T33A-1134
The growth of Rayleigh-Taylor instability under a shear-stress free top boundary condition
The separation of zones of apparent downwelling flow at the ends of the Sierra Nevada suggest a relatively large wavelength (~500km), but Rayleigh-Taylor instability for plausible rheological structures with a fixed top boundary condition require much shorter wavelength (<100km). To understand this difference we perform analytical and numerical plane-strain experiments on the stability of a dense layer overlying a less dense substratum, representing the Rayleigh-Taylor instability between the mantle lithosphere and the underlying asthenosphere, focusing on the effects of a shear-stress free boundary condition at the top. The overall effect of this condition is an enhancement of growth rate factors at long wavelengths. The enhancement depends greatly on the viscosity variation of the layer, and less so on the density distribution and viscosity of the substratum. With large variation, the viscosity at the top of the layer will be great enough that the solution approximates that with a fixed top boundary condition. With little variation, the solution approximates that for constant viscosity, which has also been shown to stifle growth at long wavelengths. The intermediate zone showing the enhancement includes ratios of layer thickness to viscosity e-folding length, h/L, of ~ 1 - 8 in Newtonian viscosity experiments, and ~ 1 - 4 in non-linear viscosity cases. The free top condition is likely to be applicable to many geologic situations where the lower crust is weak. Olivine flow laws and a gentle temperature gradient (≈ 5° C/km) place the Sierra Nevada viscosity scaling length, L, at the upper limit of this range of h/L values. Thus longer wavelengths than commonly assumed for Rayleigh-Taylor instabilities seems permissible when viscosity decreases with depth and the top surface of the layer is only weakly constrained.
T33A-1135
The Middle AsiaN Active Source (MANAS) Profile: Preliminary Results From A Deep Seismic Transect in the Tien Shan of Kyrgyzstan and China
New near-vertical deep seismic reflection data, acquired during the summer of 2007, constitute an ~350 km lithospheric transect from the northwestern Tarim Basin in China to the central Tien Shan of Kyrgyzstan. Recognized as one of the highest, youngest, and most active orogenic systems on Earth, the Tien Shan are situated internal to the Eurasian continent, removed up to 3000 km from the former plate boundary with the Indian subcontinent. Existing geologic constraints imply that up to 200 km of shortening may have occurred in Late Tertiary to Recent time. Additionally, geologic, topographic, and gravimetric data suggest that continental lithosphere of the Tarim basin may presently be subducting beneath the southern margin of the Tien Shan, in the absence of an oceanic slab. While geodetic measurements document that the Tien Shan currently record about half of the shortening between India and Eurasia, geologic data dictate that active faults are restricted to only several of the individual ranges that make up the mountain belt. Passive-source seismological studies have shown the surprising result that the orogenic crust is thickest (65-70 km) at both the southern and northern margins of the Tien Shan, and thins dramatically to ~35 km within the internal part of the orogen. Key targets of the MANAS (Middle AsiaN Active Source) Profile include (1) the top of the Tarim crust as it descends beneath the southern Tien Shan, (2) an inferred crustal-scale frontal ramp, representing where the continental plate may have broken and is now descending into the upper mantle, (3) the geometry of demonstrably active faults below the shallow depths to which they can be inferred from surface geologic constraints, (4) the topography and seismic reflection signature of the Moho, especially given the unexpected variations in crustal thickness across the orogen, and (5) the significance of both crustal and upper mantle conductivity anomalies previously identified through magnetotelluric studies. The experimental design involved roll-along of stand-alone seismometers (Reftek-125A), allowing essentially continuous CMP coverage along the entire ~350-km transect, despite the challenging terrain. These seismic reflection results will be combined with a coincident broadband array (40 stations at ~10 km spacing for two years) and magnetotelluric measurements along the profile route to provide an integrated geophysical fingerprint of the lithospheric structure of intracontinental mountain building.
