Trace element diagram comparing the siliceous metavolcanic rocks of the Bean Canyon Hanging and the Kennedy Pendant of the Kern Plateau. Color contour map showing regional variation in collected U-Pb ages of zircons from plutonic rocks of the SNB, Salinian Block, and Mojave Desert, overlaid on a DEM.
BACKGROUND AND MOTIVATION
Abbreviations: BL, Ben Lomondberg; EF, East Fork (San Gabriel Mountains); GF, Garlock fault; GH, Gavilan Hills; GR, Gabilan series; GV, Great Valley; KCF, Kern Canyon fault; MD, Mojave Desert; MM, Montara Mountain; NF, Nacimiento error;. In other words, to what extent were the upper and lower crust connected during gravitational collapse?
THESIS OUTLINE
Chapter 4, submitted to a Sierra Nevada EarthScope Project (SNEP) volume in Geosphere, addresses issues pertaining to Late Cretaceous gravitational collapse of the southern Sierra Nevada batholith and vicinity, focusing mainly on the magnitude, direction, and timing of tectonic transport of upper crustal fragments along a regional detachment system. These data provide evidence for massive devolatilization of the schist and fluid traversal of upper plate batholithic assemblages, thereby altering the isotopic composition of overlying material and triggering a magmatic flare-up in the southernmost Sierra Nevada batholith.
GEOLOGIC MAP OF THE SAN EMIGDIO MOUNTAINS
- Introduction
- Field setting
- Previous field and analytical work
- Geologic setting and summary of key basement field relations
In terms of physiography, the San Emigdio Mountains are part of the Transverse Ranges and include units south of the San Andreas fault. Geology of the San Emigdio Mountains, California: Field Trip Guidebook – Pacific Section, Society of Economic Paleontologists and Mineralogists, 48, 1-10.
INTRODUCTION
We present a model where trench-oriented channelized extrusion of the underplated schist caused a gravitational collapse and clockwise rotation of the upper plate. We propose that channel-oriented extrusion in the Late Cretaceous underplated shale led to major extension, deep exhumation, and westward deflection of the upper plate batholith above the flowing channel.
GEOLOGIC BACKGROUND
- Rand and Sierra de Salinas schists
- Kern Canyon-proto-White Wolf fault system
- Constraints on rotation and tilting
- Rand fault and Salinas shear zone
Primary zonation and structures of the Sierra Nevada batholith (top right) from Nadin and Saleeby (2008) and Saleeby et al. This uplift of the upper plate immediately preceded underplating and cooling of the Randschist.
RESULTS
Rand Mountains
The following sections describe the geology and contact relationships between upper plate and lower plate at the Rand, Tehachapi, Sierra de Salinas, and San Emigdio localities.
Sierra de Salinas
Indicators of outcrop-scale shear sense are rare in the Sierra de Salinas, but were found in thin sections of both shale and upper plate. Photographs of microstructural features in the Sierra de Salinas (a-d) and the Rim Shale of the San Emigdio Mountains (e-h).
San Emigdio Mountains
The Tehachapi-San Emigdio complex tectonically overlies the Rand shale along the polyphase Rand fault. Compositional stratification in the shale is generally parallel to that of the Tehachapi-San Emigdio Complex and dips ∼45˚ to the south.
Tehachapi Mountains
Schiff foliation data are presented as an equal-area lower-hemisphere stereographic projection of the orientation of poles to mylonitic foliation (S1) (Kamb contours at 2σ, 4σ, 6σ, 10σ, and 14σ) (Sharry, 1981). This sense of shear is consistent with that of S-C fabrics, mica fishes, asymmetric hornblende porphyroclasts and asymmetric boudins in White Oak unit mylonites east of the half window (Wood, 1997) (Figure 2.8).
Deformation temperature, strain, and flow vorticity analysis using quartz fabrics For the case of homogeneous flow, the relative proportions of pure shear and simple shear
We measured quartz CPO from three oriented mylonitic quartzites collected at the San Emigdio Shale deposit using manual electron backscatter diffraction (EBSD) analysis (Figure 2.10). Quartz CPO from sample 08SE473 is a transition between band-type fabrics and Y-maximum-type c-axis fabrics, indicating slightly higher deformation temperatures than 06SE66 (Figure 2.10). Quartz CPOs for 12 samples of Rand shale quartzitic mylonite from different structural levels in the Rand Mountains were compiled by Postlethwaite and Jacobson (1987) and Nourse (1989) and shown in Figure 2.11.
