T23A-1198
Lithospheric and Upper-Mantle Structure of the Red Sea and Arabian Peninsula
Using broadband seismic data recorded by various networks, a variety of techniques have been employed to investigate the lithospheric and upper-mantle structure of the Red Sea and Arabian Peninsula. This presentation will summarize our findings and conclusions about the tectonic evolution and current state of the Arabian Plate. S-wave receiver functions provide constraints on the lithospheric thickness and reveal very thin lithosphere (40-80 km) along the Red Sea coast, which thickens rapidly toward the interior of the Arabian Shield (100-120 km). A step of 20-40 km in lithospheric thickness is also observed at the Shield-Platform boundary. Mantle anisotropy has been analyzed using shear-wave splitting of teleseismic SKS waveforms. The consistent north-south oriented fast directions are not adequately explained by end-member models of fossilized anisotropy and present-day plate motion and have instead been explained by a combination of plate- and density-driven flow in the asthenosphere. Further constraints on the upper mantle velocity and anisotropy have been obtained by jointly inverting the receiver function constraints with frequency dependent surface wave phase delays. The results demonstrate that the thin lithospheric lid is underlain by a pronounced low-velocity zone and that anisotropy is required in both the lithosphere and asthenosphere. Attenuation and thermal estimates are also being explored and preliminary results will be presented. The combined results of these studies support a two-stage rifting history for the Red Sea, where extension and erosion by asthenospheric flow are responsible for variations in the lithospheric thickness. These lithospheric variations guide asthenospheric flow beneath western Arabia and the Red Sea, leading to a large-scale thermal anomaly that is associated with Cenozoic uplift and volcanism. This work was performed under the auspices of the U.S. Department of Energy by University of California, Lawrence Livermore National Laboratory under contract W-7405-Eng-48. UCRL-ABS-234290.
T23A-1199
The Rapid Drift of the Indian Tectonic Plate
The breakup of the supercontinent Gondwanaland into Africa, Antarctica, Australia and India about 140 million years ago and consequently the opening of the Indian Ocean was caused by heating of the lithosphere from below by a large plume whose relicts are the Marion, Kerguelen and Reunion plumes. Plate reconstructions based on paleomagnetic data suggest that the Indian plate attained a very high speed (18-20 cm/yr during late Cretaceous) subsequent to its breakup from the Gondwanaland and slowed down to ~5 cm/yr since the continental collision with Asia during the last ~50 Ma. The Australian and African plates moved comparatively lesser distances and at much lesser speed of 2-4 cm/yr. Antarctica remained almost stationary. This super mobility makes India unique compared to the other fragments of Gondwanaland. We propose that when the parts of Gondwanaland were separated by the plume, the penetration of their lithospheric roots into the asthenosphere played an important role in determining their speed. We estimated the thickness of the lithospheric plates of the different parts of Gondwanaland around the Indian Ocean using the S-receiver function technique. We found that the part of Gondwanaland with clearly the thinnest lithosphere has travelled with the highest speed - India. The lithospheric root in South Africa, Australia and Antarctica is between 180 and 300 km deep. The Indian lithosphere is in contrast only about 100 km thick. Our interpretation is that the plume that partitioned Gondwanaland has also melted the lower half of the Indian lithosphere thus permitting faster motion due to the ridge push or slab pull.
T23A-1200
The Manitoba Teleseismic Array: Examining the Westward Extent of the Superior Craton in Canada
The Superior Craton, the largest region of stable Archean crust, is exposed nearly halfway across Canada, from eastern Quebec to eastern Manitoba. Westward into Manitoba, the Superior is believed to underlie the Phanerozoic sediments of the Western Canada Sedimentary Basin, but is not exposed at the surface; further north, the contact between the Superior and the Trans-Hudson Orogen are exposed along the Thompson Nickel Belt. The Superior Province is underlain by a high-velocity upper-mantle anomaly with complex internal structure; past teleseismic studies have located features in the lithosphere corresponding closely to crustal structures, indicating long-term stability of the mantle root; however, there are indications of deformation and modification of the mantle of the eastern Superior. The lithosphere beneath the westernmost portion of the Superior and its contact with the Trans-Hudson has not been examined in detail due to a lack of instrumentation. To remedy this, we have deployed a set of seven broadband teleseismic instruments in both northern and southern Manitoba. Two southern stations (in Winnipeg and Star Lake, Manitoba) were deployed in 2006; an additional five stations were deployed in 2007 in both southern (Brandon) and northern Manitoba (Grand Rapids, Jenpeg, Thompson and Gillam). Preliminary results indicate that the unusually strong shear-wave splitting known to exist beneath western Ontario continues into Manitoba; we have also successfully applied the noise cross-correlation technique to generate a surface-wave train between the Star Lake and Winnipeg stations. Once sufficient data have been recorded, we expect to perform teleseismic tomography, shear-wave splitting and surface-wave analysis over the whole of Manitoba, extend the Superior results westward, and contribute to a better understanding of Hudson Bay as a component of the Hudson Bay Lithospheric Experiment (HuBLE).
