T31C-0576
Testing the Extensional Detachment Paradigm: A Borehole Observatory in the Sevier Desert Basin, Utah
The Sevier Desert basin, Utah represents a world-class target for scientific drilling and for the development of an in situ borehole observatory of active faulting, with potential for establishing that normal-sense slip can occur along a brittle low-angle fault and, by determining the conditions under which that may take place, for resolving the mechanical paradox associated with such structures. The Sevier Desert detachment was defined in the mid- 1970s on the basis seismic reflection data and commercial wells as the contact between Paleozoic carbonate rocks and Cenozoic basin fill over a depth range of ~0-4 km. Today, the interpreted fault is thought by most workers to root into the crust to the west, to have large estimated offset (< 47 km), to have been active over most of its history near its present 11° dip, and to be associated with contemporary surface extension (a 30- km-long zone of prominent Holocene fault scarps immediately west of Clear Lake). Although no seismicity has been documented on the detachment, its scale is consistent with earthquake magnitudes as large as M 7.0. A published alternative interpretation of the Paleozoic-Cenozoic contact as an unconformity rather than a fault has not been generally accepted. Deformation at detachment faults is commonly spatially restricted, and may have been missed in well cuttings. Exhumation of the detachment would have made it possible to remove critical footwall evidence prior to later sedimentary onlap, particularly at updip locations. The incomplete coverage and uneven quality of seismic reflection data on which the detachment interpretation depends, and an unresolved debate about stratigraphic ties to a critical well, leave room for discussion about interpretive details, including the possibility that deformation was distributed across several closely spaced faults. An apparent mismatch between stratigraphically based ages and fission-track evidence for the timing of footwall exhumation cannot be resolved with available well data. Drilling is now needed to make in situ measurements at depth, to obtain critical core of fault rocks at a down-dip site where offset should be large, and to establish more clearly the relationship between basin development and displacement along the interpreted fault. A workshop will take place from July 15-18, 2008, in Utah, under the auspices of the International Continental Scientific Drilling Program, to flesh out objectives, strategies and operational details, and to develop a consensus on the location of a drill site. http://www.ldeo.columbia.edu/sevier/icdp
T31C-0577
The Mormon Peak and Tule Springs Detachments of Southern Nevada and Their Role in Interpreting the Subsurface Structure of the Sevier Desert Basin
The eastern Sevier front is commonly overprinted by west-dipping normal faults that have been active through much of Cenozoic time. Although examples of disparate age and geometry abound, the Sevier Desert detachment stands out as being: (1) a very large, low-angle structure, (2) probably still active, and (3) at drillable depth, and hence is a prime target for scientific drilling. Among the closest surface-exposed analogs of the detachment are the Miocene Mormon Peak and Tule Springs detachments in southern Nevada, about 250 km SSW of the Sevier Desert. In both areas, the detachments developed within a few km of the basal Sevier thrust. The few traces of the southern Nevada detachments that were identified by reconnaissance mapping in the 1950s were first interpreted as underlying "rootless" gravity-slide masses. Detailed stratigraphic and structural mapping in the 1980s revealed the full extent of these detachments and their close relationship to the basal Sevier thrust. Construction and retrodeformation of cross sections through the two detachments demonstrated that neither fault could represent the base of a surficial slide mass. The key field relationship is the structural contiguity of the hanging walls of the detachments with large mountain range blocks in their down-dip directions. "Rootless" gravity-driven masses of equivalent or larger scale are widely documented, and in contrast to the southern Nevada detachments show clear evidence along their down-dip portions of either riding over the earth's surface (e.g. Heart Mountain, WY; Shadow Valley, CA) or having been internally shortened (e.g. Bearpaw Mountains, MT; Louisiana shelf). A rootless slide origin for the Mormon Peak detachment has been revived based on meso- and microstructural analysis of the fault surface (refs. 1, 2, and 3), but the hypothesis still fails the retrodeformation test at macroscopic scale. The retrodefomed sections indicate that the footwalls of the southern Nevada detachments expose a paleodepth range from near-surface conditions to more than 7 km, which will be useful for comparison with microstructures that are encountered along the Sevier Desert detachment where it is sampled at depth. 1: Carpenter et al., 1989, Geology 17, p. 774; 2: Anders et al., 2006, J. Geol 114, p. 645; 3: Walker et al., 2007, Geology 35, p. 259.
