Technical Report: Refining Estimates of the Seismic Properties and Geologic Framework of the Mojave Desert, Southern California

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Technical Report: Refining estimates of the seismic properties and geologic framework of the Mojave Desert, southern California

Emma Schneider and Alan D. Chapman Macalester College, St. Paul, MN, USA

Introduction and Objectives A thorough understanding of the composition, seismic properties, and rheology of the crust of the Mojave Desert and adjacent areas is essential to proper modeling of earthquake hazards in southern California and hence, to SCEC. While high quality seismic data from the Los Angeles Region Seismic Experiment (LARSE lines I and II), the Consortium for Continental Reflection Profiling (COCORP), and the California Consortium for Crustal Studies (CALCRUST) are available (e.g., Cheadle et al., 1986; Malin et al., 1995; Lutter et al., 1995), the data are not yet satisfactorily correlated with representative rock units of the Mojave Desert crust. As a result, the geologic framework of the Mojave Desert region, upon which several areas of ongoing and future SCEC research depend, remains incompletely understood. This work addresses this issue through characterization of the physical properties (density, compressional and shear wave velocities, seismic anisotropy, and rock elasticity tensors) of rocks representative of the upper-to- lower-crust of the Mojave Desert (namely the Rand and related San Emigdio, Tehachapi, and Portal Ridge schists) through laboratory measurements at elevated pressures and calculations based on modal mineralogy. The purpose of this work is: 1) to “ground truth” existing LARSE, CALCRUST, and COCORP data with seismic data from rock specimens, with the aim of refining our knowledge of the structure and composition of Mojave Desert crust and earthquake model input parameters and 2) through comparison of existing passive imaging results for crustal anisotropy (e.g., Tape et al., 2009; Porter et al., 2011; Schulte-Pelkum et al., 2017) with new measurements of anisotropy magnitude and symmetry, we investigate the role of rheological anisotropy in deformation patterns observed in the Mojave Desert crust Here we present new seismic data conducted on 12 rock samples of the Rand-type schist collected in the Mojave Desert and San Emigdio Mountains including both felsic and mafic schists. Using these data we suggest a new interpretation of the LARSE seismic profile (Fuis et al., 1996; Pellerin and Christensen, 1998) and what lies underneath the Mojave Desert.

Background The western Mojave Desert is underlain by granitoids of the formerly contiguous Cretaceous Sierra Nevada- Peninsular Ranges batholith and diminishing amounts of Precambrian metamorphic rocks and post-batholithic sedimentary and volcanic assemblages (Figure 1, e.g., Dibblee, 1967). Much of the Mojave Desert lacks a crustal root, and instead lies in fault contact above trench assemblages that were underplated during a Late Cretaceous episode of shallow angle subduction (Hall, 1991; Jacobson et al., 1996; 2011; Grove et al., 2003; Kidder and Ducea, 2006; Chapman et al., 2010; 2013; Chapman, 2017; Sun et al., 2017) that fundamentally reshaped the geologic framework of the Mojave and preconditioned its crust to later tectonism (Fig. 1). These trench assemblages, known locally as the Pelona-Orocopia-Rand (POR) schist crop out in a series of structural windows along the fringes of the Mojave Desert, and lie structurally below thin (generally <1 km) intervals of deeply exhumed batholithic rocks along similarly thin Late Cretaceous to early Cenozoic mylonite zones (Figure 1). Is the schist a sheet or a belt? Inspection of the curvilinear map view distribution of the schist – generally cropping out along the Garlock and San Andreas faults – leads one to assert that the schist was assembled in a belt and is not regionally extensive. However, the Rand and related Portal Ridge, San Emigdio, and Tehachapi schist are imaged seismically as a series of continuous subhorizontal reflective layers (most likely formed during assembly by tectonic underplating) that is most prominent in the western Mojave Desert, dips to the north beneath the southern Sierra Nevada, and extends eastward into the central Mojave Desert (Cheadle et al. 1986; Li et al. 1992; Malin et al. 1995; Magistrale and Zhou 1996; Yan et al., 2005; Porter et al. 2011). Isotopic values from Neogene volcanic rocks and Late Cretaceous muscovite-garnet granites match those found in the schist, suggesting that Rand and related schists extend east to at least ~116° W (Miller et al. 1996, 2000). Pellerin and Christensen (1998) led a pioneering rock physical study in which the compositional structure of the San Gabriel Mountains and Mojave Desert are inferred by comparing seismic data from LARSE I to seismic properties of rock samples collected along the seismic line. The main conclusion of this work is that the composition of the upper crust varies across the San Andreas fault from isotropic and gneissose granitoids and Pelona schist southwest of the fault to gneisses northeast of the fault. The Pelona schist is inferred to be absent beneath the Mojave Desert since measured seismic velocities from the Pelona schist are ~200 m/s lower than LARSE velocity determinations. A key difference between Rand and Pelona schists is that the Rand includes a higher proportion of metabasalt blocks, contains a higher modal abundance of garnet, and exhibits more strongly developed foliation and lineation, than the Pelona (Kidder and Ducea, 2006; Chapman et al., 2011). Hence, Rand- type schists should exhibit higher Vp, Vs, and seismic anisotropy than the Pelona variety. We hypothesize that the observed velocity structure of the Mojave Desert is best explained by the presence of Rand-type (i.e., higher velocity and anisotropy) schists. The field- and laboratory-based approach outlined below is aimed at testing the above hypothesis through characterization of the seismic properties of samples collected adjacent to the CALCRUST and COCORP traverses and LARSE lines I and II.