T23A-1201
Upper Mantle Flow Beneath the Hangay Dome, Central Mongolia
In order to investigate the upper mantle deformation beneath central Mongolia and its relation with the major structures such as the Hangay dome, the Gobi-Altai range, the Siberian platform and the Baikal rift, a NS-trending profile of broad band seismic stations has been deployed for 5.5 months in 2003 from the southern Siberian craton to the Gobi-Altai range and crossing the whole Hangay dome. Mantle flow is deduced from the splitting of teleseismic shear waves such as SKS phases and reveals a homogeneous NW-SE trending pattern. Because the observed delay times of 1.5 to more than 2.0 s suggest a coherent mantle flow over large mantle thicknesses and since the observed fast directions are parallel to the trend of the lithospheric structures and close to the trend of the plate motion, we propose that both the lithosphere and the asthenosphere may add their anisotropic effects beneath central Mongolia. Seismic anisotropy beneath the permanent station ULN in Ulaanbatar in eastern Mongolia is consistent with such interpretation since it reveals the presence of two anisotropic layers, the lower one oriented NW-SE, close to the trend of the plate motion and the upper one NE-SW, close to the trend of the lithospheric structures in this area. In order to interpret the slight clockwise rotation of the fast SKS directions with respect to the HS3-Nuvel1A plate motion vector, we propose that the sub-lithospheric flow beneath Mongolia is deflected by the thick root of the Siberian platform. In the Eurasian reference frame, the SKS splitting beneath central Mongolia show a rather good parallelism with the GPS vectors (Calais et al., 2003) suggesting that the crust escapes toward the east coherently with the lithospheric and asthenospheric mantle flow. A different behavior is observed in western Mongolia, where the GPS vectors trend NS whereas the fast SKS directions trend EW, suggesting a complete decoupling between the upper crust moving northwards and the asthenospheric mantle flowing eastwards.
T23A-1202
Joint inversion for 3-dimensional S-velocity mantle structure along the Tethyan margin
For purposes of studying the lateral heterogeneity as well as for ultimately predicting seismograms for the region which extends from the western Mediterranean region to the Hindu Kush, we construct a new 3-D S-velocity model by jointly inverting regional waveforms, surface wave group velocity measurements, teleseismic S arrival times, and crustal thickness estimates from receiver functions, refraction lines, and gravity surveys. We can expect better resolution for the resulting model than when using individual data set, because these data types have complementary resolving power for crust and mantle structures, vertical and lateral variations, shallow and deep mantle features, local and global structure, and are jointly inverted to image the complexity of this tectonically diverse area. We have fitted the waveforms of regional S and Rayleigh waves from over 3800 seismograms using Partitioned Waveform Inversion. We have accumulated over 3000 crustal thicknesses from receiver functions, gravity measurements, and refraction profiles. We have measured Rayleigh wave group velocities for hundreds of new paths recorded at the MIDSEA stations and combined them with thousands of existing paths transecting the region. We have over 5000 teleseismic S arrival times measured through cross correlation and 200,000 more from picks originally reported to the ISC. We scale the resulting S-velocity model to a P-velocity model using observed relations between S and P delay times as well as mineral physics. We then update the P-model using P delay times and compare the result to existing P-velocity models of the region. We discuss features of our new model, which includes oceanic structure, cratons, subducting slabs that penetrate into the lower mantle and others that do not, low-velocity mantle plumes, rifts, plateaus, and basins.