T31C-0578
Spatial and Temporal Distribution of Strain in the Sevier Desert Region from a Decade of BARGEN Continuous GPS Observations
A transect of four BARGEN GPS sites from 1997-2000 at latitude 39°N revealed a linear increase in velocity from 0 mm/yr on the Colorado Plateau to ~4 mm/yr in east-central Nevada. These geodetic data, when combined with paleoseismic and neotectonic observations, were used to argue for present-day strain accumulation on the Sevier Desert detachment, a seismically-imaged, low-angle (12°) normal fault that underlies a significant portion of west central Utah [Niemi et al., 2004]. A new GPS velocity solution [Davis et al., 2006], including 5 additional years of observations and a new GPS station in the eastern half of the transect, serves to both clarify and complicate our understanding of the spatial and temporal distribution of strain in this region. Geodetic velocity gradients among the three original eastern sites (from east to west, CAST, SMEL, and FOOT), and including the new site, SPIC, located between CAST and SMEL, continue to suggest a linear strain gradient from the Colorado Plateau to westernmost Utah, on a transect spanning the Wasatch, Sevier Desert, and House Range normal faults. In contrast, the baseline between FOOT and EGAN, which underwent extension from 1997-2000, began to contract in 2000, as site EGAN slowed with respect to the Colorado Plateau. This deviation in velocity continued until 2003, when EGAN began moving westward, and has, as of 2006, returned to near its pre-2000 velocity with respect to the Colorado Plateau. The eastward excursion of GPS site EGAN is not unique, and similar excursions are observed in the time series of all BARGEN GPS sites that lie west of ~114.25°W over the time period 2000-2003 [Davis et al., 2006]. The origin of this velocity anomaly is uncertain, but the magnitude and spatial extent of the excursion, as well as the observation of a deep earthquake swarm (~30-40 km depth) coincident with dramatic motion of GPS site SLID, near Lake Tahoe, in 2003 [Smith et al., 2004] suggest a deep crustal or mantle influence on the observed GPS velocities. Bright, mid-crustal horizontal reflections observed in COCORP reflection seismic data west of 114°W have been postulated to represent a detachment that could accommodate shearing along the base of the crust in Nevada [Hauser et al., 1987], a hypothesis broadly consistent with the observed GPS velocities, with the crust shearing relatively east over the subcrustal lithosphere. A deep observatory in the Sevier Desert region, transecting the Sevier Desert detachment, would complement existing paleoseismic and neotectonic studies, and the decade of BARGEN continuous GPS observations, as well as new GPS data from 8 Earthscope Plate Boundary Observatory sites installed along this transect in the past few years. Key questions that could be assessed with a deep observatory are how slip, and strain, at depth on an inclined fault are reflected in geodetic observations of strain at the surface; whether the state of stress and strain rate on the Sevier Desert detachment change through time, as suggested by long-term periodicity in strain release as recorded in the geologic record, and what effect long-lived velocity excursions, such as observed from 2000-2003, have on the regional stress state, perhaps leading to a clearer understanding of the source of these anomalies, and the aseismic tectonic behavior of the lithosphere.
T31C-0579
Magnitude, Timing, and Geometry of Extension in the Southern Sevier Desert Basin From Piercing Points, Seismic-Stratigraphic Reconstruction, and Deep well Data
Palinspastic reconstruction of Mesozoic thrust sheets provides the main constraint for an estimated 47 km of Cenozoic extensional displacement along the Sevier Desert detachment (SDD) in the central Sevier Desert Basin. Hanging wall and footwall piercing points indicate that the SDD accommodated a minimum of 35 km of extensional displacement in the narrower southern part of the basin. The piercing points for the SDD are defined by the intersection of the SDD, the Canyon Range thrust (CRT), and a regional early Cenozoic erosion surface (ES). The hanging wall piercing point lies immediately northeast of the Cricket Mountains, where the SDD-CRT- ES intersection is narrowly defined by intersecting structure maps derived from published seismic reflection data. The footwall piercing point lies in the southern foothills of the Canyon Range, where the SDD breakaway plane is well constrained by an industry seismic line that lies within 2 km of the exposed intersection of the CRT with the base of the Oligocene Oak City Formation. Timing of extension in the southern Sevier Desert basin is constrained by a kinematic reconstruction of detachment and imbricate fault displacement, footwall uplift, and supradetachment sedimentation for Oligocene, Miocene, and Plio-Pleistocene seismic sequences. The reconstruction is centered on a seismic reflection and gravity interpretation along the published Pan Canadian profiles 2 and 3 that is tied to dated intervals in six industry wells. Fault restoration indicates that Oligocene and Miocene phases of slip each accounted for about 40 percent of the total displacement. Simultaneous backstripping of the Oligocene, Miocene, and Plio-Pleistocene supradetachment sequences records hanging wall subsidence simultaneous with footwall uplift, with a footwall burial history that is consistent with published Miocene apatite and zircon fission-track ages of footwall samples. The geometric evolution of the southern SDD extensional system is consistent with its development above a broad westward-migrating "rolling hinge" zone associated with isostatic uplift of the detachment footwall. Hanging wall normal faults east of the footwall crest exhibit small post-Miocene displacement, with demonstrable Quaternary slip restricted to the crest and western limb of the uplift, most notably along the Black Rock and Clear Lake fault zones. Early abandonment of the eastern part of the detachment may explain the indistinct geomorphic and structural expression of the break-away zone at the surface. The deepest level of the southern SDD also presents a complex geometry and kinematic history. The 1996 Chevron 1-29 Black Rock Federal well through the western basin margin penetrated a normal fault that places Jurassic over lower Cambrian strata at 4650 m measured depth, well above the principal SDD seismic reflection. The fault is not correlated to any large- displacement high-angle fault at shallow levels, and may form the abandoned roof to an extensional duplex.
