T31E-01
Insights on Lithospheric Foundering from the Sierra Nevada Earthscope Project (SNEP)
Interdisciplinary studies in the southern Sierra Nevada have documented an ongoing removal of the dense residual root from beneath the southern Sierra Nevada batholith. However, many questions remain concerning the timing, spatial extent, mechanism, and consequences of this lithospheric foundering event. The Sierra Nevada Earthscope Project (SNEP) is a scientific experiment designed to investigate these questions with a 2- phase (2 year) seismic deployment of 46 broadband Flex-Array stations embedded in the existing stations of the USArray Transportable Array (TA) in the region. In the 2 phases, approximately 80 sites have been occupied from the northern edge of Kings Canyon north to Honey Lake and from the Central Valley into the Great Basin. In this presentation, we will focus on the most recent common-conversion-point (CCP) stacks of the receiver functions that provide a 3D image of lithospheric layering beneath the central and northern Sierra Nevada. Examining sequential cross-sections reveals distinctive lithospheric "reflectivity" patterns that characterize different tectonic imprints. From phase 1 data, we observed that the westernmost Basin and Range exhibits strong layering with multiple low-velocity zones in the crust and uppermost mantle and a relatively flat and strong Moho varying slowly in depth between 30 and 35 km. In the south this Basin and Range character terminates on the eastern edge of the Sierra Nevada; however, north of Big Pine the Basin and Range character intrudes progressively farther into the range and ends up more than 50 km west of the eastern edge of the range. The lithosphere beneath the southern high Sierra Nevada is characterized by a relatively transparent (homogeneous) crust and sharp Moho that disappears westward beneath the adjacent foothills. The crustal thickness in this area is mostly between 30-35 km with localized welts of thicker crust. The phase 1 observations imply that the removal process appears to be actively affecting the crust northward through at least the central Sierra Nevada. The phase 2 deployment provides coverage to the northern limit of the Sierra Nevada to determine the extent of the affected region and the character of the batholith where root removal may or may not have occurred.
T31E-02
The western extent of the Sierra Nevada batholith in the Great Valley basement and its significance in underlying mantle dynamics
An accurate understanding of the extent to which the Sierra Nevada batholith (SNB) lies beneath the Great Valley (GV) is essential in properly constraining dynamic models for underlying mantle lithosphere removal. Example: the southern Sierra Nevada mantle drip is commonly misrepresented as being strongly offset in map view to the west of the SNB, beneath the GV "forearc". A synthesis of petrographic data on over 200 GV basement cores, complimented by select single crystal U/Pb zircon ages and Nd-Sr isotopic data, shows clearly that the SNB extends westwards to at least the axis of the GV, beneath Upper Cretaceous forearc basin strata. This westernmost zone of the SNB yields zircon ages of 130 to 140 Ma, has depleted mantle Nd-Sr isotopic signatures, and consists of abundant hornblende rich mafic cumulates, as well as diorites and tonalites. Its metamorphic host rocks appear typical of the western Sierra Nevada Foothills, particularly the Middle Jurassic Smartville intra-arc igneous rift complex, Jurassic epiclastic-volcaniclastic slates and schists, and Late Jurassic deformed "Nevadan" plutons. Lower Cretaceous strata of the western GV lying depositionally above the Coast Range ophiolite (CRO) to the west are the remnants of the forearc for this westernmost zone of the SNB. This Early Cretaceous mafic to intermediate composition batholithic belt was exhumed to modest crustal depths in the mid-Cretaceous and then buried nonconformably by Upper Cretaceous forearc basin strata sourced from the felsic axial to eastern SNB. High density and strongly magnetic SNB gabbroids beneath the GV yield regional gravity-magnetic anomalies with amplitudes as high as 50 mgal and 1000 gamma. These coupled anomalies have been mistaken by numerous investigators as the mark of the eastern edge of the CRO having been "obducted" eastwards over "Sierran basement". The complete absence of depleted mantle peridotite core samples from the GV alone argues strongly against such a regional CRO obduction geometry, and in the light of mafic SNB rock types dominating core samples taken along the anomalies the CRO obduction model is rendered obsolete. An extensive westernmost mafic zone of the SNB beneath the GV has important implications for the dynamics of underlying mantle lithosphere removal. At total batholith scale, such a west to east primary compositional zonation pattern would impose a strong primary horizontal density gradient that would favor nucleation of convective instability of the lower crust and upper mantle along the west side of the SNB, as observed. Compounded on this effect is that western zone SNB mafic cumulates, as exposed at 1 GPa levels in the southwesternmost SNB, show preferential copious garnet production during cooling through solidus to hot sub- solidus conditions. Hornblende breakdown is the key to the observed reaction, and solidus to sub-solidus garnet modes tend to mimic protolith hornblende modes. This reaction, as observed in the southwesternmost SNB, could represent a window into incipient eclogitization of lower SNB crust preferentially developed in hornblende rich cumulates. Such a primary compositional control on lower crustal eclogitization would amplify the regional transverse horizontal density gradient across the SNB, and further assist in the preferential nucleation of convective instability to the west. Curiously, the southern Sierra Nevada mantle drip is centered over the largest known concentration of hornblende rich SNB mafic and ultramafic cumulates.