T33A-1136
Uplift and Extensional Deformation Caused By Delaminated Lithosphere of the Eastern Anatolian Plateau
The process of delamination has been invoked to explain the anomalous uplift of various disparate locales such as the Sierra Nevada, Colorado Plateau, Andes, and Alboran Sea/Rif-Betics. In this study, we examine the possible role of delamination in the tectonic evolution of the Eastern Anatolian plateau. This is an enigmatic region of high topography that is absent of mantle lithosphere; and it is bounded by plate convergence but shows appreciable extension across the plateau. Our geodynamic models demonstrate that a rapid delamination of mantle lithosphere (in 7 Myrs) results in broad surface uplift of 2 to 3 km through isostatic adjustment and flow stresses. The amplitude and form of uplift is close to the present day surface topography profile of the Eastern Anatolia along 42° E. Zones of crustal shortening, thickening, and enhanced uplift flank the uplifted plateau. These are caused by a combination of imposed plate convergence in the models and delamination-driven effects. More notably, the delamination also drives a region of extension and crustal thinning within the model plateau, even in the midst of the broader plate convergence. When the plate convergence rate is set to zero, the delamination effects are isolated and we observe amplified extension and thinning, but decreased plateau uplift and shortening/thickening at the plateau flanks. When the convergence rate is doubled, extension/thinning is muted and the contraction and thickening is amplified but is still localized to the sites determined by the delamination. The results suggest that mantle lithosphere delamination may reconcile the presence of extensional structures and half-grabens across Eastern Anatolia within the larger-scale contractional system. Furthermore, the model may be used to understand similar styles of syn-convergent extension at the Apennines- Tyrrhenian, Himalayas and Alboran Sea/Rif-Betics.
T33A-1137
Spatial Relationship Between Crustal Structure and Mantle Seismicity in the Vrancea Seismogenic Zone of Romania
Active crustal deformation and subsidence in the Southeast Carpathian foreland has previously been attributed to active foundering of thickened continental lithosphere beneath the Carpathian bend region (Knapp et al, 2005). The present study involves integration of active and passive-source seismic data in order to place constraints on the duration, timing, and scale of crustal deformation in the Carpathian foreland, and in particular to assess the genetic relationship with the Vrancea intermediate-depth seismogenic zone (VSZ). Relocated crustal earthquakes and focal mechanisms were correlated with four deep industry seismic profiles, the reprocessed DACIA PLAN deep seismic profile, and the DRACULA (Deep Reflection Acquisition Constraining Unusual Lithospheric Activity) II and III profiles. Projection of foreland crustal hypocenters onto the deep seismic lines correlates well with previously identified crustal faults such as the Trotus and Sinaia, as well as the newly identified Ialomita Fault. Specifically, results of this study (1) image the full crustal and uppermost mantle structure of the Focsani Basin in the close proximity of the VSZ, (2) show evidence for a sub-horizontal, slightly east-dipping Moho in the vicinity of the VSZ and thinning of the crust towards the Carpathian orogen, (3) illustrate the conspicuous absence of west-dipping fabrics or structures in the crust and across the Moho, (4) present evidence that the Trotus Fault is a crustal-scale active fault with a dextral sense of motion, (5) suggest that the Paleozoic age Peceneaga-Camena and Capidava-Ovidiu Faults have not been active in post-Paleozoic time, and (6) show evidence for a new active crustal scale sinistral fault, named the Ialomita fault. Both the seismogenic Vrancea body and deformation in the Focsani Basin appear to be concentrically bound by the Trotus Fault in the north and east and the Sinaia-Ialomita Fault in the south, suggesting a coupled deformation between the VSZ and the foreland deformation, possibly accommodated on these two major fault systems. These results contradict both the "subduction-in-place" and "slab- break-off" hypotheses as feasible explanations for VSZ intermediate-depth seismicity, and lend additional support to a lithospheric delamination model to explain both the origin of the VSZ and the crustal architecture of the Southeast Carpathian foreland.
