Tomography of potential downwellings--论文代写范文精

底部中生代花岗岩类岩石和老变质围岩,最初被认为与一个通风的地壳均衡有关。尽管后来证实了重力测量的补充,最高的一部分被发现只有35公里厚,略厚的死亡谷地区大约2公里低。下面的report代写范文将进行一部分的详述。

Abstract

Teleseismic P-wave tomography using the Sierra Nevada Earthscope Project (SNEP) deployment, older temporary deployments in the Sierra, and broadband stations from permanent and USArray Transportable Array (TA) stations was derived from starting models either lacking lateral variation (one dimensional [1-D]) or created from three-dimensional (3-D) surface-wave models. The use of multiple starting models permits examination of the robustness of different features while limiting the inherent ambiguities of teleseismic body-wave tomography. Our results confirm that mafic residuum of the Mesozoic Sierran batholith has been removed from the eastern Sierra north to at least 39°N.

INTRODUCTION

The Sierra Nevada of California, stretching ∼500 km in length and rising to peaks above 4 km in elevation (Fig. 1), has proven to be a tectonic enigma. Underlain by Mesozoic granitoids and older metamorphic wall rocks (e.g., Fig. 2; Evernden and Kistler, 1970), the range’s elevation was initially thought to be in isostatic equilibrium with an Airy crustal root (Byerly, 1937; Lawson, 1936). Although later gravity measurements confirmed the presence of a compensating mass deficit (e.g., Oliver, 1977), the highest part of the range was found to have a crust only ∼35 km thick, only slightly thicker than that of the Death Valley region roughly 2 km lower (Fliedner et al., 1996; Jones and Phinney, 1998; Jones et al., 1994; Ruppert et al., 1998; Wernicke et al., 1996). Paradoxically, thicker crust is found under the low western foothills (Fliedner et al., 1996, 2000; Jones and Phinney, 1998; Ruppert et al., 1998). Support for at least the southern part of the range relative to the western foothills is therefore thought to at least partially originate in the mantle, an inference consistent with Quaternary xenoliths, magnetotellurics, and seismic wavespeeds (Ducea and Saleeby, 1996; Fliedner et al., 1996; Jones et al., 1994; Lee et al., 2000; Park, 2004) indicating the presence of asthenospheric material at or only a short distance below the modern Moho.

Compensation in the mantle indicated that material was removed from beneath the eastern, high part of the Sierra in the late Cenozoic. Initially, some workers had suggested that the mantle lithosphere had been extensionally thinned or removed from under the eastern Sierra (Jones, 1987; Jones et al., 1994; Mavko and Thompson, 1983), but most xenoliths from 8 to 12 Ma volcanics in the southern Sierra indicated that the crust had been perhaps 65 km thick at that time, with the lower half being a mafic rock containing considerable amounts of garnet (Ducea and Saleeby, 1996, 1998). Such xenoliths are absent from volcanics 3 Ma and younger as well as the 8.6 Ma site at Blue Knob, which do contain xenoliths from this depth range. Thus Ducea and Saleeby (1996, 1998) inferred that this dense material was removed since 8–12 Ma, a scenario that would cause the Sierra to gain elevation in the absence of any cryptic crustal thinning (e.g., Jones et al., 2004). Removal at ca. 3–4 Ma has been suggested because of a short-lived burst of potassic volcanism with unusual isotopic characteristics (Farmer et al., 2002; Lee et al., 2000; Manley et al., 2000). A late Cenozoic age for removal of this material is also consistent with heat-flow measurements in the Sierra that are far too low to be in steady state with such shallow asthenosphere (Crough and Thompson, 1977; Saltus and Lachenbruch, 1991).

With the suggestion that dense material had been removed from the southern Sierra, attention turned to where this material might be at present. A high-wavespeed body in the upper mantle under the southeastern Great Valley and southwestern Sierra had been identified previously (Benz and Zandt, 1993; Biasi and Humphreys, 1992; Humphreys et al., 1984; Jones et al., 1994; Raikes, 1980); variously termed the Southern Great Valley anomaly, the southern Central Valley anomaly, and the Bakersfield anomaly, we will call it the Isabella anomaly (reflecting its first identification by Raikes [1980] at station ISA). Data from a 1997 deployment led to new images (Boyd et al., 2004) and an interpretation that the Moho was distorted by this body (Zandt et al., 2004). The geometry of this body has varied between studies, from being near vertical (Benz and Zandt, 1993; Biasi and Humphreys, 1992) to perhaps plunging somewhat to the west (Jones et al., 1994) to plunging to the east or southeast (Boyd et al., 2004; Yang and Forsyth, 2006). Differences in the geometry are largely due to differences in coverage and resolution of the different studies.