T31C-0580
The Cave Canyon Detachment: A Standard for Deformation Expected of the Sevier Desert Detachment
The Cave Canyon detachment (CCd)is a Neogene normal fault in the Mineral Mountains of west-central Utah. The Mineral Mountains are located directly south of the Sevier Desert basin. Because of its location, orientation and age constraints the Cave Canyon detachment has been proposed to be a non-active splay of the Sevier Desert detachment. The Sevier Desert detachment (SDd) is thought to be one of the largest displaced active normal faults in North America accruing as much as 47 km of displacement. There are some fundamental differences between the CCd and the SDd, one of those being that the CCd places Paleozoic limestone directly on a granitic pluton whereas the SDd beneath the basin places Neogene terrestrial sediments directly on Paleozoic limestones. Nevertheless, the temperature and depth at the time of movement are similar, thus making the CCd an appropriate analog to what might be expected of material potentially recovered by scientific drilling of the SDd. A suite of samples from above and below the CCd yields a distinctive pattern of brittle deformation. In the lower plate granites deformation is mostly brittle, and in the upper plate there is only a minor zone a few cm thick of mylonitized carbonate. The lower plate granites are characterized by an aureole of healed microfractures that decrease in abundance from the contact to background at over 100 m. Cataclasis is observed well over 100 m from the detachment, and is so intense at the contact that no grain is unaffected. Significant growth of new minerals and cross-cutting quartz veins are observed within a few meters of the detachment. In upper plate carbonate rocks, a cm-thick zone of dynamic recrystallization is observed, above which there is a meter or so of extensive undulose extinction of grains. Outward from this is a zone of strongly preferred oriented twinning. At a distance of 15 m from the contact only minor carbonate veinlets distinguish fault zone rock from unaffected limestone. If the SDd is an active or recently active fault zone with displacement as much as an order of magnitude greater than that of the CCd, and appreciably more deformation would be expected within the fault zone both above and below the detachment surface.
T31C-0581
Extension in the Colorado Plateau/Basin and Range Transition Zone, Central Utah: An Active or Passive Process?
The Colorado Plateau-Great Basin transition zone (TZ) in central Utah is a presently extending lithospheric block composed of previously stable Proterozoic lithosphere. TZ extension may be driven by high topography resulting from overthickening during the Laramide Orogeny and passive plate boundary forces similar to the Great Basin. However, high TZ topography coupled with a thinned lithosphere may indicate dynamic mantle upwelling and active processes acting within the TZ. To investigate the active vs. passive rifting hypotheses we have collected 56 new broadband and 9 long-period MT stations to merge two existing MT lines for a combined length of 400~km (124 sites) covering the eastern Great Basin into the Colorado Plateau at a latitude of ~38.5°N. We have also collected over 300 new heat production and thermal conductivity measurements across the southwest to develop a thermal model of the eastern Basin and Range and Colorado Plateau. MT data show a semi-contiguous highly conductive body in the lower crust of the eastern Great Basin that rises to a shallow depth of <20~km beneath the TZ. This conductive layer appears to be connected to the surface by a series of symmetric rift related normal faults mapped at the surface. These normal faults may be acting as pathways for large-scale fluid connection between the upper and lower crust. MT inversion results suggest an electrical anisotropy factor >3 in the upper mantle with an enhanced conductivity in a N-S orientation that is in agreement with observations of fast seismic SKS split direction. We propose that anisotropy observations may be due to small degrees of interconnected partial melt aligned with a N-S geologic strike beneath the Basin and Range. A thermal model, combining measured thermophysical properties with existing heat flow data, MT measurements, and estimates of seismogenic depth is used to predict lithospheric thickness and mantle temperature variations along the MT profile. Lithospheric thicknesses is approximately ~150~km, beneath the resistive core of the Colorado Plateau and ~60~km in the Great Basin. Geodynamic modeling using the new thermal and resistivity constraints may reveal insight into the active/passive nature of the rift.