Figure 1. Geologic map of southern California basement rocks based on USGS Open-File Report 2005-1305 (Ludington et al., 2007) and modified after Luffi et al. (2009) and Chapman et al. (2011), showing location of Los Angeles Region Seismic Experiment (LARSE) lines I and II, as well as CALCRUST and COCORP profiles in red. Abbreviations: CA, California; EF, East Fork (San Gabriel Mountains); GF, Garlock fault; KCF, Kern Canyon fault; LA, Los Angeles; NV, Nevada; O, Orocopia Mountains; PRB, Peninsular Ranges batholith; PR, Portal Ridge; R, Rand Mountains; SAF, San Andreas fault; SE, San Emigdio Mountains; SC, Santa Catalina Island; SNB, Sierra Nevada batholith; SP, Sierra Pelona; TC, Tehachapi Mountains; WWF, White Wolf fault. Methodology Methods were modeled after Pellerin and Christensen (1998). Oriented samples were collected from regions in the Mojave Desert specified in Table 1 and cut using facilities at the California Institute of Technology. Hand samples and billets for thin sections were obtained. Three perpendicular minicores (x, y, z), 2.5cm in diameter and 2.7-7.2cm in length, were cored from each sample. In anisotropic samples, x- and y-cores were cut parallel to foliation and perpendicular to one another (Fig. 2). If lineation is present, the axes of x-cores are parallel to the strike. The axes of z-cores are oriented normal to foliation. Hand samples and thin sections were obtained and examined to determine approximate mineral composition and strength of foliation.

Figure 2. Model of mutually perpendicular cores taken with respect to foliation and lineation. The x-direction is parallel to foliation and lineation, the y-direction is perpendicular to lineation and parallel to foliation, and the z- direction is perpendicular to foliation. The axis of each core is parallel to one of these directions and takes its name from this, such as the X-Core with an axis in the x-direction. Compressional waves travel along this axis. Shear waves travel perpendicular to this axis in the other two directions, such as Vs1 in the z-direction and Vs2 in the y- direction in the X-Core.

Physical properties of the samples were measured at the Rock Physics Laboratories of the Earthquake Science Center at the USGS campus in Menlo Park, California. Cores were trimmed and ground until the ends were parallel, then measured and weighed to determine length, density, and porosity. Cores were prepared for velocity measurements by jacketing the sample in three layers of heat-shrink tubing secured to aluminum heads with metal clamps in order to isolate the sample from the confining fluid of kerosene while under pressure. The aluminum heads contained an assemblage of transducers to generate compressional and shear wave pulses on one end and receive the pulses on the other end. Special care was taken to orient the shear wave transducers at a 90° angle to one another and to align one shear wave (S2) parallel to foliation when present. The apparatus was first tested without a sample to measure the baseline transmission time between the aluminum heads at various pressures (10, 50, 100, 150, 200 MPa). A trendline was fit to these points and used to back calculate a baseline transmission time for each pressure. Transmission times of each wave through each sample were recorded at pressures of 20, 50, 100, 150, and 200 MPa and corrected using corresponding baselines. Compressional and shear wave velocities were calculated using the measured sample length and the corrected transmission times. Seismic anisotropy was determined by finding the difference between the minimum and maximum velocities as a percentage of the mean velocity for each sample. Velocity measurements were taken at confining pressures up to 200 MPa to determine and mitigate the influence of microcracks on determined velocities. The LARSE velocity model loses resolution below 6 km depth (Lutter et al., 1995) corresponding with a confining pressure of around 170 MPa, so we will examine seismic properties at 150 MPa to correlate our data effectively with LARSE data and data from Pellerin and Christensen (1998) and to greatly reduce the effects of microcracks. Table 1 lists the velocity, density, and anisotropy of each minicore at 150 MPa.