A decrease in Rxz from the top of the shale downwards would still require a decrease in Wm away from the Edge Fault, although this scenario would imply a larger decrease than in the case of constant Rxz (Figure 2.12).
DISCUSSION
- Tectonic model
- Rotation of the southern Sierra Nevada batholith
- Schist Exhumation Mechanisms
- A comparison with Cordilleran metamorphic core complexes
- Do the schist localities preserve a subduction interface?
- Dynamics of schist exhumation
- Late Cretaceous Regional Cooling
Replacement of the sub-batholith cap lithosphere with relatively weak shale led to gravitational collapse of the thickened upper plate and regionally extensive trench-directed flow in the shale (Saleeby, 2003). It is important to reiterate here that fabrics at the base of the upper plate and in shale are generally parallel. We suggest that the flowing shale exerted traction at the base of the upper plate, driving the upper plate extension and rotation.
The consistency of schist transport to the SSW highlights the role of the schist as an active agent of tectonic rotation and large-scale extension in the upper plate.
CONCLUSIONS
Metamorphic petrology
- Tehachapi-San Emigdio complex
- San Emigdio Schist
The second unit is informally called the San Emigdio quartz diorite orthogneiss and abbreviated the San Emigdio gneiss (Figure 3.3b). The San Emigdio gneiss is structurally overlain by the Marginal Fault and consequently exhibits a highly weakened structural fabric characterized by anastomosing ductile to brittle shear zones. Photographs of structural and petrological features in metasandstone (a, b, f and g) and mafic (c, d and h) San Emigdio schist. a) Altered kyanite porphyroblast surrounded by graphitic plagioclase poikiloblasts; xpl.
Postlethwaite and Jacobson, 1987; Jacobson 1997; Kidder and Ducea, 2006; Jacobson et al., 2007), the San Emigdio Shale has abundant garnet except at the lowest structural levels.
Mineral zonation
- Analytical methods
- Schist
- Upper plate
At structural highs (< 150 m from the Rand fault) in the extreme eastern and westernmost shale exposures in the San Emigdio Mountains, garnet resorption features and diffusive relaxation of major and trace element growth zoning are common (Figs. 3.5 and 3.6). . Except at high structural levels, garnets lack "Mn kick-ups" (Kohn. a) X-ray maps and (b) zonation profiles in shale garnets. Fractured garnet whorls are compositionally identical to idioblastic garnet rims (Figures 3.5 and 3.7) and are interpreted, based on the sharpness of the interface between the compared compositions, to represent growth zonality.
Profile location in grain 1 shown in (a); grain 2 not shown. a) X-ray maps and (b) zoning profiles in garnet from upper slab sample 06SE14A.
Thermobarometry and inverted metamorphic field gradient
Calculated pressures and temperatures for San Emigdio shale and upper plate San Emigdio gneiss, using THERMOCALC v. Calculated thermal gradients increase with decreasing exposed structural thickness from east to west (Figure 3.2), suggesting that structural attenuation is at least partly responsible for 1100 ˚C / km value.
Thermodynamic modelling and P-T paths
Pseudosection shown in Figure 3.10a with contours of (a) H2O volume percent in solids, (b) garnet volume fraction, and (c-f) garnet composition expressed as mole fractions overlaid. Pseudosection shown in Figure 3.10b with contours of (a) H2O volume percent in solids, (b) garnet volume fraction, and (c-f) garnet composition expressed as mole fractions overlaid. Modeled profiles for the prograde P-T trajectory shown in Figure 3.11 (sample 06SE23) for (a) homogeneous equilibrium crystallization, (b) 10%.
The retrograde pathways shown in Figure 3.15 involve the formation of clinozoisite/epidote and/or chlorite, which require the presence of H2O to occur.
Diffusion modelling of broken garnet
Along these retrograde paths, each sample encounters contours of increasing H2O content, implying that the assemblage becomes fluid-absent, unless fluid is available along grain boundaries or externally derived (Guiraud et al., 2001) (Figure 3.14). We exploit the diffusional annealing of the hypothetically originally sharp assemblage steps in fractured garnets of sample 06SE23 to estimate the amount of time required for their relaxation. Garnet endmember compositions reach plateau values adjacent to the diffusion zone in both grains (Figure 3.7).
Inter Diffusion coefficients were calculated for each profile using thermo barometric constraints on sample 06SE23 (Table 3.1) at P = 10 kbar and Tch ≈ 0.97 Tpeak = 645 ˚C.