T23A-1203
Investigating the Abitibi-Grenville Region, Eastern Canada: Improvements in Resolution From a New Seismograph Network, Central Quebec
Teleseismic studies of the Superior Craton and neighbouring Proterozoic Grenville Province in eastern Canada have revealed significant variability in lithospheric structure on a sub-provincial scale. Of particular interest is a northwest-southeast trending anomaly of lowered seismic velocity imaged by regional-scale body wave tomography and continental-scale surface wave tomography. Until now, the regional-scale studies have met restrictions in their ability to constrain the 3D geometry of the low-velocity feature, mainly due to a lack of seismograph station coverage in the central part of Quebec. In summer 2007, five new broad-band seismographs were deployed across central and western Quebec. The layout of the array was designed to address the problem of seismograph coverage for lithospheric structure studies across the region. The stations use local AC power and Internet telemetry to provide seismic data in real time. We use P-wave travel-time data from teleseismic earthquakes from the new network in conjunction with POLARIS and CNSN stations in eastern Ontario and western Quebec to study the deep lithospheric structure of the Abitibi-Grenville area and to provide new insights into the tectonic history of the region, including the interaction of cratonic lithosphere with mantle thermal anomalies.
T23A-1204
3D Radial and Azimuthal Anisotropic Structure in North America
We recently developed a 3D tomographic model of the upper mantle beneath North America that includes both isotropic S velocity structure as well as radial and azimuthal anisotropy (Marone et al., 2007; Marone and Romanowicz, 2007). This model was constructed from a joint inversion of fundamental and higher mode surface waveforms together with constraints on azimuthal anisotropy derived from SKS splitting measurements. This model showed evidence for the presence of two layers of anisotropy beneath the stable part of the North American continent: a deeper layer with Vsh>Vsv and with the fast axis direction aligned with the absolute plate motion direction suggesting lattice preferred orientation of anisotropic minerals in a present day asthenospheric flow and a shallower lithospheric layer likely showing records of past tectonic events. Under the tectonically active western US, where the lithosphere is thin, the direction of tomographically inferred anisotropy is stable with depth and compatible with the absolute plate motion direction. We here present an updated model, which includes a larger waveform data set (three more years of data including new data from US Array) and a larger SKS splitting dataset. In addition, we are going one step further in that we now combine the results from inversion for radial and azimuthal anisotropy, to recover the distribution of the direction and amplitude of the fast axis in 3D, under the assumptions of hexagonal symmetry. We discuss the results in the light of possible causes of anisotropy in the cratonic lithosphere.
T23A-1205
Crustal thermal regime and its relationship to seismogenic layer thickness: Comparisons among Japan, California, and Kamchatka
Temperature at bottom of seismogenic layer is one of the most important and still controversial information for geological phenomena. Spatial variations in the maximum depth of seismicity have been correlated with crustal temperature. However this correlation is shown at geographically restricted area. In order to overcome this spatial limitation, determination of the basal depth of magnetic layer, Curie point depth, based on spectrum analysis of magnetic anomaly data was applied to estimate regional thermal structure. This analysis is still controversial and this depth does not necessarily represent an isotherm, however, previous studies suggested that there was an inverse correlation between these depths and heat-flow measurements. Recently, the correlation between the basal depth of magnetized layer and the seismogenic layer showed at Japan [Tanaka and Ishikawa, 2005] and California [Ross et al., 2006]. The centroid of depth of magnetized layer also showed a good correlation with the seismogenic layer beneath Kamchatka region [Tanaka, 2007]. To address how thermal regime relates to seismogenic layer, we compare relationship between depths of magnetized and seismogenic layers at three active plate boundaries: Japan, California and Kamchatka. Relationship between depths of magnetized and seismogenic layers for each region is clear, however each region has own cluster; the basal depth of magnetic sources of California region are significantly deeper than those of Japan, and the centroid of magnetic layer of Kamchatka region are deeper than those of Japan. Previous work has attributed the concept of thermal structure as a fundamental parameter for determining the thickness of the seismogenic zone, which might be strain rate, lithology, stress-state, and pore fluid pressure dependent. Each region consists of different physiographic provinces; therefore another factors are required to explain why each region has its own clusters. The Pacific/North America plate boundary dominates right-lateral shear along the San Andreas Fault in California, whereas Japanese Islands are mainly located in east-west or northwest-southeast compressional area. This difference of tectonic setting may cause the cluster shift between Japan and California. The difference in slab depths beneath Kamchatka and Japan may explain the difference.