Results and Interpretations Results are reported here at 150 MPa and for the following analysis to mitigate the influence of microcracks and to allow for direct comparison with data from LARSE and Pellerin and Christensen (1998). Measured compressional velocities range from 4.36 - 6.69 km/s. Mafic and felsic samples yield average compressional velocities of 6.16 and 5.82 km/s, respectively. Shear wave velocities range from 2.50 - 4.12 km/s and show little variation in average shear wave velocity between mafic and felsic samples, measured at 3.51 km/s and 3.59 km/s respectively. These results yield Poisson's ratios ranging from 0.066 - 0.305. Mafic samples have an average Poisson’s ratio of 0.255 while felsic samples have an average ratio of 0.180. Compressional anisotropy ranged from 0.8-18.9% with an average of 16.4% for mafic samples and much lower 6.6% average for felsic samples. Shear wave anisotropy ranged from 7.7 - 30.3% with similar averages of 14.9% and 14.6% for mafic and felsic samples, respectively. Our results overlap Pelona-Orocopia Schist data as determined by Pellerin and Christensen (1998). Values from the Rand Schist fell in a similar range to those reported for the Pelona-Orocopia Schist. Similar patterns were noted in data from each, including increasing density corresponding with increasing velocity, and increasing compressional anisotropy corresponding with increasing shear wave anisotropy. Our results also follow a comparable pattern of exponential increase of velocity as pressure increases until around 100 MPa, when the relationship becomes more linear, indicating the sealing of microcracks. These similarities in data are expected as these are grouped together as the Pelona-Orocopia-Rand Schist, but also verify the validity of our methods and results.

Percent Density Vp Vs Poisson's Compressional Shear wave Orientation Mafic (kg/m3) (km/s) (km/s) Ratio (σ) Anisotropy Anisotropy x - direction 10% 2.72 6.04 3.79 y - direction 10% 2.71 5.90 3.80 z - direction 10% 2.72 5.62 3.39 Total Rand Schist 10% 2.72 5.85 3.59 0.188 7.6 14.6 Total Rand Schist 15% 2.73 5.87 3.58 0.191 8.1 14.6 Total Rand Schist 5% 2.70 5.84 3.59 0.184 7.1 14.6 Table 1. Weighted average densities, velocities, Poisson’s ratios, and seismic anisotropy at 150 MPa (a depth of ~5 km) are listed as a function of direction in which the wave traveled or percent composition.

The Rand Schist is comprised of approximately 10% mafic and 90% quartzofeldspathic schist with some locations having a proportion of mafic schist as high as ~15% (Jacobson, 1995; Chapman and Saleeby, 2012; Chapman, 2017; Dawson and Jacobson, 1989) We calculate weighted values using a composition of 10% mafic and 90% felsic schist. Weighted averages using ratios of 5% or 15% mafic schist did not differ greatly from our chosen weighted averages, as seen in Table 1, so uncertainty in the exact ratio should not affect results greatly. Weighted average velocities, densities, Poisson’s ratios, and seismic anisotropy at 150 MPa (a depth of ~5 km, assuming a crustal density of 2.8 g/cm3) are listed in Table 1. The Rand schist yields a weighted average density of 2.72 g/cm3, Poisson’s ratio of 0.188, compressional seismic anisotropy of 7.6%, and shear wave anisotropy of 14.6%. The weighted average compressional velocity is 5.85 km/s and shear velocity is 3.59 km/s. Average velocities vary greatly by wave orientation. We calculate weighted average compressional velocities of 5.62 km/s perpendicular to foliation (‘Z’ cores), 5.90 km/s perpendicular to lineation and parallel to foliation (‘Y’ cores), and 6.04 km/s parallel to foliation and lineation (‘X’ cores). Average shear waves moving perpendicular to foliation (‘z’ direction), perpendicular to lineation and parallel to foliation (‘y’ direction), and parallel to foliation and lineation (‘x’ direction) have velocities of 3.39 km/s, 3.80 km/s, and 3.79 km/s respectively. The Rand schist displays strong seismic anisotropy due to moderate to strong foliation and preferred mineral orientation, causing velocities to vary depending on the direction in which the wave travels relative to the metamorphic fabric. Compressional and shear wave velocities travel slower perpendicular to foliation and compressional waves travel the fastest parallel to foliation and lineation. Field and seismic data indicate that mineral stretching lineation in the Rand schist is oriented parallel to the LARSE line and that lineation lies approximately within the plane of foliation (Chapman, 2017). Given these orientations, we can use the compressional velocity of 6.04 km/s when interpreting the LARSE velocity model in Fig. 3. The velocity variations northeast of the San Andreas fault are best represented by the Rand Schist. Therefore, we suggest that the Rand Schist lies beneath the Mojave Desert as a sheet with fairly constant orientation of metamorphic fabric.

Figure 3. Constant-pressure model presented at 150 MPa. LARSE velocity variations represented by the bold line. Weighted lithologic averages are plotted as constant-velocity lines. Modified from Pellerin and Christensen (1998).

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