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Ten-million years of activity within the Eastern California Shear Zone from U–Pb dating of fault-zone opal
Earth and Planetary Science Letters 521 (2019) 37–45
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Earth and Planetary Science Letters www.elsevier.com/locate/epsl
Ten-million years of activity within the Eastern California Shear Zone from U–Pb dating of fault-zone opal Perach Nuriel a,b,∗ , David M. Miller c , Kevin M. Schmidt c , Matthew A. Coble a , Kate Maher a a b c
Department of Geological Sciences, Stanford University, Stanford, CA 94305, USA Geological Survey of Israel, 32 Yeshayahu Leibowitz St., Jerusalem, 9692100, Israel U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, USA
a r t i c l e
i n f o
Article history: Received 28 November 2018 Received in revised form 28 May 2019 Accepted 31 May 2019 Available online 18 June 2019 Editor: A. Yin Keywords: U–Pb SHRIMP opal fault dating Eastern California Shear Zone
a b s t r a c t Reconstructions of long-term fault activity are essential for understanding both the mechanisms controlling fault behavior and accurate earthquake hazard assessments. Increasing evidence for temporal variations in strain accumulation suggests non-uniform strain rates over a range of historic to geologic timescales. The paucity of long-term records of fault activity has limited our ability to resolve these variations. We present a method for constraining long-term fault activity based on U–Pb dating of fault-related opal from secondary fault segments within the Eastern California Shear Zone (ECSZ). The presence of sheared and breccia-cemented opaline silica within well-exposed faults at near-surface conditions suggest that opal formation is associated with high-magnitude earthquakes capable of surface rupture (>6 M). Temporal constraints from massive and sheared syntectonic opal (n = 74) on related secondary faults from this study provide new insights on the timing of fault initiation, reactivation, and longevity. The oldest dates obtained indicate that ECSZ activity commenced at or before 10 Ma. Multiple deformation events dated within a single structure on episodically deposited and sheared opal (up to six generations), demonstrate that fault reactivation occurred over 105 yr timescales (0.7–0.1 Myr). Relative probabilities of dated deformation events can be used to evaluate changes in fault activity in the past 2.5 Ma (n = 60). This analysis indicates enhanced fault activity starting at 2 Ma and peaking around 1 Ma, possibly due to fault-interactions and distribution of deformation between the ECSZ and the San Andreas Fault. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Knowledge of the early phases of brittle fault activity is restricted by available paleoseismic records, which mainly span the last 50 ka. Furthermore, for long-lived active fault systems, such as the San Andreas Fault (SAF) and the Eastern California Shear Zone (ECSZ), only the recent fault history is represented by paleoseismic records (e.g. Rockwell et al., 2000; Dolan et al., 2007; Oskin et al., 2008). Consequently, slip history of complex fault systems, including fault initiation, tectonic quiescence, and reactivation, is not well established. Seismic hazard assessments within the ECSZ rely on such knowledge for the evaluation of longterm displacement rates, identification of active faults, and changes in deformation style through time (Dokka and Travis, 1990b; Friedrich et al., 2003; Bennett et al., 2004). The spatial activity and timing of fault initiation in the ECSZ is currently debated. Schermer et al. (1996) argue for ∼10 Ma initi-
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Corresponding author. E-mail address: [email protected] (P. Nuriel).
https://doi.org/10.1016/j.epsl.2019.05.047 0012-821X/© 2019 Elsevier B.V. All rights reserved.
ation although others suggested younger onset of deformation. For example, Langenheim and Powell (2009) argue for initiation before ∼7 Ma; Oskin and Stock (2003) suggested ∼5–6 Ma, coincident with opening of the Gulf of California; and more recently, ∼3.8 Ma is suggested for the initiation of the west part of the ECSZ (Andrew and Walker, 2017). Schermer et al. (1996) established that locally slip began in the northeast sector of the ECSZ after an ∼12 Ma tuff bed was deposited. They noted that if vertical-axis rotation of blocks bounded by left-lateral faults is extrapolated from shortduration records [∼4◦ /Ma; (Pluhar et al., 1991)], the observed full rotations are consistent with ∼10 Ma fault initiation. Average slip rates for faults differ depending on assumed inception ages and variation in inception ages may inform on progressive evolution of the ECSZ. In addition, large discrepancies between geodetic and geologic strain-accumulation rates in the ECSZ (Gan et al., 2000; Oskin et al., 2007, 2008) suggest that strain rates likely varied over geological timescales. Some of the observed discrepancies may be due to earthquake clustering patterns from the past 20 ka that indicate fault-network switching behavior, whereby seismicity al-
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10 Ma) record of fault activity, including age constraints on (1) early phases of activity; (2) reactivation and longevity of specific structures; and (3) relative probability of activity of secondary faults associated with larger regional block-boundary faults. The results of this study and following studies can help to better understand fault reactivation behavior over geological timescales. 2. Geologic setting and sampling
Fig. 1. Fault-zone opal in the ECSZ. Neotectonic map of the Mojave Desert block ECSZ (https://earthquake.usgs.gov/hazards/qfaults/), showing locations of sampling sites within the Cave Mountain (CMF), Cady (CaF), Manix (MF), and Camp Rock (CRF) fault zones. Lockhart-Lenwood (LWF), Pisgah (PF), Calico (CF), and Helendale (HF) faults are also shown. Surface ruptures associated with the 1992 Landers (Unruh et al., 1994) and the 1999 Hector Mine (Treiman et al., 2002) earthquakes are shown with yellow and light blue lines, respectively. Opal sampling sites within secondary faults in the Cady, Camp Rock, Manix, and Cave Mt. faults are shown in red circles. White rectangles indicate location of Fig. 2A–D. (For interpretation of the colors in the figure(s), the reader is referred to the web version of this article.)
