existing data to constrain velocity structure and local earthquake sources. Results provide new information on crustal thickness across the transitions between subduction and collision and on the morphology of the shallow part of the slab near this transition. Precise locations and focal mechanisms illuminate active structure in the crust and within the slab. These results help to address critical questions: (1) what happens to the slab and the overriding plate as they are forced by rollback into the narrowing space between Adria and North Africa? (2) What are the location and nature of the transition between the seismically active thrust belt north of Sicily and the normal faulting along the Apennines and Calabria? (3) Is rollback still in process, or has it stopped? (4) What are the relationships between crust and mantle seismicity and the fine-scale structure of the upper mantle? And (5) what is the relationship between destructive shallow earthquakes in southern Italy and the large-scale dynamics of the slab? We will use travel-time and body-waveform tomography as a proxy for temperature variations related to lithospheric involvement in the subduction and crustal extension, shear-wave splitting as a proxy for vertically integrated mantle deformation and flow pattern, regional surface wave inversions for crustal and upper-mantle azimuth-dependent velocities to provide constraints on the depth distribution of anisotropy related to mantle deformation, and receiver functions to determine crustal structure. Preliminary results of this ongoing experiment will be shown.
T53B-0503 1340h
Constraints from Moho Geometry and Crustal Thickness on the Geodynamic Origin of the Vrancea Seismogenic Zone (Romania)
Reprocessing of industry deep seismic reflection data (Ramnicu Sarat and Braila profiles) from the Eastern Carpathian foreland of Romania provides important new constraints on geodynamic models for origin of the intermediate depth Vrancea Seismogenic Zone (VSZ). Mantle (70-200 km) earthquakes of the VSZ are characterized by high magnitudes (greater than 6.5), frequent occurrence rates (approximately 25 years), and confinement in a very narrow (30 by 70 by 200 km) near vertical body atypical for a Wadati-Benioff plane. Crustal seismicity of magnitudes less than 6 occurs over a much broader region (150 by 200 by 45 km) in the Southeastern Carpathian foreland and is both vertically (25 km) and horizontally (50-100 km) offset to the E from mantle seismicity. The foreland basin in front of the 110 degrees bend of the Southeastern Carpathians is characterized by (1) an 18 km thick Miocene-Quaternary sedimentary cover (2) late Pleistocene-Quaternary extensional faults, and (3) unusually low topography. Two deep (20 s) seismic reflection profiles (64 km total length) from the SE Carpathian foreland reveal (1) a high-amplitude, gently east-dipping reflection across most of the section from what we interpret to be the Moho at 15 s (45 km), (2) a thick sedimentary cover increasing in thickness from east (4 s; 8 km) to west (7 s; 14 km), (3) an eastward increase in crustal thickness from 38 km (above the VSZ) to 42 km, and (4) seismic and topographic evidence for a newly imaged, possibly seismically active basement fault with a surface offset of 30 m. Along with the relocation of crustal events with magnitude greater than 3, which are then projected onto the seismic sections, this research aims to identify the distribution, type and geometry of active crustal faults and their possible continuation into the upper mantle. This will help evaluate the spatial relationships between crustal and mantle seismicity, and implicitly, between the mechanical relationship of the crust and uppermost mantle lithosphere. These observations appear to argue against recent models for west-dipping subduction of oceanic lithosphere at or in the vicinity of the Vrancea seismogenic. Since there seems to be a mechanical coupling between the foreland crust and upper mantle, one possible explanation for the geodynamic origin of the Vrancea zone could be partial delamination of the continental lithosphere in an intra-plate setting along a sub-horizontal lithospheric interface in the Carpathian hinterland that involves a remnant lithospheric coupling between the crust and uppermost mantle in the foreland.
T53B-0504 1340h
Echoes from the Transylvanian Lithosphere: Preliminary Results From the DRACULA I Profile and Implications for Geodynamics of the Vrancea Zone
Preliminary examination of new deep seismic reflection data in the Transylvanian Basin of Romania, collected through Project DRACULA, reveals strongly layered subhorizontal reflectivity throughout the middle and lower crust, as well as the upper mantle. Collected with 20 kg shots every 1 km recorded to 60 s by a 32-km active spread (640 channels at 50 m) with Reftek-125 seismometers in roll-along mode, the DRACULA I profile consists of a 240-km transect extending NW from the Vrancea Seismogenic Zone. Single-fold shot gathers repeatedly image (1) a strong reflector (basement cover contact?) at around 1.6-2.0 s TWTT (~3-5 km) within the Transylvanian basin, (2) strong sub-horizontal reflections beneath the Persani Mountains that may represent either structural elements or igneous intrusive features (3) suhorizontal to gently E-dipping reflections beneath the Eastern Carpathians at 6.0-8.0 s TWTT that may be the downdip continuation of regional detachment faults of the Carpathian nappes, and (4) strong sub-horizontal and laterally continuous reflectivity, beginning at 6-10 s TWTT (15-25 km) and continuing downward in some cases to as much as 30 s (greater than 100 km), clearly within the upper mantle. An interpretation for the strongly layered upper mantle, and the absence of a clear break in reflectivity across the Moho is still premature, but it would appear that geologic processes in the Transylvanian mantle have imparted reflection fabrics similar to those seen in the lower crust on many deep reflection profiles. Of particular significance is the exclusively and pervasively subhorizontal geometry of reflective layering throughout the Transylvanian crust and upper mantle, seemingly precluding the presence of a former subduction zone anywhere within the 240-km trace of the profile. These observations would appear to argue against a subduction origin for intermediate depth seismicity in the Vrancea Zone.