T33A-1138
Segmentation and mechanism for subsidence across the western Calabria continental margin, south-eastern Tyrrhenian Sea
The most relevant geologic features across the continental margin offshore western Calabria and its Neogene to Recent sedimentary and tectonic evolution are reconstructed using high-resolution coupled with high-penetration seismic lines. Data and interpretations we present offer an opportunity to investigate the influence of the roll-back and retreat of the NW-dipping subduction of the Ionian plate underneath Calabria on vertical movements in the overlying upper crust. Seismostratigraphic analysis was used to define seismic units. To assign ages to the sedimentary units, a key role was played by the Messinian horizon. Across the margin the major sedimentary units vary in thickness, number of seismic units and post late-Messinian tectonic deformation. On this basis, the margin has been divided into three distinct segments separated by narrow sub-vertical discontinuities. The NW- segment extends from the continental shelf of Calabria to the morphological high bordering to the west the Paola Basin, a NNW-SSE elongated basin filled by Plio-Pleistocene sediments more than 4.0 s thick. Here, we have distinguished four seismic units corresponding to sedimentary or crystalline rocks, pre late-Messinian in age, and three seismic units of Plio-Quaternary age. The central segment extends along the central sector of the margin. Here four seismic units have been identified. The lower one, tentatively attributed to the crystalline and metamorphic rocks and their Cenozoic terrigenous cover related to the Kabilian-Calabrian units, is unconformably overlain by the Messinian horizon. A number of closely spaced, mainly land- and sea-ward dipping listric faults with limited vertical slip, offset Plio-Pleistocene and older horizons. The SE-segment reaches to the west the oceanic sector of the Marsili Basin. Here, the Plio-Pleistocene seismic units appear to be strongly deformed by post – Late Messinian compressional or transpressional tectonic events. Geometries of reflectors in the continent-oceanic transition zone suggest that they represent the seismic expression of buried volcanic body. Data we presented suggests that: limited vertical slip occurs along normal and reverse faults detected at the border and inside the sedimentary infilling of the Paola Basin. Thus, mechanisms for subsidence affecting this area since the Late (?) Neogene significantly differ from extensional and/or compressional processes responsible for formation of basins along the Sardinia and in the north Sicily continental margins. Taking into account the pattern of the sedimentary infilling, the shape of the basin and its position in the regional context, we propose that the Paola Basin developed approximatively near the northern edge of the Ionian slab where tearing of the lithosphere is expected. Moving toward the continent-oceanic transition zone, the Calabrian margin may be partitioned into three segments characterised by different post Late-Messinian tectonic deformation and separated by localised strike-slip fault zone. It has been proven that strain across continental margin in oblique plate convergent setting is frequently partitioned in discrete strike-slip fault system and zone of diffuse deformations. Therefore, segmentation of the margin could be correlated with changes along strike in the dip and/or age of the Ionian oceanic lithosphere subducted underneath Calabria.
T33A-1139
Deformation History of the Susques Basin (~23°S, 66°W), Puna Plateau, NW Argentina: New Constraints by Apatite (U-Th)/He Thermochronology and 40Ar-39Ar Geochronology
New geochronologic and thermochronologic data from the Susques basin in the central Puna plateau of northwest Argentina provide information about the timing of deformation and exhumation in this region. Seventeen samples for 39Ar/40Ar geochronology were collected in the Susques basin and the adjacent Sierra del Tanque. These include ashes in deformed and undeformed sections, as well as reworked units and pristine, undisturbed units. Single grain analyses of locations previously dated by multi-grain analyses or whole-rock K-Ar methods reveal the presence of several vastly different age populations within reworked samples, forcing a re- assessment of existing geochronology in this area of the Puna. The oldest undeformed ash yields an age of 6.59 ± 0.027 Ma, indicating that deformation ceased prior to this time. Samples for Apatite (U-Th)/He dating were collected from 2 transects in the basin-bounding Sierra del Tanque and Sierra de Cobres. Helium ages range from ~47-29 Ma in the Sierra del Tanque, and from ~32-21 Ma in the Sierra de Cobres. Geochronology from the ashes intercalated within deformed and undeformed basin sediments and (U-Th)/He thermochronology from the two transects suggest that: 1. Deformation in the Susques basin, which occurred primarily by thrust and reverse faulting, ceased by ~6.5 Ma, and 2. Less than 2 km of exhumation has occurred over the past ~20 Ma, and only ~500 m of exhumation occurred between 20-50 Ma. The cessation of tectonic activity was rapidly followed by the deposition of large-volume, undeformed ignimbrites between ~6.5 Ma and 900 ka. No balanced structural sections exist in this area. However, the limited exhumation documented by the (U-Th)/He system suggests that regional deformation was also likely limited. Since significant amounts of shortening are not recorded in the unroofing history of the basin-bounding ranges, a mechanism other than recent distributed crustal shortening must be invoked to explain the high elevation and thick crust of this area of the Puna plateau. Our observations, combined with the presence of voluminous, late Cenozoic ignimbrites, suggest that a mechanism that involves addition or removal of material from the lower crustal and/or mantle lithosphere may play a significant role in creating the high topography of this area.