T31E-03
Sierra Nevada river incision from apatite 4He/3He thermochronometry
Published erosion rates suggest that acceleration of river incision beginning some time before 3 Ma initiated formation of the deep river canyons in the southern Sierra Nevada. Such acceleration signals a change in erosional efficacy but its initial timing is poorly constrained. Increased erosional efficacy caused by elevation gain is predicted by scenarios such as block faulting, mantle lithosphere removal, and passage of a slab window. The timing and magnitude of elevation gain may be used to distinguish between competing mechanisms. As in many landscapes, the small magnitude (< 1.5 km) and antiquity of river incision in the Sierra Nevada make the timing of landscape evolution and its relation to tectonic scenarios inaccessible by most methods. Until recently, we have lacked the potential to 'see' erosional events that exhume less than several kilometers and that occur over several to several tens of millions of years. We present apatite He concentration profiles revealed by the recently developed 4He/3He method. The sensitivity to near surface temperatures of the apatite He concentration profile bridges the gap between bulk (U- Th)/He ages and cosmogenic ages, producing a continuum of long term and short term geomorphic rates. We analyzed a series of samples from a vertical profile in Kings Canyon where cooling was not instantaneous and where the helium concentration profile constrains a time-temperature path through roughly the last kilometer of exhumation. Preliminary data suggest that small amounts of bedrock incision (< 1 km) produced thermal perturbations that are resolvable by this approach. We explore various thermal models that satisfy the helium age and concentration profile data alone and compare these results to geomorphic constraints and other rates derived from geologic and cosmogenic data. We also assess the reproducibility of 4He/3He data by analyzing replicate apatites from the same samples. Multiple samples from a vertical profile and replicate data from individual samples allow us to establish the self-consistency and reproducibility of the resulting cooling paths.
T31E-04 INVITED
Stable Isotopic Constraints on the Cenozoic Topographic Evolution of the Sierra Nevada
The Sierra Nevada with mean elevations of 2500 in the north and 3000 m in the south produces a modern rain shadow as a result of the interception of moisture sourced in the Pacific. This rain shadow is reflected in a gradient in the oxygen and hydrogen isotopic composition of precipitation across the mountain range, with a west to east difference in δD of ~40 per mil and δ18O of ~5 per mil. The persistence of this isotopic gradient in the past allows us to place constraints on the surface elevation history of the Sierra Nevada. With the aim of reconstructing the rain shadow development over time, we undertook three different stable isotopic paleoaltimetry studies of the Sierra Nevada. These studies involved: 1) reconstructing the isotopic rain shadow on the east-side of the Sierra using the oxygen isotopic composition of smectite from weathered ashes and calcite from paleosols from Miocene to Recent sediments (Poage and Chamberlain, 2002; Tectonics); 2) determining the hydrogen isotopic composition of kaolinite from weathered Eocene stream deposits along an elevation gradient on the west-(windward-)side (Mulch et al., 2006; Science); and 3) examining the hydrogen isotopic compositions of hydrated volcanic glasses from Miocene to Recent ashes found on both the east- and west-side of the Sierra Nevada (Mulch et al., in review). All of these studies give consistent results and indicate that the Sierra Nevada has existed as an orographic barrier since the mid-Miocene and in the north may date back into the Eocene with no significant surface uplift (>1 km) since then. Taken together, these results suggest that the Sierra Nevada formed the edge of a pre-Eocene continental plateau and the current mountain range proper was formed by tilting and down-drop along its eastern edge by later Basin and Range faulting and maybe already earlier during the incipient demise of the plateau. Our results do not support models calling for greater than 1 km of surface uplift in the Pliocene as a result of removal of dense mantle lithosphere. The fact that different methods and models for reconstructing surface elevation histories (e.g. geomorphic analysis, thermochronology, cosmogenic radionuclides, stable isotope paleoaltimetry, geodynamic modeling, tilting and sedimentation analysis) give conflicting conclusions with regard to the elevation history of the Sierra Nevada is disconcerting, particularly since these methods have been applied in much more detail here rather than anywhere else on Earth. It is critical, therefore, for the scientific community to fully understand what these methods are telling us about surface elevation histories and perhaps the Sierra Nevada offers the ideal area to compare and reconcile these different approaches.