T23A-1206
Seismic Imaging Beneath the Region of the Wallowa Mountains in Northeast Oregon
I use seismic imaging to resolve and constrain the geometry and physical properties of the Columbia River Basalt (CRB) magma chamber, as well as the velocity structure in the crust and the upper mantle beneath the Wallowa batholith in Northeast Oregon. Northeast Oregon is composed of accreted oceanic terranes and is both tectonically and magmatically unusual. This is where Earth's most recent flood basalt erupted, the ~16 Ma old CRB. The source of the CRB is around the Wallowa batholith, ~ 500 km north of the Yellowstone hotspot track, which indicates the existence of a magma chamber beneath this area. The current elevation of the Wallowa batholith is up to ~2 km, standing in the center of a circular region of post-eruption uplift, indicating major changes in isostatic forces related to major modifications in lithospheric structure associated with these eruptions. The Wallowa region is a good place to study the process of CRB flood basalt and the related lithospheric effects. However, the tectonic activity, deep structure and magmatic processes in this area are not well understood. Twenty broadband three-component seismometers were borrowed from PASSCAL in Fall 2006. All seismometers have already been installed around the Wallowa Mountains. The array, oriented NW-SE, crosses the major structures of interest, covering SE Washington, NE Oregon and part of western Idaho. Currently we collect teleseismic data for 9 months. I also use data from Gene Humphrey's former project, US Array and the Pacific Northwest Seismograph Network (PNSN). In the next several months, by analyzing those data I will do receiver function imaging and SKS splitting to image variations in the crust and upper mantle structure with high resolution.
T23A-1207
Shear wave Splitting beneath the East European Craton
The method of shear-wave splitting provides a unique possibility to identify seismic anisotropy and to measure orientations of fast and slow wave propagation directions. Seismic anisotropy results from the lattice-preferred orientations of anisotropic minerals. Though the depth of the anisotropic layer is less well-constrained there is evidence that most of the splitting occurs in the asthenosphere and/or lithosphere. Lithospheric anisotropy is often referred to as "frozen" anisotropy as it renders past tectonic events. On the other hand, asthenospheric anisotropy results from the deformation associated with plate motion. That deformation aligns the minerals, in particular olivine, in the direction of relative motion. Previous investigations in central Europe indicate fast directions aligning along the craton margins. This might perhaps indicate asthenospheric flow around the thick lithospheric keel, but other causes such as fossil anisotropy can not be excluded for that region to the west of the East European Craton. We investigate the continuation of this shear wave splitting pattern further to the East, using 18 broadband stations located on the East European Craton. Several arguments support the presence of lithospheric anisotropy under the craton: 1) large-scale coherence within each of the 4 constituing blocks but significant variations between the blocks; 2) a lacking correlation with absolute plate motion vectors; 3) reasonable correlation between fast axes orientations with magnetic anomaly alignments, the latter reflecting only crustal features. The good correlation between these crustal features with the mantle-induced shear-wave splitting strongly supports the idea of vertically coherent deformation throughout the upper mantle and crust. The observed splitting orientations reflect thus the last tectonic events of each block, frozen-in into the lithosphere.
T23A-1208
A density model of the Euro-Mediterranean mantle by Integrated Inversion of GRACE gravity data
We present preliminary results about the density structure of the mantle below the Euro-Mediterranean area, down to 1000 km depth. The velocity variations of the P-wave tomography of the Alpine-Mediterranean area (PM0.5, Piromallo and Morelli, 2003), together with the Bouguer anomalies of the satellite-derived GGM02C (Tapley et al., 2005) high-resolution global gravity model, are inverted with the method of sequential integrated inversion (Tondi and de Franco, 2006). The approach uses a probability density function, where the information given by the seismic model and the information on the physical correlation among density and velocity parameters limit the model space within which the inversion of gravity data can operate. The density model parametrization, which uses polyhedral bodies, whose density is linearly dependent on the three coordinates (Pohànka, 1998), leads to a perfect match between the density and the velocity models, and takes into account the presence of structures characterized by a gradual increase in density with depth. Considering the gravity data resolution, we define the cell model with a horizontal regular spacing of 1° x 1°. The model horizontal dimensions are approximately 6600 km in E-W direction, and 3900 km in N-S direction. The vertical spacing is 50 km. The results, compared to existing 3D density models of the area derived from ship-borne and on land data (e.g. Makris et al., 1998) provide an indication of the extent to which GRACE data can resolve specific deep lithospheric and mantle structures.