T33A-1140
ALPASS: Teleseismic Tomography of the Eastern Alps
The Eastern Alps were formed by the north-south directed collision of the Adriatic (African) and European plates and a subsequent tectonic escape of crustal fragments to the unconstrained margin in the east, represented by the Pannonian Basin. Recent controlled source seismic experiments (TRANSALP, CELEBRATION 2000, and ALP 2002) revealed significant internal structures of the crust and the Moho topography. However, deeper plate tectonic structures (e.g. subducting slab) are still under debate. ALPASS is a passive seismic monitoring project aiming to reveal lower lithosphere and upper mantle beneath the wider Eastern Alpine region, and to contribute to a better understanding of the geodynamic processes at work. By cooperation of Austria, Croatia, Finland, Hungary, Poland, and USA 57 temporary seismic recording stations were deployed from May 2005 until May 2006. The layout was designed to extend the efforts of earlier experiments (e.g. TRANSALP) and to support two other passive seismic experiments (BOHEMA, Carpathian Basin Project), which are overlapping in the investigation area. Additionally, data from permanent networks was collected to improve coverage of the investigation area. 144 events (50% with M > 5.6) from epicentre distances between 30° and 100° were selected for teleseismic inversion. Travel time picking of P-wave arrivals has been done by a semi-automatic correlation technique. Crustal corrections benefit from the high resolution velocity model of the crust and the new Moho map derived from CELEBRATION 2000 and ALP 2002 data. First results of teleseismic inversion will be presented and discussed with respect to crustal structures revealed by the controlled source experiments, tomographic models generated during earlier studies, and their consequences for the conception of plate tectonics in the Eastern Alps.
T33A-1141
The Eocene to Oligocene Landscape of the Northern Sierra Nevada
To gain a better understanding of the Cenozoic tectonic and landscape evolution of the northern Sierra Nevada, well-preserved Eocene to Oligocene sedimentary and volcanic units form the focus of a detailed stratigraphic study which incorporates geochemical and stable isotopic analyses. Widespread silicic ash-flow tuffs (31-28 Ma) crop out across the northern Sierra from near paleo-sea level at the eastern edge of the Great Valley across the modern crest of the range into Nevada. On the western flank of the northern Sierra, they cap Eocene prevolcanic fluvial sediments of the ancestral Yuba and Feather Rivers. The Eocene fluvial system was dominantly controlled by bedrock structure, consisting of two types of coeval valley morphologies: steep, narrow high-energy valley segments and broader, lower-gradient braided stream valley segments. The braided fluvial sequence contains four upward-fining cycles: coarse lower intervals consist of gravel-sand dunes and lateral accretion elements deposited within higher energy channels; upper fine intervals consist of 1-5 m thick lignite-bearing clay and silt marsh deposits. Full-valley width exposures of clay eliminate the possibility of autocyclic controls, indicating that the multiple upward-fining cycles reflect base-level change. Two distinctive overlying ash-flow tuffs were identified and correlated by trace and rare earth element composition of volcanic glass and lithologic criteria. Hydrated glass was used as a proxy for hydrogen isotopic composition of precipitation to determine the paleoelevation gradient in the Oligocene. The δD of ancient meteoric waters, which scales at a predictable rate with change in elevation, decreases steadily across a range-perpendicular transect, from -125‰ ± 1‰ in the west to -160‰ ± 4‰ in the east. This 35‰ decrease in the δD of precipitation is similar to the compositional gradient of the range today, and reflects an increase in ancient mean elevation along the transect. These results suggest that the northern Sierra Nevada stood as an area of high topography in the early Oligocene, similar to the modern range. This relatively steep topographic gradient is consistent with the landscape reflected in the stratigraphic sequence: steep high-energy fluvial channels emptying into structurally-controlled broad braided stream valleys.
T33A-1142
Paleoseismicity of the California Wash Fault, Southern Nevada: Implications for neotectonics and seismic hazard
The Quaternary (Q) California Wash Fault (CWF) poses a seismic threat to the greater Las Vegas area, thus, detailed study is imperative. The CWF lies 60 km NE of Las Vegas and forms the North Muddy – Muddy Mountain range front. Our purpose is to document and analyze the paleoseismicity, structural geology and seismic hazard of the CWF. We collected new 1:12,000 scale map data, a trench log, clast counts, scarp profiles, drainage offsets, and 14C samples from colluvial wedges. Map, profile, drainage, and trench data show that the fault is a down-to-the west, high-angle, multi-stranded, segmented normal fault. The 14C and profile data show the MRE occurred ~1900 cal year BP. The trench log and profile show up to five earthquake events with throws of 0.7 to 3.1 m indicating events that cluster around ~M6.9. Map data show a surface rupture length of 32.2 km, also consistent with an earthquake of ~M6.9. A M6.9 event just 65 km away from Las Vegas and a few km away from a major electrical power generating plant could cause substantial damage to humans and infrastructure. The CWF is one of the southernmost active faults in the Basin and Range province and lies near the latitude where strike-slip faults motions split to the San Andreas fault system and the Eastern California Shear Zone / Walker Lane belt.. It does not appear to reactivate an older fault near the surface where it offsets Quaternary to Paleozoic units. It has been active at least since the time when some of the strike-slip motion associated with the San Andreas / plate boundary stepped east of the Sierra Nevada Mountains. We suggest that extension along the CWF, and other similar faults in the region, is part of a space accommodation associated with the northwestward motion of the Sierra Nevada relative to the Colorado Plateau.