ternates between the ECSZ and Los Angeles regions (Dolan et al., 2007). However, resolving spatial-temporal deformation processes requires longer records of fault activity, from initiation to present. The deformation within the ECSZ is also distributed between two main strike-slip fault domains (Fig. 1). North- to northweststriking right-lateral faults (e.g., Calico, Camp Rock, Lenwood, Helendale, and Paradise faults), and east-striking, left-lateral faults (e.g., Cady, Manix and Cave Mountain faults and faults farther north and south) both indicate activity over long time periods (Dokka and Travis, 1990b; Miller et al., 2007; Miller, 2012), however, the spatial-temporal evolution of the two contrasting fault sets is not well understood. Fault-network switching of the SanAndreas and ECSZ systems, and of the different strike-slip domains within the ECSZ may thus contribute to the observed discrepancies between geodetic and geologic records. Geochronology of authigenic and syntectonic material within fault zones can potentially extend the record of activity within fault systems into the Pliocene and Miocene. This can significantly improve constraints on duration and fluctuations in fault activity and how strain is distributed spatially over geological timescales. For example, U–Th and U–Pb ages of syntectonic calcite have been recently used to constrain the activity of many fault systems around the world (Nuriel et al., 2012, 2017, 2019; Ring and Gerdes, 2016; Roberts and Walker, 2016; Hansman et al., 2018; Parrish et al., 2018). Others have used (U–Th)/He dating of synkinematic hematite to date pulses of fault activity in the southern SAF and the Wasatch fault in Utah (Ault et al., 2015; McDermott et al., 2017). Previous studies on opal-filling fractures from Yucca Mt. NV, using secondary ion mass spectrometry (SIMS) U–Pb dating, helped to constrain the timing of deformation (Neymark et al., 2002; Dublyansky et al., 2010; Paces et al., 2010). These studies demonstrated the potential of using ages of syntectonic opal deposition in order to constrain the timing of brittle fault activity. Despite the potential to obtain long-term constraints using authigenic minerals, these approaches have not yet been attempted for the ECSZ. In this study we combine microstructural observations with high spatial resolution U–Pb geochronology of fault-related opal from secondary faults in the ECSZ to provide a long-term (ca.
We sampled opaline silica from more than 20 individual outcrops along the Camp Rock (CRF), Cady (CaF), Manix (MF), and Cave Mountain (CMF) fault systems (Fig. 1). The faults investigated are both right-lateral and left-lateral strike-slip offset, and span ∼60 km west to east, encompassing much of the ECSZ (Fig. 1). Each fault in detail is composed of one to three primary strands that are part of a fault set that is enveloped by a zone of less pronounced faults as wide as 600 m (Fig. 2). These <600 m wide fault rupture zones are comparable to surface damaged zones associated with the 1992 Landers (Unruh et al., 1994) and the 1999 Hector Mine (Treiman et al., 2002) earthquakes (Fig. 1). The northernmost surface rupture from the Landers earthquake terminated just south and west of the CaF. We observe that cores of primary fault strands typically consist of complex breccia and clay gouge with no through-going opal-filled fractures that can be sampled for geochronology. Adjacent strands of secondary faults with shorter segment lengths and varying orientations, however, commonly host calcite or opal bands that are both sheared and undeformed, in many cases in complex stacked sequences. These secondary fault strands are kinematically coordinated with the primary strands as indicated by anastomosing map patterns and sub-horizontal slickenlines. In the following section we provide descriptions of the fault segments, supplementing recent mapping in the area with detailed studies of secondary faults and associated opal deposits. The Cave Mountain fault (CMF) strikes ∼east and is left lateral (Fig. 2A). It is a recently recognized fault (Miller et al., 2007) with estimated 4–6 km of total offset (Miller, 2017). Minor strands on either side of the primary strand exhibit sheared opal and calcite. Strands north of the primary strand cut sandstone thought to be middle Miocene in age (Byers, 1960) whereas strands south of the primary strand cut conglomerate of probable Pliocene age (Miller et al., 2014). The Manix fault (MF) is a left-lateral fault south of the Cave Mountain fault (Fig. 1). It branches near its east end into two or more major strands (Reheis et al., 2014) as well as into minor strands that we have