T23A-1209
Upper mantle structure of the European and Mediterranean region from inversion of surface wave group velocity
We present a new transversely isotropic shear wave velocity model of the upper mantle in the European and Mediterranean region, obtained by analysis of surface wave group speed. Our data set consists of fundamental mode Love and Rayleigh wave group speed measurements, taken by multi-filter analysis on wave groups isolated by phase matched filtering on seismograms recorded at regional distance. Group velocity maps for periods ranging from 35 to 150s are then obtained by linear inversion. Data coverage in this region is not uniform, as it is highly influenced by the uneven distribution of stations and events. We therefore stabilize the inversion using a priori information in the form of a smooth global reference model we previously derived by inverting a phase velocity dataset (Ekstrom et al. 1997) on a global grid, where our regional grid is embedded. This greatly improves the coverage near the borders of our region. In each pixel - approximately 120 km in size - the set of group velocities are then used to find the best-fitting vertical shear velocity profile by nonlinear inversion. In this step, we make use of a priori knowledge from PREM and CRUST2.0. We present results in terms of Voigt average velocity variations, and radial anisotropy, and compare them with other results from the literature. The resulting model confirms the larger-scale deep geological features known for the region, and add great detail due to the inclusion of measurements on shorter paths.
T23A-1210
Shear Wave Velocity Structure Beneath the Western Basin and Range Province, Eastern California: Implications for Crustal-scale Tectonic Models
Broadband seismic data from three seismic stations located within the western Basin and Range Province of eastern California were analyzed in order to image the shear wave velocity structure of the crust and upper mantle in the region. The stations include: (1) Manual Prospect Mines, Trona, (2) Cottonwood Creek, Lone Pine, and (3) Slate Mountain, Trona. The area of the study is characterized by a complex geologic history predominantly involving the Mesozoic emplacement of the Sierra Nevada batholith and Cenozoic extension, uplift, and volcanism. Lateral and vertical velocity variations deduced from receiver-function analyses indicate complex lithospheric structure in this region. The crustal configuration beneath the stations depicts a low-velocity zone (LVZ) that is present between 16-18 km depth and the Moho at about 30-km depth. These results are consistent with extensional models of the region involving lateral flow of mid- to lower-crustal material and/or delamination of the lower crust.
T23A-1211
Very low heat flow near the center of the Canadian Shield
Extrapolation of presently available heat flow measurements suggests that heat flow is very low in the northeastern part of the Canadian Shield over a region covering Hudson Bay, northern Quebec, and Labrador. The basement of this region includes several Archean blocks that were welded together during the PaleoProterozoic (ca. 1.8Ga). High seismic velocities to a depth of ~ 300km have been inferred for the same region. So far, the existence of a low heat flow region was based on extrapolation of trends in the extremely scarce data so far available. The presence of this region has been confirmed by new measurements obtained in the James Bay area, to the southeast and to the southwest of Hudson Bay, and at the tip of the Ungava peninsula, to the northeast. We shall present new heat flow and heat production measurements from deep boreholes at six sites in the Archean Superior Province and in the PaleoProterozoic Cape Smith Belt. The heat flow values determined so far range between 22 and 34 mW~m-2 compared with a mean of 41 mW~m-2 for the entire Superior Province. These lower than average heat flow values were measured over various rock types, including granitic rocks that are not depleted in radio-elements. Our preliminary interpretation thus requires low temperatures in the lithospheric mantle at the southern and eastern edges of Hudson Bay.
T23A-1212
Integrated crustal models, characterisation of crustal lithology and geodynamic implications - southwestern Barents Sea.
Our 3D model covers the offshore extension of the Norwegian Caledonides, which constitutes of a series of basement high and deep basins. Our 3D model integrates density and magnetic modelling with the interpretation of seismic refraction and reflection data and helps to characterize the different basement units. Furthermore, the 3D model allows characterizing the crustal configuration and contributes to our understanding of the geodynamics processes that affected the region and are expressed in the basement lithology variability. The model is based on interpretation of profiles constrained by refraction and reflection seismic. Well data were integrated to adjust the depth of the sedimentary layers. The geological interpretations are based on onshore- offshore links and onshore petrophysical sampling. The distribution of density, magnetic susceptibility and Q- ratio values allows distinguishing the different basement units (Precambrian gneiss, Caledonian Nappes, granitic plutons and mafic intrusions). One model explains the long-term persistence of the Loppa High as a high since Devonian times. The 3D model allows mapping the gneiss dome sitting along the west side of the Loppa High and other localised intrusions. At the east of the Loppa High, the model highlights a thickening of the Caledonian nappes important enough to overprint the Precambrian basement magnetic signature. Our basement unit map raises the question why the magnetic signal of the Caledonian basement is so representative at the east of the Loppa High whereas onshore its signal is weak and does not cover the Precambrian basement signal. The combination of our regional geophysical modelling with a more detailed study of the seismic interpretation allows proposing a geodynamic scenario for the Loppa High installation. We link the Loppa High geometry and composition to the installation of a gneiss dome syn-tectonic with the development of the Bjørnøyrenna Fault Complex. Initiated in Devonian times, the development of the Bjørnøyrenna Fault Complex and installation of the gneiss dome installations led to a rotation of the basement blocks rotations. As a consequence the Caledonian nappes in the east have been preserved, while the Caledonian layers have been eroded due to an uplift of the western part.