T33A-1143
Fault Slip Partitioning in the Eastern California Shear Zone-Walker Lane Belt: Pliocene to Late Pleistocene Contraction Across the Mina Deflection
Two different mechanisms have been proposed for fault slip transfer between the subparallel NW-striking dextral- slip faults that dominant the Eastern California Shear Zone (ECSZ)-Walker Lane Belt (WLB). In the northern WLB, domains of sinistral-slip along NE-striking faults and clockwise block rotation within a zone of distributed deformation accommodated NW-dextral shear. A somewhat modified version of this mechanism was also proposed for the Mina deflection, southern WLB, whereby NE-striking sinistral faults formed as conjugate faults to the primary zone of NW-dextral shear; clockwise rotation of the blocks bounding the sinistral faults accommodated dextral slip. In contrast, in the northern ECSZ and Mina deflection, domains of NE-striking pure dip-slip normal faults, bounded by NW-striking dextral-slip faults, exhibited no rotation; the proposed mechanism of slip transfer was one of right-stepping, high angle normal faults in which the magnitude of extension was proportional to the amount of strike-slip motion transferred. New geologic mapping, tectonic geomorphologic, and geochronologic data from the Queen Valley area, southern Mina deflection constrain Pliocene to late Quaternary fault geometries, slip orientations, slip magnitudes, and slip rates that bear on the mechanism of fault slip transfer from the relatively narrow northern ECSZ to the broad deformation zone that defines the Mina deflection. Four different fault types and orientations cut across the Queen Valley area: (1) The NE-striking normal-slip Queen Valley fault; (2) NE-striking sinistral faults; (3) the NW-striking dextral Coyote Springs fault, which merges into (4) a set of EW-striking thrust faults. (U-Th)/He apatite and cosmogenic radionuclide data, combined with magnitude of fault offset measurements, indicate a Pliocene to late Pleistocene horizontal extension rate of 0.2-0.3 mm/yr across the Queen Valley fault. Our results, combined with published slip rates for the dextral White Mountain fault zone (0.3-0.8 mm/yr) and the eastern sinistral Coaldale fault (0.4 mm/yr) suggest that transfer of dextral slip from the narrow White Mountains fault zone is explained best by a simple shear couple whereby slip is partitioned into three different components: horizontal extension along the Queen Valley fault, dominantly dextral slip along the Coyote Springs fault, and dominantly sinistral slip along the Coaldale fault. A velocity vector diagram illustrating fault slip partitioning predicts contraction rates of <0.1 to 0.5 mm/yr across the Coyote Springs and western Coaldale faults. The predicted long-term contraction across the Mina deflection is consistent with present-day GPS data.
T33A-1144
Shear Wave Splitting and Mantle Flow beneath The Colorado Plateau and the Colorado Plateau-Great Basin Transition
Shear-wave splitting measurements are determined using data collected from LA RISTRA 1 and 1.5 (Colorado pLAteau RIo Grande Rift/Great Plains Seismic TRAnsect) to study the origin of seismic anisotropy in the mantle beneath the Colorado Plateau and the Colorado Plateau-Great Basin transition. Results show that, on the average, the fast polarization directions are sub-parallel to North American absolute plate motion within the central Rio Grande Rift and Colorado Plateau until it reaches the western rim of the plateau where the fast direction becomes NNE-SSW, then rotated westward to N-S in the transition zone, and then to NNW-SSE along stations in western Great Basin. The location of the onset of change of SKS splitting fast direction corresponds exactly to the position where the S-wave velocity perturbations change from about -8 percent beneath the Great Basin to 4 percent beneath the western Colorado Plateau. This location coincides approximately with the Proterozoic-Paleozoic lithospheric boundary (Wasatch line) and the eastern edge of the Marysvale volcanic field. The anisotropy beneath the Colorado Plateau and central Rio Grande Rift shows a remarkably consistent pattern with a mean fast direction of about 45 degrees. Delay times average from 1.5 s beneath the central Colorado Plateau to 0.8 s near the transition zone. Delay times for stations beneath the Great Basin are about 1.2 s. For the central Colorado Plateau, we suggest that anisotropy is controlled predominantly by a combination of sub- continental asthenospheric flow and "frozen" lithosphereic fabric. The observed pattern of rotating westward anisotropy from the western Colorado Plateau through the Colorado Plateau-Great Basin transition and into the Great Basin can be explained by asthenospheric flow around and deflected by a lithospheric keel beneath the Colorado Plateau or by the combined effect of North America plate motion and the edge driven small-scale convection.