The Isabella anomaly had previously been interpreted in many ways, including being a fragment of slab (Benz and Zandt, 1993) or a convective downwelling (Aki, 1982) of Great Valley lithosphere (Zandt and Carrigan, 1993), sub–Tehachapi Mountains lithosphere (Jones et al., 1994), sub-Sierran lithosphere (Biasi and Humphreys, 1992), or Coast Ranges and Mojave Desert lithosphere (Biasi, 2009). Subsequent to the recognition of late Tertiary removal of a mafic lower crust from the Sierra Nevada to the northeast, suggestions focused on the Isabella anomaly being that material, having been removed convectively (Molnar and Jones, 2004; Zandt, 2003) or through something closer to delamination (Le Pourhiet et al., 2006). These suggestions have been bolstered by the inference of Saleeby and Foster (2004) and Saleeby et al. (2013) that the Sierran foothills and San Joaquin Valley above and adjacent to the Isabella anomaly have subsided in Quaternary time. Even among these interpretations, there is disagreement about what exactly is in the Isabella anomaly: Zandt (2003) suggested that the upper mantle anomaly was lithospheric mantle entrained by removal of mafic crust that is now at much deeper levels in the mantle, but Saleeby et al. (2003) argue that the mafic crust must reside within the upper mantle anomaly. Biasi (2009) argues against a sub-Sierran source from calculated discrepancies in volumes between sources and sink; he instead prefers some downwelling induced by convergence in the mantle. A sub-Sierran origin has also been challenged by Wang et al. (2013), who resurrect the inference that this is a piece of Farallon slab (part of the mostly subducted Monterey subplate).

Because the geologic and geophysical constraints on lithospheric foundering were largely in the southern Sierra, focus on upper-mantle anomalies was directed to the adjacent Isabella anomaly, but a somewhat similar anomaly had been imaged in northern California by Benz and Zandt (1993). They interpreted this Redding anomaly as part of a stagnating slab associated with subduction of the adjacent Juan de Fuca plate, but Jones et al. (2004) suggested that the anomaly, which is unusually large in magnitude and volume when compared with images of the slab to the north, might also contain descending sub-Sierran lithosphere.

The possibility that the Sierra Nevada is the product of foundering of mantle lithosphere and/or mafic lower crust and that this foundering material might still remain distinct in the upper mantle led to a pair of major initiatives: the Sierra Nevada Earthscope Project (SNEP) focused on seismological imaging of the lower crust and upper mantle under the Sierran region, and the Sierra Nevada Drips Continental Dynamics Project focused on the geologic history, geodynamic constraints, and magnetotelluric imaging of the range. The overall goal of these projects is to determine the extent of lithospheric foundering, its impact on the geology of the region, and the mechanism of removal. We report here on the P-wave tomography facet of SNEP. A companion work growing out of the same field experiment has found that the Moho geometry of the southern Sierra appears to characterize much of the range, with a deep, poorly defined and probably gradational Moho under the western foothills contrasting with a sharp, shallower Moho under the eastern Sierra (Frassetto et al., 2011).

The main goals in the tomographic component of SNEP are (1) to determine the extent of the range currently lacking the Mesozoic mafic lower crust identified by Ducea and Saleeby (1996, 1998) and (2) to determine the characteristics of the Isabella and Redding anomalies and their possible relationship to the Sierra Nevada. To that end, 84 new sites were occupied for at least a year with broadband seismometers from the FlexArray pool from EarthScope (Supplemental Table 11; Figs. 1 and 2). Seismograms from these stations and USArray Transportable Array (TA) stations in the region were picked for P-wave arrival times, which were merged with arrival times to older, more localized deployments in the southern Sierra in order to develop a broader and more uniform picture of the wavespeed variations in the lower crust and upper mantle through the Sierra and adjacent areas. A companion S-wave study is still under way.

Nearly all of the 84 locations occupied during the SNEP deployment had three-component, broadband CMG-3T seismometers recorded at 40 sps (samples per second) by Reftek R130, 24-bit, three-channel digitizer data acquisition systems. The exceptions were four 1-s CMG-40T seismometers used from September 2006 to January 2007 and a few stations set to 20 sps when access was expected to be too infrequent for flash disks to be replaced before filling. In general, one year’s worth of data was collected at each site; a few stations were occupied for two years to overcome equipment failures in an attempt to achieve this goal. Eleven additional broadband seismometers were deployed in the summer of 2007 along the Tioga Pass Road in Yosemite National Park at 5 km spacing. All SNEP data are archived with the Incorporated Research Institutions for Seismology Data Management Center (IRIS DMC) as data set XE in 2005–2007 (http://www.iris.edu/gmap/XE?timewindow=2005-2007).

The 2005–2007 SNEP deployment was supplemented with seismograms from permanent stations lying within a large polygon for the time period of May 2005–September 2007 (Figs. 1 and 2). The area of study measured ∼700 km east to west and 900 km north to south. Seismograms came from networks operated by USArray Transportable Network; Berkeley, California Institute of Technology; University of Nevada, Reno; University of Washington; University of Oregon; U.S. National Seismic Network; and the Leo Brady Network (Supplemental Table 22). The sampling rate from these stations was usually 40 sps, but a few stations were sampled at 25 sps or 50 sps. Such seismograms were resampled to 40 sps using Gary Pavlis’s dbresample routine for Antelope 4.8 (xcor 1.2) (Antelope contributed software.

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