T23A-1213
Secular Evolution of Mantle Heat Flow in Precambrian Terrains
We complied a global continental average heat flow - heat production data (Q-A plot) for 32 Precambrian terrains. A linear fit to the data gives Q = 10 X + 32 . There is no correlation between reduced heat flow and age. On the global average Q-A plot, high average surface heat flow in Precambrian terrains correlates well with the high average heat production in the crust. Mantle heat flow predicated by the average Q-A relationship is 25 ± 5 mWm-2 for Archean, Proterozoic and Paleozoic terrains. Using the characteristic depth D =10 km as empirically determined by the global average Q-A plot and applying a decay correction, we generate the past surface heat flow versus age best fit line (32 terrains) as Q = 64 +1.9t (t time in Ga), in contrast to uncorrected relationship which is best fit line to the present surface heat flow versus age as of Q = 64 - 5.3t . This shows that the present best fit line to the surface heat flow versus age is mainly controlled by the decay of radioactive elements. Consequently, the lower surface heat flow predicated for terrains in the age range of 3.9 - 2.5 Ga is a result of decay of crustal radioactivity to ~ ≥ half its initial value, as the half life of crustal radioactivity is comparable to the ages of Archean cratons ~ (2.5 - 3.9 Ga) . Thus it is clear that variations in surface heat flow are mainly controlled by shallow crustal radioactivity rather than deep lithosphere thermal processes. Modeling of the crustal radioactivity in the Archean crust through time compared with the present best fit line to the surface heat flow versus age, also favors Archean crustal radioactivity in the range of ~ 0.4-0.5 μ Wm-3, assuming constant mantle heat flow. What we found is in contrast to the concept that Qo = 0.6Q, (where Qo is the reduced heat flow) i.e., that the radioactivity is same in terrains of different ages and variations in surface heat flow are mainly controlled by variations in the mantle heat flow. Using linear fit to the surface heat flow Q = 65 - 9t and reduced heat flow Qo = 48 - 9t versus crustal age of Precambrian terrains, it has been proposed that there is secular increase in the mantle heat flow with decreasing age. In the Archean crust mantle heat flow is 10 - 30 mWm-2 and in the late Proterozoic crust it is 30 - 45 mWm-2, with a total variation of 10 - 45 mWm-2 . A surface heat flow difference of ~ 15 mWm-2 has been implied by the difference in the mantle heat flow between Archean and Late Proterozoic terrain which in turn is inferred to be due to lithospheric thickness contrast. Seismic tomography implies a difference of ~ 100 - 200 km in the thickness of lithosphere between Archean cratons (3.9 - 2.5 Ga) and post -Archean terrains (< 2.5 Ga). However this analysis shows that the low surface heat flow in the Archean cratons is due to the decay of radioactivity in comparison to the model of low mantle heat flow due to thick lithosphere. Therefore, heat flow data complements xenolith data which do not support proposed lithospheric thickness differences between PreMesozoic terrains of different ages.
T23A-1214
New 3D Gravity Model of the Lithosphere and new Approach of the Gravity Field Transformation in the Western Carpathian-Pannonian Region
The 3-D forward modeling was performed for the Western Carpathians and the Pannonian Basin system. The density model includes 31 cross-sections, extends to depth of 220 km. By means of the combined 3-D modeling, new estimates of the density distribution of the crust and upper mantle, as well as depths of the Moho were derived. These data allowed to perform gravity stripping, which in the area of the Pannonian Basin is crucial for the signal analysis of the gravity field. In this region, namely, two pronounced features (i.e. the deep sedimentary basins and shallow Moho) with opposite gravity effects make it impossible to analyze the Bouguer anomaly by field separation or filtering. The results revealed a significantly different nature of the Western Carpathian- Pannonian region (ALACAPA and Tisza-Dacia microplates) from the European Platform lithosphere (i.e. these microplates to be much less dense than the surrounding European Platform lithosphere). The calculation of the transformed gravity maps by means of new method provided the additional information on the lithospheric structure. The use of existing elevation information represents an independent approach to the problem of transformation of gravity maps. Instead of standard separation and transformation methods both in wave-number and spatial domains, this method is based on the estimating of really existing linear trends within the values of complete Bouguer anomalies (CBA), which are understood as a function defined in 3D space. An important assumption that the points with known input values of CBA lie on a horizontal plane is therefore not required. Instead, the points with known CBA and elevation values are treated in their original positions, i.e. on the Earth surface.