T33A-1145
Geologic Evidence for Eruption of Voluminous High-K Magmas at the Onset of Walker Lane Transtensional Faulting, Central Sierra Nevada: Birth of a Plate Margin, Not Root Delamination
Two major NSF projects have been using dominantly geophysical techniques to consider the role of lithospheric foundering in the tectonic evolution of the Sierra Nevada over the past 10 My. Yet some of the best constraints on our understanding of Sierran landscape evolution have come from field studies of dateable Cenozoic strata in the Sierra, largely preserved as volcanic-volcaniclastic fill of paleochannels. Our work, supported by two new NSF grants, is providing detailed stratigraphic, structural, geochronogical and geochemical constraints on the Cenozoic evolution of the central Sierra Nevada and adjacent areas. These provide a test of the high- K/delamination link. We infer that three episodes of Cenozoic uplift occurred in the central Sierra, in addition to Cretaceous uplift. These correspond to: (1) weak uplift associated with the onset of arc magmatism (at 15 Ma), (2) major uplift during the onset of Walker Lane transtension (at 10 Ma), and (3) renewed uplift at the arrival of the triple junction and cessation of arc volcanism (at about 6 Ma). We present evidence that the onset of Walker Lane transtension was accompanied by eruption of 10 - 9 Ma voluminous high-K volcanics at the Little Walker Center, the largest Miocene volcanic center in the Sierra Nevada. We propose that the Little Walker Center formed at a right (releasing) stepover in a dextral transtensional fault system that includes the longest fault in the central Sierra. Our 40Ar/39Ar dates and field data show that this fault zone became active for the first time only 270 - 30 Ka before the onset of high-K volcanism. Transtensional activity in this short time frame included tilting of Oligocene to Miocene strata 40 degrees toward the range front, with offsets up to 700 m on multiple faults, as well as widespread landsliding, with slide blocks up to several kilometers long being shed onto downthrown blocks. The immediately ensuing 10 - 9 Ma high-K volcanism was accompanied by continued transtension. We interpret the Little Walker Center as a pull-apart basin, with faults that penetrated a lithospheric plate with a thick crustal section, thereby tapping low degree partial melts generated at relatively great depth (Putirka and Busby, Geology, in press). In contrast, the 15 - 6 Ma andesites and trachyandesites that enclose the 10 - 9 Ma high-K volcanic rocks were vented along shorter faults, with normal to weakly transtensional offset, thereby tapping magmas at shallower depths. High-K volcanism in the Sierra Nevada thus records the birth of the transtensional plate boundary, rather than lithosphere delamination. http://www.geology.ucsb.edu/faculty/busby
T33A-1146
High K volcanism in the Sierra Nevada: A signal for the initiation of Walker Lane Faulting, and range uplift, not lithosphere delamination
K2O contents have long been recognized as a potential indicator of tectonic processes, and based upon models developed for the Andes (Kay and Kay, 1993) and Tibet (Turner et al., 1996), high-K volcanism has been related to lithosphere delamination, by partial melting of a K-metasomatized lower crust or upper mantle (Feldstein and Lange, 1999; Manley et al., 2000). However, new data from the central Sierra Nevada cast doubt on this K2O-delamination link. Instead, high-K volcanism is better explained as low degree partial melts (F), where low F magmas are preferentially erupted over thick crust, under conditions of high tensile stress. In the central Sierra, a high tensile stress regime was imposed at the onset of Walker Lane transtension, at the eastern edge of the Basin and Range province. We surmise that high K volcanism is similarly controlled by the onset of tensile stresses throughout the Sierra, recording the initial phase of Sierra Nevada uplift. These conclusions stem from several observations. First, K2O contents are highly correlated with Th (R=0.82), Ba (R=0.83), U (R=0.85), Rb (R=0.88) and Pb (R=0.83), and other highly incompatible elements, suggesting a general enrichment mechanism, such as low degree partial melitng. Second, volcanic rocks with the highest K have the highest La/Nb and the lowest 143Nd/144Nd, indicative of a mantle lithosphere source - inconsistent with delamination. Third, maximum K contents increase from north (near Lassen) to south, following in increase in crustal thickness and the (87Sr/86Sr) i of basement granitoids, suggestive of a crustal control on volcanism. Finally, field evidence in the central Sierra shows that the pulse of high K2O volcanism there was synchronous with the development of a pull-apart, along a series of right-stepping dextral transtensional faults, at the onset of Walker Lane faulting. Partial melting calculations verify that primitive magma compositions from Lassen to the southern Sierra, can all be explained by partial melting of a single mantle source, with Cordilleran-type enrichments, but no special K enrichments in any particular region. Moreover, just as low F melts are enriched in K, Pb, or U, they will also be enriched in water, which greatly reduces magma density (Ochs and Lange, 1999). Such differences in water contents provide a mechanism for regional variations in volcanic compositions: depth-integrated density models show that dry mafic magmas have insufficient buoyancy to erupt from beneath thick crust, but low F (water-enriched) melts are sufficiently buoyant to allow eruption. Theoretical models (Takada, 1994) further indicate that tensile stress regimes favor the transport of low F melts. Thus, where the crust is thick, such as in the southern Sierra Nevada, only low F magmas can erupt, due to their natural water enrichments and added buoyancy, and even then are probably only erupted when tensile stresses favor their segregation from their source region. In our alternative interpretation then, high-K volcanism reflects the inception of transtensional stresses, recording the birth of a plate boundary.