T53B-0505 1340h
Carpathian Rotation During Slab Roll Back and Detachment
As part of a collaborative project focusing on Mediterranean geodynamics, we report paleomagnetic results that constrain the development of the Carpathian arc (Romania). While tectonic rotations are evaluated through analysis of paleomagnetic directions, age control on sedimentary formations is obtained using high-resolution magnetostratigraphy performed on several sections from the Eastern and the Southern Carpathians. In Badenian to Sarmatian strata (16.4-9.9 Ma), statistical analysis of paleomagnetic site-mean directions indicate ca. 30 degrees clockwise rotation of the Southern Carpathians while remagnetization is suggested by a negative fold test in the Eastern Carpathian. During Late Miocene to Early Pliocene, magnetostratigraphic results indicate over 7 km-thick continuous deposition between 7.0 and 2.5 constrained by unambiguous correlation to the astronomical polarity time-scale (APTS). These strata record no rotation indicating that the rotations stopped everywhere by 7.0 Ma except in the Bend Area between the Southern and the Eastern Carpathians where rotation still occurred after 5.4 Ma. Our results are compared to a compilation of existing Tertiary paleomagnetic data throughout the Carpathian orogen. The outcome of this analysis is interpreted in a three stage development of the Carpathian arc: (1) up to 80 degrees clockwise (counterclockwise) rotation of the Eastern (Western) Carpathians during early eastward slab roll back; (2) 30 degrees clockwise rotation of the Southern Carpathian during Sarmatian southward slab detachment and (3) recent rotation in the bend area associated to final slab detachment and break off. To understand slab-related processes driving arc development, the Carpathian arc is compared to the Calabrian, the Hellenic and Gibraltar arcs where similar tectonic rotation patterns are observed during the Neogene.
<a href='http://www.geo.uu.nl/~forth/' >http://www.geo.uu.nl/~forth/
T53B-0506 1340h
Active Foreland Deformation of the Southeastern Carpathians from Deep Seismic Reflection Profiles DRACULA II and III: Genetic Relationships with the Vrancea Seismogenic Region
Project DRACULA (Deep Reflection Acquisition Constraining Unusual Lithospheric Activity), a collaboration between the University of South Carolina, the University of Bucharest, and the National Institute for Earth Physics in Romania, involved the acquisition of 320 km of deep (60 s) multichannel seismic reflection data in the summer of 2004. Project DRACULA involved the acquisition of three transects (DRACULA I, II, and III) surrounding the Vrancea zone of Romania in order to evaluate the existing competing models for the origin, lithospheric structure, and geodynamic setting of this highly disputable intermediate-depth (70-200 km) seismogenic region. While the DRACULA I profile, acquired in the hinterland of the southeastern Carpathians, is presented elsewhere, here we discuss preliminary results from lines DRACULA II and III collected in the foreland, east and south of the seismogenic area. DRACULA II and III profiles were designated to image the crustal structure of the foreland across the Pecineaga-Camena, and respectively, Intramoesian Faults, two of the active foreland faults, in order to place constraints on the duration, timing, and scale of foreland deformation that appears to be a direct response to the Vrancea seismogenic body. The DRACULA II line shows strong subhorizontal reflections from 14.5-15.0 (approx. 45-48 km) in the eastern side of the Pecineaga-Camena fault that we interpret to be the Moho. Unusually layered reflectivity is seen down to 20 s in this part of the line. A very intriguing high-amplitude subhorizontal reflector, approximately 7 km long, is observed at 35.5 s (approx. 130 km). The western side of the line, across the fault, displays a fairly transparent lower crust and uppermost mantle, possibly indicating the depth penetration of the Pecineaga-Camena fault across the Moho. The DRACULA III line displays a very high amplitude reflection at 14.0 s (aprox. 42 km) in the south-west that dips slightly to 14.5-15.s (45-48 km) in the north-east. This reflection is interpreted to be the Moho, but it is premature to comment whether this variation in its depth is related to the penetration of the Intramoesian fault into the lithospheric mantle. Although preliminary, results from the DRACULA II and III profiles indicate that the active crustal faults in the foreland that have significant surface expressions (up to 100 m escarpments) continue in the uppermost mantle, and may be related to the Vrancea seismogenic zone.
T53B-0507 1340h
CONVERSION OF BOUGUER GRAVITY DATA TO DEPTH, DIP AND DENSITY CONTRAST WITH COMPLEX ATTRIBUTES ANALYSIS TECHNIQUE IN THE AREA OF GREECE.
The complex attributes analysis is an operator used in the extracting parameters of the buried structures with susceptibility and density contrasts distributions, which lead to the gravity and magnetic anomalies in the region of interest. In this paper is presented the complex attributes analysis of gravity field filtered for wavelengths lower than 50 km in the territory of Greece. The area o Greece has a complex tectonic history and fault system dominated by the subduction of the African plate beneath the Eurasia. A Low-pass filter is used on the Bouguer Anomaly to cut off wavelengths lower than 50Km in order to delineate the major faults structures of interests at big depths. The complex attributes technique aids in interpretation of potential field anomalies, because it can delineate the edges of concealed targets. In obtaining the source parameters from the complex attributes like the local depth, strike and dip, the assumption of sloping contact for the subsurface model is used. The estimated local parameters are in agreement with results obtained by previous interpretations. They can be used in combination with other method to interpret the anomalous field.