T31E-05
The Possible Role of Mantle Instabilities in Initiating Back-Arc Spreading
Back-arc extension and spreading has been splitting apart the central North Island of New Zealand since the Pliocene. But the impressive volcanism and geothermal activity linked to this extension belies the extensive period of compression, and thrusting that dominated the pre-Pliocene tectonics of the central North Island. Up to 100 km of Oligocene-Miocene shortening is estimated from both deep seismic lines and plate reconstructions beneath the western North Island. Crustal uplift, erosion and sedimentary deposition easily account for the removal of the shortened crust during this time. Yet, the consequences of shortening ~ 100 km of mantle lithosphere are not so obvious. We present a series of seemingly disparate geophysical and geological observations that appear to be most easily explained by thickening of the mantle lithosphere and its ultimate disposal by viscous flow and detachment. These observations include: 1. Parallel zones of late Miocene extension and volcanism that developed each side of the central region of compression; 2. A rapid, domal, surface-uplift of ~1 km for western and central North Island that is dated at ~ 5 Ma and is over a region 200 by 200 km in extent; 3. Low Pn and Sn wave-speeds indicating the mantle lithosphere is missing; 4. Lack of Pn and SKS anisotropy in the western North Island despite anisotropy being pervasive elsewhere in New Zealand; 5. Unusually thin crust (~ 25 km) over much of the western North Island; 6. High-K volcanism over the western North Island during the Pliocene. 7. 600 km deep earthquakes beneath the western North Island. Any one of these 7 observations could be explained by the plate tectonic paradigm. It is, however, the explanation of all 7 observations by a single process that makes the concept of convective removal of thickened mantle lithosphere so compelling. If correct, the late-Miocene removal of the thickened mantle lithosphere was a possible kick-start mechanism for consequent Pliocene back-arc extension and spreading within the central North Island.
T31E-06
Lithospheric Gravitational Instability Beneath the South-East Carpathians
The Carpathians are a major mountain system of the Central and Eastern Europe, which in the South-east surround the Transylvanian basin. Located on the oroclinal bend of the Carpathian mountains, the Vrancea region is characterised by a localised (~ 30 km x 80 km in the horizontal plane) zone of seismic activity to depths of 200 km. This phenomenon has been attributed to subduction of oceanic lithosphere. However, there is no obvious zone of subduction associated with the Vrancea deep earthquakes. An alternative explanation to this deep seismicity is the downwelling of the continental lithosphere in the form of a Rayleigh Taylor instability. The fault plane solutions, for a time interval of 40 years, indicate maximum vertical extension rates on the order of 14% per Myr in the depth range 50-100 km, decreasing by about an order of magnitude in the depth range 100- 150 km. Such rapid rates of deformation clearly represent a recent development, that could not have persisted for a period of time much greater than 5 Myr, and cannot be clearly attributed to recent subduction. Three dimensional finite deformation models of the gravitational instability of the continental lithosphere, based on the finite element method, demonstrate that the Rayleigh Taylor mechanism can explain the present distribution of deformation within the downwelling lithosphere, both in terms of distribution of seismicity and amplitude of strain rates. The spatial width of the high stress zone that corresponds to the seismically active zone is realistically represented when we assume that viscosity decreases by an order of magnitude across the lithosphere. The mantle downwelling is balanced by lithospheric thinning in an adjacent area which would correspond to the Transylvanian basin. This type of planform is inherently three dimensional and is triggered in these experiments by a harmonic perturbation in the form of a first order Bessel function (with m=1 asymmetry). In these models mantle downwelling is associated with crustal thickening but the lithospheric thinning beneath the adjacent basin is associated with only minor crustal thinning.