T31C-0582
The MOLE Drilling Project: Laboratory at Depth on an Active Fault in Central Italy
Several fundamental questions concerning: i) the geophysical and geochemical processes controlling normal faulting and earthquake ruptures during moderate-to-large seismic events and ii) the low angle normal fault paradox, still need to be fully answered. In this work we aim to present an example of low angle normal fault (Alto Tiberina Fault) located in the Northern Apennines (Italy) showing conclusive evidence of its seismic activity. This fault is a likely target of an international project: the MOLE (Multidisciplinary Observatory and Laboratory of Experiments) Drilling project. Indeed, under the auspices of the International Continental Scientific Drilling Program a workshop is being organized in Italy next spring 2008, to promote the creation of an international multidisciplinary team of scientists, to discuss the project in detail and also to prepare a full proposal for ICDP. This project wants to investigate the inner structure of normal faults in Central Italy to get physical constraints on the processes controlling faulting and earthquake mechanics. The Umbria-Marche sector of Northern Apennines offers a unique opportunity to reach a complex system of normal faults among which we selected two possible targets. 1) The active Colfiorito fault dipping about 45° toward SW which ruptured during the 1997 earthquake sequence; 2) the Alto Tiberina low angle normal fault dipping 15°-25° towards ENE, which moves through a combination of aseismic creep and repeating microearthquakes. Drilling the Colfiorito active fault at a depth of about 2-3 km allows targeting the high coseismic slip patch of the 1997 earthquake M=6 seismogenic structure. Drilling the Alto Tiberina Fault at a depth of nearly 5-6 km will target a micro seismicity source. We aim to collect new original data through borehole logging and sampling and to set up a permanent observatory at depth for a multidisciplinary monitoring to characterize these active normal fault zones. This will allow to understand how such faults behave and to create more realistic models of: earthquake nucleation, seismicity pattern, stress interactions and earthquake triggering at local and regional scale. Both drilling targets present relevant technical issues that should be discussed from different points of view before selecting the starting drilling site.
T31C-0583
Continuous GPS constraints on active extension in the Umbria-Marche Apennines, northern Italy
A recently established network of 20 continuously operating GPS stations in the Umbria-Marche region of the northern Apennines, Italy, records strain accumulation associated with a system of active normal faults. Crustal velocities reveal roughly 3 mm/yr of relative motion accommodated between the Tyrrhenian and Adriatic coasts, but extensional deformation is mostly concentrated in a fairly narrow ~30 km-wide belt which lies just west of the Apennines topographic high. An apparent NW directed component of velocity, possibly related to broader scale geodynamic processes associated with Nubia-Eurasia convergence, or other causes, is superimposed upon the extensional deformation signal. The locus of the extensional component of deformation coincides with the low angle (~20°) NE dipping Altotiberina fault, which is clearly imaged in seismic reflection profiles (CROP03) and highlighted by microseismicity (0.6 ≤ M ≤ 3.0). However, larger earthquakes in the region (~M5-6) appear to occur on antithetic normal faults with high angle (~60°) SW dip (e.g., 1979 Norcia, 1984 Gubbio, and the eight 1997-8 Colifiorito earthquakes). Low angle normal faults such as the Altotinerina fault are widely documented, but whether or not displacement along low angle normal faults is mechanically possible remains an important unresolved issue. Collettini and Holdsworth (Journal of the Geological Society of London, 161, 2004) proposed a model for low angle Altotiberina fault slip by aseismic frictional-viscous creep at fairly shallow crustal levels. Based on the statistical characteristics of our preliminary data analyses, we show that the new continuous GPS network in the Umbria-Marche Apennines will be well suited to test this hypothesis as more data are collected and horizontal and vertical rates become more precise.
T31C-0584
Seismic Profiling of a Breakaway Zone in Southeast Arizona and Implications for Models of Core-Complex Development
The Basin and Range Province of western North America represents a broad zone of Cenozoic crustal extension characterized by various styles of extensional deformation and development of zones of extreme extension manifested as metamorphic core complexes. The Models concerning core complex development define a breakaway zone, along which extension is initially accommodated in the upper crust in which normal fault systems slip in the direction of transport. Slip is initiated as a listric or low-angle normal fault system, which then feeds slip into a subhorizontal or low-angle shear zone at depth. The Models suggest that the breakaway zone normal fault system to have slipped at low-angle during the earlier stages of core complex development. It's been suggested that folding/back-tilting of the original breakaway zone result from the isostatic rebound of the footwall due to unloading of upper plate rocks and exhumation of a portion of the ductile shear zone. The San Pedro trough in Southeast Arizona is an elongate structural depression flanked on the southwest by the Catalina core complex and bounded by the relatively undeformed Galiuro Mountains in the northeast. The detachment system is believed to have initiated from the breakaway zone located on southwestern flank of the Galiuro Mountains. Cenozoic crustal extension revealed through seismic reflection investigations helps us understand the fault geometries and upper crustal structure of the study area. 2-D seismic reflection data in Southeast Arizona shows that the San Pedro Trough contains thin sedimentary cover above Paleozoic and older rocks. Though The detachment fault in the breakaway zone dips SW toward the San Pedro Trough and can reasonably be modeled as a moderate-angle fault that cuts through mylonitic fabric to mid-crustal depths; if this fault forms low-angle shear zone that crosses the San Pedro Trough at shallow depths and re-emerges due to folding/back-tilting during the emplacement of the metamorphic core complex, the fault surface must apparently form the bedrock floor of the trough. Field relations, outcrop data and borehole data also help constrain the structure of the breakaway zone and development of the core complex. Keywords: Metamorphic Core Complex, breakaway zone, crustal extension, San Pedro Trough, detachment fault