T33A-1147
High-Resolution 3-D Regional Double-Difference Tomography of the Reno-Tahoe-Carson City Region
We have performed a high-resolution tomographic inversion using ~200,000 P and S absolute travel times from nearly 10,000 local sources recorded at the Nevada Seismological Laboratory from 1990-2006. An additional nearly 200,000 P- and S-wave high-quality cross-correlation differential travel times were also used in the inversion for double-difference relocation of earthquakes. Results indicate prominent low velocities in the Reno, Washoe Valley and Carson City basins. Shallow high velocities comprise the regions west of the high Sierra Nevada crest, whereas lower-than-average velocities are found in a broad region from Truckee to Reno to Carson Valley to just west of Lake Tahoe in the shallowest portion of the crust. These upper crustal features persist to about 5 km depth, where P velocities lose much of the correlations with surface features, averaging ~6 km/s and slowly increasing with depth. Although the bulk of seismicity lies shallower than 15 km depth, we have incorporated a unique deep sequence of events that occurred in 2003-2004 at 25-30 km depth under northern Lake Tahoe that has been previously interpreted as due to a magma intrusion from seismic and geodetic evidence. We image P velocities at 30 km depth in the low 7 km/s range, consistent with expected lower crustal velocities in the region. Vp/Vs is high within the basins, as expected in alluvial areas. Notably, we also image high Vp/Vs directly above the deep Tahoe sequence in the 16-23 km depth range. This latter observation may be due to the presence of fluids in the middle crust in this region. Low Vp/Vs primarily occurs under the high Sierra Nevada which could be indicative of their high silica content even at depth. Several seismicity lineaments are imaged in the high-precision double-difference earthquake locations. The 1995 Double Spring Flat (Mw 5.8) aftershock zone delineates multiple narrow zones of activity, possibly due to complex interactions among multiple faults that were activated as a result of this event. The deep Tahoe sequence is also sharply defined in volumetric extent; the subsequent shallower Tahoe sequence is imaged as a steeply SSE dipping lineament from ~9-15 km depth. Other features of interest include lineaments of seismicity near Carson City, northeast of Lake Tahoe to Antelope Valley, and northwest and south of Reno.
T33A-1148
Mysterious Moho Beneath the Southern Sierra Nevada, Analyzed with Beam-Formed Receiver Functions
Single station receiver functions from the southern Sierra Nevada, California (36-38 N latitudes) show considerable variation in amplitude of Moho P-S conversions. Specifically, in the range's western foothills, a region termed the "Moho hole" (Zandt et al., 2004), the Moho is absent on single station receiver functions, proposed to be the result of a dramatic cusp on the Moho that destroys Moho P-S coherence. I explore the possibility that signal generated reverberations have sufficiently obscured the seismograms to disguise the Moho P-S conversion. Using data from the 1997 Sierra Paradox Experiment, seismograms from groups of 3 stations spaced ~20 km apart are slant-stacked to make the vertical and horizontal components of the "beam" used in deconvolution. Exploiting improved signal-to-noise ratio, radial receiver functions calculated for the beams better recover from the Moho and intracrustal discontinuities while reflections off of structure near individual stations interfere destructively before deconvolution and are thus not imaged. In some cases, the linear (post- deconvolution) stack is incoherent while the receiver function calculated by first beam forming is clear and interpretable. Of paramount interest is the use of this technique in the region interpreted as the "Moho hole"; here even small amplitude yet coherent receiver functions can be used to constrain crustal geometry. Calculations agree with previous models based on active source seismology and do not invoke dramatic topography at the base of the crust. To investigate the cause of the severely diminished Moho signals, the relative Moho P-S-to- direct P ratio, I have calculated predictions for these relative ratios. Models match very closely with observations, with the low amplitudes under the western foothills primarily the result of coupled geometric considerations and low impedance contrasts across the Moho. Therefore, no dramatic cusp on the Moho is necessary to explain receiver function observations, and overall crustal architecture is consistent with the removal of a large mafic root from the base of the Sierra, strengthening the case that modern topography of the southern Sierra Nevada has resulted from a ~3.5 Ma delamination event.