T23A-1215
Lithospheric thickness and mechanical strength of the Indian Shield
The relative mechanical strength of the Indian subcontinent is determined by a robust coherence method based on multitaper spectral analysis on overlapping windows of equal size. Representing the elastic properties of the lithosphere by a transitional coherence wavelength bypasses the need to input rheological variables such as Young's modulus or Poisson's ratio, which have been shown to vary on other continents. The measurements are compared in terms of age and find that regions underlain by Proterozoic basement are on average stronger than regions of Archaean age. This is in stark contrast to the common assumption that mechanical strength should increase progressively with time. While the transition wavelengths show a good separation between both age groups and the North and South Indian shields, suggesting they are indeed mechanically distinct, inversions for absolute effective elastic thickness show a greater overlap. http://www.facweb.iitkgp.ernet.in/~rajesh/
T23A-1216
The use of Free air Gravity Data to Construct a 3D Density Model for the Region of Libya
We present a new 3D density model for the Libya region that includes the topographic relief and five individual subsurface layers, the deepest being at a mantle depth of 50 km. The model was derived utilizing over 6,000 gravity measurements in conjunction with other geophysical and geological data obtained from oil exploration, published research, and other academic institutions. To develop the density model, we used "SURFGRAV", a newly developed general purpose gravity modeling program. To evaluate the validity of our model, we computed the predicted Free air gravity anomalies for the study area and compared our results to the observed Free air gravity anomalies. The gravity modeling showed that our new density model is able to predict large portions of the observed Free air anomalies, especially in uplifted areas. With the exception of portions the northwestern and northeastern regions where data are sparse and thinning of the crust would improve the fit the residual Free air gravity is less than 50 mGals, indicating that a more detailed model is required to produce a better match between observed and calculated gravity values.
T23A-1217
Terrestrial heat flow distribution in Japan area based on the temperature logging in the borehole of NIED Hi-net
Measurements of heat flow and geothermal gradient using boreholes on land are often concentrated in specific areas such as geothermal region, so the measured heat flow data in non-volcanic area is poorly mapped. We newly measured thermal gradients and heat flow all over Japanese Islands by using the National Research Institute for Earth Science and Disaster Prevention's (NIED) high-sensitivity seismograph network (Hi-net) boreholes. NIED Hi-net is composed of about 800 borehole stations installed almost homogeneously over the Japanese Islands with an average spacing of 20km. Although majority of the Hi-net stations have the boreholes of 100- 200m in depth, 55 deep (300-1000m) and 29 very deep (1000-3500m) observation wells were constructed at some specific sites if necessary. Because these Hi-net boreholes are designed for a long-term observation, these are structurally stable with using casing pipe. These borehole temperature profiles contain an influence of climate change, so we correct temperature profile for climatic change, the rapid warming of 1950-2000A.D. Comprehensive heat flow distribution in Japan is as follows: low heat flow in forearc region, high heat flow in back-arc region, and there seems to be the correlation between the seismogenic layer thickness in upper crust and terrestrial heat flow. In the volcanic chain area, very high terrestrial heat flow over 200mW/m2 are observed. In the fore-arc area, low heat flow under 50mW/m2 are observed. In the South Kanto area where is metropolitan Tokyo and the Hokkaido Hidaka area, very low heat flow under 40 mW/m2 are observed. In the southern Kii Peninsula, there are many non-volcanic hot springs, so around this area high heat flow over 150mW/m2 are observed. In Shikoku area also high heat flow over 100mW/m2 are observed. Geographical distributions of terrestrial heat flow show that high heat flow stations are observed along the region where non-volcanic long-period tremors occur about 30km deep. In SW Japan, the slab-derived fluids, which cause fracturing within the crust, result in easier transfer of fluids, mixed with mantle helium, to the surface (Notsu et al., 2006). This movement of the slab-derived fluids also transports the heat of mantle wedge, which cause the terrestrial high heat flow anomaly.