T31C-0585
Insights to the Distribution of Slip Directions along Normal Faults from Three Dimensional Finite-Element Models
Normal faults in nature that are isolated and strike perpendicularly to the direction of extension exhibit a systematic variation of fault-slip direction along their length. Pure dip-slip is taking place at their centres while oblique slip is observed near their tips. The distribution of the slip vectors along the fault trace forms a symmetric converging pattern with the hanging wall moving towards the fault's centre. This pattern is commonly attributed to along- strike stretching of the hanging wall and the local stress distribution near the fault tips. We use three-dimensional finite element models of normal faults with different dimensions and dip angles to evaluate the variation of slip direction along their strike. The results show a nearly linear increase of the strike-slip component for the first three quarters of the distance from the fault centre to the fault tips. Consequently the obliqueness of the slip vectors is increasing from the fault centres to the fault tips. Based on the model data, the relationship between lateral slip and fault trace length can be approximated by a power law function for most of the fault length. Furthermore the amount of lateral slip is proportional to the amount of dip slip at the centre of the fault and inversely proportional to the fault dip-angle. These findings are summarized into simple mathematical relations that give the amount of lateral slip and the rake of the slip vector along the fault trace in relation to the geometry of a normal fault. The finite element models reveal local perturbations of the stress field near the fault tips as the main factors behind the occurrence of oblique slip away from the fault centres. A comparison of the model results with field data from Greece and Italy shows that the displacement profiles and the slip vectors along the modelled faults mimic those of real normal faults reasonably well. This has implications when slip data along normal faults are used to ascertain regional stress regimes.
T31C-0586
Sequential Development of Interfering Metamorphic Core Complexes: Numerical Experiments and Comparison to the Cyclades, Greece
The Cycladic extensional province (Greece) contains classical examples of metamorphic core complexes (MCCs), where exhumation was accommodated along multiple interfering and/or sequentially developed syn- and antithetic extensional detachment zones. Previous studies on the development of MCCs did not take into account the possible interference between multiple and closely spaced MCCs. In the present study, we have performed new lithosphere-scale experiments in which the deformation is not a priori localized so as to explore the conditions of the development of several MCCs in a direction parallel to extension. In a narrow range of conditions, MCCs are closely spaced, interfere with each other, and develop in sequence. From a comparison between numerical results and geological observations, we find that the Cyclades metamorphic core complexes are in good agreement with the model in terms of Moho geometry and depth, kinematic and structural history, timing and duration of core complex formation and metamorphic history. We infer that, for Cycladic MCC-type to develop, an initial crustal thickness prior to the onset of post-orogenic extension between 40 and 44 km, a boundary velocity close to 2 cm/yr and an initial thermal lithospheric thickness of about 60 km are required. The latter may be explained by a significant heating due to delamination of subducting continental crust or vigorous small-scale thermal convection.
T31C-0587
Microstructural Evaluation of Strain Localization Along the South Mountains Detachment Fault System, Arizona, USA
Detachment faulting is widely recognized as a mechanism of extension in the Basin and Range province, but the strain localization processes thought to contribute to the development of a discrete detachment fault surface are variable within the province. Though some detachment faulting has been attributed to deformation purely within the brittle regime, other fault systems, such as the South Mountains core complex in Arizona, are associated with fabric development within both the ductile and brittle regimes. In the South Mountains, Middle Tertiary (25-22 Ma) magmatism was accompanied by detachment faulting, which resulted in the development of mylonitic and cataclastic fabrics in intrusive rocks of the footwall. We examine granodioritic fault rocks from the detachment fault footwall and fault surface to identify specific deformation mechanisms that promoted strain localization along the South Mountains detachment fault. We use field observations, hand sample observations and microstructural analyses to describe fabric development in the footwall rocks. This work is important because the fabrics developed in the syntectonic plutonic rocks provide insights into the rheology of continental crust during extension. We focused our investigation in a region of the footwall adjacent to the only exposure of the detachment fault surface. We collected 15 oriented samples over a region equivalent to the uppermost 104 meters of footwall beneath the low-angle detachment fault surface. The samples from the lower 83 meters of the sampled shear zone exhibit protomylonitic fabrics characterized by microfaulted porphyroclasts of plagioclase and alkali feldspar within a matrix dominated by quartz and biotite. Within the zone of protomylonites, the proportion of matrix increases structurally upwards towards the detachment fault surface. In contrast, the samples from the upper 21 meters of the sampled shear zone exhibit cataclastic textures characterized by clasts of protomylonite within a matrix of chlorite and epidote. Preliminary results of these analyses suggest that quartz is rheologically weaker than the plagioclase and alkali feldspar during crystal plastic flow, and that brittle deformation may be enhanced by the growth of alteration minerals.