T33A-1149
Shear wave anisotropy beneath the Sierra Nevada range: Implications for lithospheric foundering and upper mantle flow
Recent work asserts that the garnet-rich Sierra Nevada batholith root has undergone foundering since the early Cenozoic. The Sierra Nevada EarthScope Project (SNEP), undertaken to gain a better understanding of this phenomena, consists of a network of ~80 broadband seismometers spaced at ~25 km from ~37.0N to 40.5N. We use the Silver and Chan method to determine shear wave splitting parameters (dt and φ) for teleseismic SKS phases recorded at SNEP and US Array Transportable Array stations in the region. We find dt>1.1s and φ approximately in the E-NE direction over most of the batholith. Splitting of this magnitude cannot be accounted for solely in the crust, and our results, therefore, have significant implications for upper mantle flow beneath the region. At latitude ~39N to 40N, from the western Sierra Nevada range across our study area to central Nevada, we observe dt<1s at some stations. Smaller dt measurements in this region may be consistent with a vertically oriented anisotropic fabric resulting from asthenosphere upwelling into the slab window south of the southern edge of the Gorda-Juan de Fuca Plate. Such a flow pattern is also consistent with the circular pattern of splitting measurements that exist in the broader California and Western Nevada region. We observe subtle variations in splitting parameters as a function of backazimuth primarily at stations situated on the western foothills of the Sierra Nevada. These complexities may be indicative of either a two-layer or dipping layer structure beneath the batholith that may be associated with on- going lithospheric foundering beneath the Sierran range. Additionally, in the southern part of our study area, we note a reduction in dt for arrivals that sample the high Vp Isabella anomaly – an upper mantle downwelling thought to be a result of recent lithospheric foundering.
T33A-1150
Teleseismic Travel-Time Tomography of the Sierra Nevada and its Foundering Lithosphere
Inferences of foundering lithosphere under the southern Sierra have mostly been based upon xenolith petrology and seismic tomography. To better evaluate the extent and geometry of any such unstable lithosphere, we timed teleseismic bodywave arrivals from approximately 500 teleseisms at 75 stations occupied for 1 or 2 years by the Sierra Nevada Earthscope Project (SNEP) and about 15 Earthscope Transportable Array (TA) stations in the region of the Sierra. With the SNEP footprint, stations are spaced about 25 km apart and extend roughly 400 km along the range and 150 km normal to the range. Additional TA stations extend the aperture of the array to about 600 km. Events were chosen to get the best signal-to-noise ratio while optimizing the backazimuthal coverage. P- wave arrival times were determined simultaneously across all stations for each event using a waveform correlation technique developed by G. Pavlis (dbxcor). Initial examination of the travel-time residuals indicates that no substantial lithospheric downwelling exists between the previously recognized "Isabella anomaly" at the southern end of the range and the approximate position of the south edge of the Gorda plate. Residuals in the Basin and Range are generally small but consistently late compared to stations to the west, but large differences in residuals (>1 s) in the western Sierran foothills can occur over short (~50 km) distances, suggesting substantial heterogeneity in the crust or uppermost mantle. Results of a 3-D isotropic inversion will be presented, and any systematic residual patterns remaining will be evaluated with an eye towards identifying any anisotropy.
T33A-1151
Magnetotelluric Transect Through Yosemite National Park Provides a First Look at Deep Crust and Upper Mantle Electrical Conductivity
The first of three east-west regional transects has been completed as part of an NSF-supported study to examine the nature of the lithosphere beneath the Sierra Nevada. This transect from the Nevada border to the San Joaquin Valley through Yosemite National Park consists of 15 long period magnetotelluric soundings spaced approximately 20 km apart. Data were recorded with instruments from the EMSOC (Electromagnetic Studies of Continents) consortium and processed using both local and distant remote referencing. Broadband data were also acquired at each long period site, so magnetotelluric transfer functions at periods from 0.01 to 20,000 s were sampled. Initial models of the electrical structure reveal the extent to which Basin and Range extensional processes encroach on the Sierra Nevada batholith.