T31E-07
Edge-driven convection along the Colorado Plateau – Great Basin Transition: Implications for the morphology and dynamics of the Plateau
High mountains and extensive middle to late Cenozoic magmatism rim the margins of the Colorado Plateau (CP) resulting in a bowl-shaped morphology, with the interior of the CP being relatively unaffected by Laramide compression and Cenozoic extension and magmatism. We present new passive seismic imaging and modeling results from the RISTRA 1.0-1.5 transect across the CP that reveal ongoing crustal and uppermost mantle processes affecting the western CP and Great Basin (GB) transition. The CP crust exhibits variable thickness (42- 50 km) near its centre and thins gradually from 40 km to 30 km thick at the western edge, where the highest elevations and the roughest topography are also observed. However, isostatic calculations suggest that less than 50 percent of central CP elevations can be explained by thickened crust alone, with CP margins requiring nearly total mantle compensation. The responsible mechanism for margin topography is edge-driven convection in the upper mantle induced by a step in lithosphere thickness at the western edge of the CP. This step probably was created by the collapse of the enriched and weak Great Basin Phanerozoic lithosphere abutting the depleted and stronger Proterozoic CP lithosphere. In this interpretation, edge-driven convection results in upward flow of material below the transition zone and downward flow below the interior edge of the plateau. The mantle flow causes dynamic uplift of the transition zone and drives a progressive inward migration of Cenozoic volcanism at the CP edge of ~7 mm/yr. Horizontal flow velocities at the margins related to the convection cell are maximally 3 cm/yr at about 200 km depth. Downwelling below the CP interior may additionally influence the history of regional Colorado River system drainage and entrenchment. A similar process may have occurred during the opening of the Rio Grande Rift on the southeastern margin of the CP.
T31E-08
3-D Tomography of the Gulf of California Extensional Province by Rayleigh Wave Inversion
The Gulf of California, separating the Pacific and North American plates, is an excellent area to study the continental rifting process, including magmatism, lithosphere deformation and crustal thickness distribution. To create a 3-D image of this rift zone, we carry out a Rayleigh wave dispersion study, which can sense velocity structure at different depths using different period surface waves. We use fundamental modes waves ranging in period from 20 to 140 s generated by teleseismic sources and recorded by the NARS-Baja seismic array and stations in southern California. Surface waves traveling along the Gulf or Baja California have typically undergone waveform distortion before they reach the study area by propagating along continental margins and subduction zones that induce scattering and multipathing. Further waveform distortion is caused by heterogeneities in the extensional province. In other words, the incoming waves are no longer well represented by a simple plane wave. Instead, we employ an array-processing method (Forsyth and Li, 2005), which describes the incoming wave field as the interference of two plane waves which travel along slightly different paths from the great circle paths and employs finite frequency response kernels (Zhou et al. 2004) to represent scattering within the study area. In southernmost California, there is a low-velocity anomaly in the shallow upper mantle dipping eastward from beneath the Peninsular Range toward the Salton Sea. At depths greater than 150 km, there is a high velocity anomaly beneath the Peninsular Range. Together, this pattern resembles the anomaly pattern beneath the southern Sierra Nevada, where the lower lithosphere is thought to have delaminated and been replaced by upwelling asthenosphere. One of the primary goals of this study is to determine how far south this pattern continues. Another goal is to detect remnants of the Farallon plate that may still be attached to the lithosphere beneath Baja California, left over from the stalling of spreading before the ridge subducted. Results show: 1) Peninsular Range anomalies do not continue into Baja California ; 2) there is a consistent high velocity anomaly in short periods in Guadalupe plate area, which indicates one of the remnants of Farallon plate.