T31C-0588
Late Miocene cooling and extension identified on Serifos, western Cyclades: Development of an Aegean metamorphic core complex
The island of Serifos is located in the Western Cyclades archipelago, a region noted for Miocene extension, where slab roll-back of the African-Arabian plate and gravitational collapse of a thickened crust has resulted in north-south to northeast-southwest directed extension. Bedrock lithologies on the island are composed of calc- silicates, schists, gneisses, and marbles metamorphosed to greenschist- to lower amphibolite-facies conditions. Project ACCEL (Aegean Core Complexes along an Extended Lithosphere) has recently documented a major low-angle shear zone consisting of (ultra)mylonitic marbles and orthogneisses with stretching lineations and kinematic indicators that record consistent SSW-directed extensional shear. The orthogneisses found within this shear zone are thought to be a syn-kinematic granite that yield preliminary U-Pb zircon rim ages of c. 37 Ma, suggesting a Late Eocene magmatic and deformation event. The southern part of the island is dominated by a largely undeformed, I-type granodiorite pluton of Late Miocene age. Serifos contains all the hallmarks of a classic Aegean metamorphic core complex, and a thermochronology campaign is underway to elucidate the timing of extension and unroofing. Analysis of fourteen mica separates using Ar-Ar thermochronmetry reveals two distinct cooling age populations, separated by a steep age gradient that is coincident with the well-defined shear zone in the northern region of the island. Samples taken from the southern units of the island, within the shear zone, and adjacent to the granodiorite pluton, give cooling ages of 8-9 Ma, illustrating bedrock cooling and exhumation coeval with intrusion of the granodiorite. Lower grade lithologies from the northern portion of the island yield mica cooling ages of 32-35 Ma. Eocene cooling ages present on Serifos are identical to deformation ages reported from the Cycladic Blueschist unit (CBU), present on nearby islands in the Cyclades and Evvia. The cooling and crystallization ages from Serifos reported here demonstrate the protracted nature of extensional deformation since the Eocene, commencing with probable extrusion of CBU-equivalent units. Extensional shear was localized along the site of extrusion during Late Miocene development of a metamorphic core complex, which overprinted much of the earlier metamorphic history. Our results suggest a new domain in the Western Cyclades that has been exhumed by an uncharacteristic south-directed extensional regime since the Late Miocene.
T31C-0589
Structural controls on the Surprise Valley fault system, northwestern margin of the Basin and Range, based on combined geologic, potential-field, and seismic reflection modeling
Surprise Valley of northeastern California is a major high-angle fault-bounded basin that marks the northwestern margin of the Basin and Range province. On the western margin of the valley, the east-dipping Surprise Valley Fault separates the valley from the Warner Range and accommodates at least 7 km of normal slip. On the eastern margin, a west-dipping normal fault has exhumed the Hays Canyon Range, but offset along this fault decreases significantly to the north. In between these two major faults, numerous Holocene fault scarps and hot springs suggest that the area is still actively extending. Previous workers have mapped several NW-SE-trending structures cutting across the valley that control the geothermal system, but the role of these structures in basin evolution and their relationship to the range-bounding faults is poorly understood. The Surprise Valley fault system is of particular interest given its location at the margin of the Basin and Range, its relative isolation from other major normal fault systems, and the recent offset along the faults. Our focus is on identifying and characterizing intra-basin and basin-bounding faults and structures and constraining the basin geometry through a combination of geological mapping, potential field modeling and seismic reflection modeling. We have collected nearly 800 new gravity stations, ~75 line-kilometers of truck-towed magnetometer data, and ~17 km of foot-traversed gradient magnetometer data within and surrounding Surprise Valley for potential field modeling. These data, combined with new 1:24,000-scale geologic mapping and a ~16 km vibrator-source reflection survey collected in the summer of 2004, confirm the importance of the NW-trending structures not only in the distribution of hot springs, but also the location and orientation of accommodation zones between N-S trending, antithetic normal fault systems. The NW trend of these structures, including the Lake City Fault Zone and Fandango Valley, parallels a prominent regional system of faults that includes the Eugene-Denio and Mt. McLoughlin fault zones to the north and the Likely Fault to the south. Field relationships imply that both N- S and NW-SE trending fault sets have been active since the late Tertiary. The widespread, NW-trending fabric, locally present in the Lake City fault zone may be accommodating strike-slip motion predicted in recently published GPS data.