Inverted metamorphism
Ducea et al. rapid en masse subduction of cold schist beneath a hot upper plate (i.e. the 'hot iron effect'; Peacock, 1992), and 3) tectonic underplating and progressive cooling beneath an initially hot upper plate after the onset of shallow subduction ( Peacock, 1992; Kidder and Ducea, 2006). This could be accomplished by the cessation or slowing of subduction, for which there is no evidence, or by the relocalization of the megathrust below the shear-top plate contact. Kidder and Ducea (2006) argue, based primarily on evidence of partial melting and ductile deformation of quartz, that the schist is unable to support shear stresses of 50 to 90 MPa required by the shear heating hypothesis. 3) The upper plate was hot, with peak temperatures of 680 to 790 ˚C in the TSE complex (Pickett and Saleeby, 1993; this study).
Kidder and Ducea (2006) claim, based on a 1D thermal model, that high temperatures in the upper levels of the shale (about 700 ˚C) can be reached in about 0.3 Myr when exposed to an upper slab of 800 ˚ c. 4) Scenario 2 requires subduction of the entire schist section as a single tectonic disk.
Metamorphic convergence between upper and lower plates
Hornblende Ar/Ar systems were affected only in pervasively mylonitized rocks along the basal halo of the upper plate. In total, we interpret the San Emigdio gneiss garnet patterns to indicate residence in the deep crust at elevated temperatures (zone 1), followed by cooling (zone 2) and warming (zone 3) of the base of the upper plate adjacent to the rising shale along the Edge Fault. Uplift, contractile deformation, and erosional denudation of the upper plate is followed by deposition and subduction of San Emigdio Schist protolith.
Gravitational collapse of the upper plate drives trench-directed channelized extrusion of the schist (Chapman et al., 2010).
P-T path
Prograde elevated stresses and strain rates and rapid cooling following peak conditions
Fracturing and deformation of clinopyroxene twins along the Salinas shear zone are cited as evidence for large (140 to 1000 MPa), quasi-instantaneous, localized stresses accompanying seismic events (Ducea et al., 2007). If large paleoseismic events are responsible for cataclastic deformation of San Emigdio Schist garnets, these events must have occurred near peak conditions, i.e. during the reversal from subduction to eduction (Chapman et al., 2010). These cooling rates are consistent with geochronological and thermochronometric studies of the shale (Grove et al., Saleeby et al., 2007; Jacobson et al., in press).
In the San Emigdio Schist, the time interval between the youngest age of zircon-U-Pb (i.e. the maximum possible depositional age) and the cooling ages of muscovite 40Ar/39Ar is < 3 Ma (Grove et al., 2003).
Evolution of the thermal structure of a newly initiated flat subduction zone
Palinspastic map showing pre-Neogene positions of San Emigdio and associated shales after Grove et al. The timing of schist deposition and metamorphism decreases systematically from northwest (San Emigdio Schist, Sierra de Salinas and Portal Ridge Schists, and Rand Schists) to central and southeastern exposures (Pelona and Orocopia Schists) (Grove et al., 2003; al ., in press) (Figure 3.17). However, there is no correlation between the San Emigdio Schist and the older (ca. 120 to 115 Ma), structurally shallow, high-grade (upper amphibolite facies) levels of the Catalina Schist (Grove et al., 2008 and references in. Jacobson et al., in press).
Partial melting in the Catalina Shale is explained by subduction of part of the Cretaceous forearc beneath the NW Peninsular Highlands batholith (Grove et al., 2008).
SUMMARY AND CONCLUSIONS
Cretaceous batholithic belts
Due to differences in impact composition, emplacement age of plutons, and geochemistry, four distinct zones are delineated in the SNB (Nadin and Saleeby, 2008 and references therein). From west to east, we define the following zones: 1) the midoritic zone, a collection of mafic assemblages dominated by quartz diorite, gabbro, and tonalite, with limited outcrop along the westernmost Sierra Nevada and extensive subculture beneath the San Joaquin Basin (May and Hewitt, 1948; Williams and Curtis, 1976; Ross, 1989; Saleeby, 2009a, 2011 and unpublished data);. Initial 87Sr/86Sr (Sri) increases systematically eastward through the Cretaceous zones from ~0.703 to ~0.709, reflecting the increasing component of the continental crust progressing from the dioritic to the granitic zone.
Block diagram illustrating the petrological, isotopic, and age zoning of the SNB and the distribution of Paleozoic wall rock terranes and joints of Lower Mesozoic sedimentary and volcanic protolith sequences (the "sequence of kings") immediately prior to Late Cretaceous extensional collapse and SSD activity.
Pre-Cretaceous plutons and metamorphic framework