T23A-1218
Upper mantle anisotropy beneath Indochina block and adjacent regions from shear wave splitting analysis of Vietnam array data
Indochina block is located at the junction of Eurasia, Indian and Pacific plates. It has a close relationship with the uplift of Tibet plateau and the spreading of South China Sea basin in the geological evolution histories. The anisotropy is considered to directly relate to deformation in the mantle, and therefore provide constraints on tectonic and geodynamic processes. In this study, we present upper mantle anisotropy beneath Indochina block and adjacent regions from shear wave splitting analysis of Vietnam array and IRIS data. The data used here recorded by Vietnam array during the period between February 2000 and October 2005. The array consists of 6 broadband seismometers. We employed a compact HDD for data storage, which enables long-term and stable observation. We also used data form three IRIS stations around Vietnam for the same period. All records are band-pass filtered between 0.02 and 1.0 Hz to eliminate background noise. The dominant time windows used are 10-15 s around SKS phases based on the predicted travel times from the iasp91 model. We used the methodology of Silver and Chan (1991) as modified by Walker (2004) to determine the fast polarization direction and the delay time between the fast and slow components. In order to find the best splitting parameters, a grid search over possible values is performed to linearize the shear wave particle motion when the effect of the anisotropy if removed. The error estimation of each combination of splitting parameters is given by 95 percent confidence level of F test. We are able to characterize the splitting patterns in greater detail because of the use of new data. The average value of delay times in Indochina block is about 1.35 s, larger than that in adjacent regions. Tomographic studies also revealed low velocity anomalies in upper mantle beneath study area (Lebedev and Nolet, 2003). We estimate that the Indochina block might be affected by the mantle convection produced by the spreading of the South China Sea. Fast polarization directions vary significantly from south to north, indicating complex deformation in the upper mantle beneath study area. We suggest that the Indochina block might be divided into two sub-blocks according to the historic seismicity and tectonic activities. References Silver P.G., and W.W. Chan, Shear wave splitting and subcontinental mantle deformation, J. Geophys. Res., 96: 16429-16454, 1991. Lebedev, S., and G. Nolet, Upper mantle beneath Southeast Asia from S velocity tomography. J. Geophys. Res. 108, 2048-2074, 2003. Walker K.T., G.H.R. Bokelmann, and S.L. Klemperer, Shear-wave splitting reveals mantle upwelling beneath eastern Nevada, USA, Earth Planet Sci. Lett., 222, 529-542, 2004.
T23A-1219
Heat flow distribution in Chinese continent and its adjacent areas
Using a compilation of 6980 heat flow measurements, we produce a new heat flow map for the Chinese continent and its adjacent areas. We develop an objective and integrated method to interpolate the heat flow data, taking into account both the uniformity within geological units and coherency of regional heat flow. The geologic units are outlined based on Zhang et al.'s (2003) active tectonic block model. Our heat flow model is presented in two formats: a contour map and a heat flow dataset with values on a 1° × 1° grid in the Chinese continent and its adjacent areas, reflecting detailed variations at some regions. Also provided is a resolution map which helps understand the reliability of the heat flow model. Our results reveal that: (a) Heat flows in the east part of the Chinese continent are relatively higher than those in the west part except that in the Tibetan plateau area. (b) Heat flows in the Ordos and North China blocks are around 60 mW/m2, and are 50 ~ 55 mW/m2 in South China except in the continental marginal sea regions. (c) Heat flow is the lowest in the Junggar basin, only 35 ~ 45 mW/m2, and is45 ~ 55 mW/m2 in the Tarim basin. The results of this study provide an important dataset for studies on thermal and rehological structures of the Chinese continent and its adjacent areas.
T23A-1220
Heat Transport in Fluid Layers Heated From Within and Below
The heat transport scaling relationships for fluids heated from within and below is established based on numerical calculations of isoviscous convection at infinite Prandtl number. The internal temperatures scale in a way that is similar to the internally heated case, but with an offset equal to the average of the two boundary temperatures, reflecting the underlying temperature structure of bottom heated convection. The heat flux through the boundaries scales as a linear combination of the end-member modes of heat transport, consistent with the effects of internal heating on the interior temperature. The boundary layer thicknesses, however, depend on H and Ra in a non-intuitive way. As the internal temperature increases with the addition of internal heating, the upper thermal boundary layer thickens despite the increased temperature drop across the layer (the reverse is true for the bottom boundary layer). This is inconsistent with the idea that the boundary layer thickness is controlled by a stability condition on the local Rayleigh number, which would predict that the boundary layer would thin as the temperature drop increases. Deriving boundary layer thicknesses from the scalings for heat flux and boundary layer temperature drop provides an excellent fit to the model results and reveals the importance of plumes arriving from the other boundary layer in establishing the boundary layer thickness. This suggests that, though widely used, boundary layer stability analysis is not an accurate description of the processes controlling boundary layer thickness.