T31C-0590
Deciphering monazite ages in the Menderes Massif in Western Turkey, using cathodoluminescence, electron microprobe and ion microprobe techniques
The Menderes Massif is a key Aegean metamorphic core complex, exposing over 40,000 km2 of metamorphic and igneous rocks via large-scale extension. Granites cut by the Alasehir detachment surface, which bounds the northern edge of the core complex, contain monazite grains than range from 9.6±1.6 Ma to 21.7±4.5 Ma (±1σ). The grains were dated in thin section using an ion microprobe (Th-Pb). To understand the reason for the ~10 m.y. discrepancy, we applied backscattered and secondary electron imaging, cathodoluminescence (CL), and X-ray element (REE, Th, Y, Ca) mapping. Monazite does not luminesce, however surrounding minerals show CL evidence of fluid flow that indicate the likelihood of dissolution/reprecipitation reactions and aid in the interpretation of the monazite ages. The CL images show the granites contain plagioclase with corroded cores and distinct rims, and quartz in some samples show evidence for brecciation and subsequent recrystallization. Feldspars are not only zoned in CL from core to rim, but also show differences in color along twin planes. Radiation haloes surround many monazite grains. The CL was also useful for identifying shear sense indicators not always seen using light microscopy or electron microprobe imaging. The dated monazite grains are chemically zoned, but the mineral's composition does not always represent a different age. The granites visibly range in degree of alteration, depending on their proximity to the Alasehir detachment. Whole and trace element rock chemistry from the granites show they experienced a complicated magmatic history, consistent with the CL images. The combination of electron microprobe, ion microprobe and CL techniques are useful for deciphering the complicated history of the granites exposed along the Alasehir detachment, and can be applied to elsewhere to regions that have experienced a polyphase magmatic and tectonic history.
T31C-0591
Right-Stepping North-South Sub-basins Linking the Dixie Valley and Fairview Valley Structural Basins, Nevada, USA, Identified From new Land Gravity Measurements
In 1954, a sequence of major earthquakes produced a widely-spaced, north-south trending, left-stepping surface rupture pattern distributed over the Transition Zone (TZ) between the Dixie Valley Structural Basin (DVSB) and Fairview Valley Structural Basin (FVSB). Although the seismological and neotectonic aspects of this event are well studied, particularly in the DVSB, the previous published exploration geophysics data (specifically gravity) across the TZ are too sparse and inconsistent to reveal the basement structural relief. We present results and structural interpretations for a recently completed 300 station gravity survey covering 1,000 square km, which was designed to investigate the structural linkage between the DVSB and FVSB and the associated left-stepping 1954 Dixie Valley – Fair View Peak rupture pattern. The complete Bouguer gravity anomaly map shows what appears to be a remarkably clear picture of the basement structure in the TZ and adjoining DVSB and FVSB. Depths are interpreted assuming and Tertiary alluvium and volcanic basin fill with Mesozoic metasedimentary basement. Results show, within the study, the DVSB and FVSB are more than 2 km deep and have approximately northwestern trending terminations at their respective southern and northern extremes. These terminations effectively define the boundaries of the TZ, which measures 20 km north to south. Structurally projecting from the DVSB's southwest and FVSB's northeast termination quadrants are a roughly mirror image pair of north-south trending, right-stepping grabens (2 to 4 km wide and 500-700 m deep) which attenuate in relief toward a central overlap area. This overlap area is located 7 km east-northeast of where Elevenmile Canyon exits the Stillwater Range. The right-stepping grabens appear to represent a zone of maximum extension across the TZ, which also aligns with a southward projection of the maximum extension reported by other studies on the western structural boundary of the DVSB. The area defined by the graben pair's overlap and maximum depth attenuation aligns on a northwestern trend that we suggest may represent a crustal shear separating two the two major basin systems. The relation of these grabens to the neotectonic surface faulting across the TZ is not clear at this time, but is under study.
T31C-0592
Tectonic significance of southward shear sense indicators in central and southern Menderes Massif, western Turkey
The Menderes massif of the Western Anatolia Extended Terrane in western Turkey contains four major extensional shear zones which are, from north to south, Simav, Alasehir, Buyuk Menderes and Kayabuku (Selimiye) shear zones. It has been well-established that the Simav of the northern Menderes Massif and Alasehir of the northern Margin of the Central Menderes massif contain only top to the north shear sense indicators. In recent years, we have conducted detailed structural analysis along the Buyuk Menderes shear zone of the southern margin of the central Menderes massif and the Kayabuku shear zones of the southern Menderes massif. The footwall of the Buyuk Menderes shear zone is made up of quartzite and marble intercalated micaschists. Its hangingwall contains the gneissic metamorphic rocks and the Early-Miocene sedimentary rocks, usually referred in the area as the Haskoy Formation. The footwall rocks contain both top to the North and top to the South shear sense indicators. The south-dipping Kayabuku shear zone of the southern Menderes massif also contains well-developed mesoscopic and microscopic shear sense indicators, and separates orthogneiss and augengneiss in its footwall from schist and marble rock units in its hanging wall. The shear zone contains two ductile sense of shearing; 1) top to the north and 2) top to the south. Our petrographic analysis of the shear sense indicators along the Buyuk Menderes and Kayabuku shear zones suggest that the top to north shear sense indicators are overprinted by top to the south shear sense indicators. This suggest to us that both the Buyuk Menderes and Kayabuku shear zones were developed in two stages.
T31C-0593
The Termination Of The Northwest Basin And Range Into A Northwest Trending Extensional Fault System
Extensional provinces terminate along-strike where extension dissipates to zero, at transform faults, or at triple junctions. Termination of the northern Basin and Range province in western North America has long been thought to be controlled by an intracontinental transform fault, the Brothers Fault zone (BFz). New mapping in the boundary region between a major Basin and Range fault, the Hart Mountain fault system (HFz), and the BFz was conducted to test this model and to determine the structural and temporal relationship between the two fault zones. Stratigraphic separation and topography were used to determine fault orientations, displacement gradients, and sense of motion in the two fault zones. N-S trending faults of the HFz show predominantly normal displacement. Displacement in the HFz decreases northward from a maximum of ~940 m at Hart Mountain to ~173 m to 0 m at the BFz. Fault orientations smoothly change from the N-S Basin and Range trend in the south to the NW BFz trend in the north. Topography and stratigraphic separation indicate that the BFz is a series of isolated faults with maximum slip of ~106 m and lengths of up to a few kilometers. Piercing points within the BFz indicate that down-to-the-north motion predominates with the fault system and that strike-slip motion is minimal. Uncertainty on observed fault displacements is + 5m. Stratigraphic separation of the Rattlesnake Tuff indicates that activity on the NW trending fault system post-dates the 7 Ma tuff. That displacement along the HFz effectively goes to zero at the BFz suggests Basin and Range faults have propagated northward into an existing northeast- facing extensional province represented by the BFz.
T31C-0594
Strike-Slip displacement along the Furnace Creek Fault Zone, southern Basins and Ranges, Death Valley, California
The southern Basins and Ranges contain several strike-slip fault zones in addition to predominant normal faults. One of the strike-slip faults is the Furnace Creek fault zone (FCFZ) which extends from the Amor¬gosa Valley in eastern California northwestward continuously about 200 km and termi¬nates in the Fish Lake Valley in Nevada. The fault zone is a part of the Eastern California Shear Zone. Although the right-lateral sense of strike-slip movement along the FCFZ is undisputed, the magnitude of displacement has been controversial since the 1970s. Recently, we have mapped conglomerates exposed in the Travertine point area of the Furnace Creek Wash of the Death Valley region. The conglomerates are composed of Paleozoic clasts from the following formations: Bonanza King, Nopah, Pogonip, Eureka Quartzite, Hidden Valley, and Ely Springs Dolomite. Our analysis of these breccias showed that they are made out of clasts of one composition and a matrix that was slightly different. This observation and our microscopic analysis suggest to us that these breccias were formed as fault breccias along the Furnace Creek fault zone. We have also mapped breccias in the Desolation Canyon on the southwestern side of the FCFZ where the Bonanza King Formation is brought into structural contact over the Ely Spring Dolomite and Eureka Quartzite suggesting the presence of a thrust fault. We correlate this thrust fault with a similar structural setting along the Clery Thrust of the southern Funeral Mountains on northeastern sides of the FCFZ where the Clery thrust brings the Cambrian Bonanza King Formation over the Eureka Quartzite and Ely Spring Dolomite in the southern Funeral Mountains. These observations suggest to us that the thrust fault in the Desolation Canyon area is the continuation of the Clery Thrust of the southern Funeral Mountains. If this interpretation is correct, the strike-slip displacement along the FCFZ is about 30 km.
T31C-0595
Structural Insight of the Eastern Marmara Sea by Combined Multichannel Seismics and Refraction Tomography
At least the eastern 2/3 of the Marmara Sea constitute a well-documented seismic gap since they are the only part of the North Anatolian strike slip system that was not ruptured during the 20th century. The 60 km-long Cinarcik Basin is the easternmost basin of the Marmara Sea, immediately east of which the rupture of the destructive 1999 Izmit earthquake has stopped. The seismotectonic regime within the Marmara Sea has been subject to debates, that have for a large part concerned the Cinarcik basin, and whose implications are important in terms of seismic hazard in the densely populated area of Istanbul. Different hypotheses have been proposed, essentially based on shallow geophysical studies. Deep investigations are a necessary complement to gain a better insight of the structures and their evolution. We present here the results obtained from data collected during the SEISMARMARA cruise in 2001. A dense set of 81 multichannel seismic lines has been acquired whose shots were also recorded by an array of 13 ocean bottom seismometers deployed throughout the basin. Wide-angle data were processed by refraction tomography that provided a 3D model of P-wave velocities spanning over 66x36 km. The MCS data were classically processed through a stacking and migration procedure, and eventually depth-converted using the velocity model. This provides us with both a quantitative assessment of the broad structures and a geometrically accurate image of the fine structural setting down to ~ 8 km depth. The interpretation of these data brings an insight of the crustal processes responsible for the formation of the basin and its subsequent evolution, recorded by the sedimentary in-fill. This analysis reveals high velocities within the basement underlying the basin, possibly implying a thinning of the lithosphere by removal of the upper crust. Within the sedimentary basin, a strong spatial variability and a significant tectonic complexity is observed; this is directly related to the dominant strike slip motion of the North Anatolian system and to the geometric complexity of structures with respect to this general kinematics.