mapped as splays with NNE strikes (Fig. 2B). The strand we sampled cuts two gravel units of presumed Pliocene or early Quaternary age as well as middle Quaternary deposits of Lake Manix (Reheis et al., 2014). The lowest of the units, which hosts the opal we sampled, recently yielded a tephrochronology age from ash recovered nearby. The ash is one of the Alturas sources and 4.8–5.0 Ma in age (E. Wan written commun, 2018). The Manix fault has greater than 5.2 km total offset (Miller, 2017 and references cited therein). The Camp Rock fault (CRF) is a right-lateral, northwest-striking fault, part of which ruptured during the Landers earthquake (Fig. 1; Unruh et al., 1994). It terminates to the northwest in a complex compressive left step that involves Mesozoic granite and its overlying Early Miocene strata (Fig. 2C; Dibblee, 1964; Dokka and Travis, 1990a). Younger gravels overlying the Miocene strata are folded and faulted, with main fault sets striking NNE and SE (Fig. 2C). Several of the small faults are mineralized with opal, offering opportunities to date fault materials hosted by Early Miocene rocks as well as Pliocene and early Quaternary materials. Maximum total offset on the fault is about 4.6 km (Miller, 2017).
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Fig. 2. Geologic maps of the four fault systems we sampled. (A) Cave Mountain fault area. Samples are from opal-bearing faults cutting Miocene and Pliocene materials. (B) Manix fault area near Afton Canyon. Unit Qi includes lake deposits as well as alluvium. Samples from undated early Pleistocene or Pliocene gravel. (C) Camp Rock fault area, Daggett Ridge. Samples are from Miocene and probable Pliocene deposits. (D) Cady fault area, Cady Mountains. Samples are from old gravels underlying intermediate alluvium (Qi), exposed in deep arroyos. All geologic mapping conducted by the authors except map B, Manix fault, modified from Reheis et al. (2014). Sample labels are abbreviated and pertain to individual fault systems, see Table 3 in supplementary material.
The Cady fault (CaF) is an assemblage of dominantly eaststriking left-lateral faults (Fig. 1). Quaternary surficial deposits mantle the underlying Pre-Tertiary granitic basement as well as Miocene volcanic and sedimentary rocks (Fig. 2D). The faults are flanked to the south by higher topography of the bedrock dominated Cady Mountains. One fault displays evidence of Holocene activity where young alluvial fan apexes (Qy in Fig. 2D), dated by infrared-stimulated luminescence (IRSL) to 10.7 ka, are offset by roughly 7 m from their upslope watersheds (Schmidt et al., 2012). Schmidt and Langenheim (2012) estimate a total cumulative offset of ∼6 km over a long slip history but with no direct constraint on the timing of initiation. At its eastern terminus, the CaF intersects or merges with an unnamed east-vergent, westdipping, N-NE striking thrust. Immediately east of this thrust fault lies an extensive, deeply incised, alluvial sequence composed of sediments with a clast composition consistent with a local source from Cady Mountains. This alluvial sequence is displaced by numerous discontinuous, steeply west dipping (65–85◦ ), low-strain faults ranging in structural orientation from N-NW to N-NE, situated a distance of ∼1 to >2 km east of the eastern terminus of the CaF. Abundant, conspicuous opaline precipitates display evidence of multiple-generations of syntectonic precipitation. These
same faults produced back-facing scarps evident on local topographic highs of fan surfaces and likely represent horst-like structures associated with east-vergent shortening of this alluvial fill. Deformation features in this accommodation zone are consistent with shortening associated with a right step in a broader leftlateral fault system. 3. Methods In order to resolve the ages of sub-millimeter features within the fault-filling opal we used a micron-scale spatial resolution geochronological approach. In the past decade U–Pb and U-series techniques have been successfully applied to date opaline silica (Neymark et al., 2000; Maher et al., 2007; Neymark and Paces, 2013), and amorphous silica coating associated with glacial polish (Blackburn et al., 2019) providing important temporal constraints on paleoenvironments. In this study, we apply in-situ U–Pb geochronology to opals using SHRIMP-RG (Sensitive High-Resolution Ion Microprobe – Reverse Geometry). Unlike bulk solution analyses, the method can resolve age-averaging and age-mixing problems that arise from slow-growth or complex textures at the sub-millimeter scale
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(Neymark et al., 2000; Neymark and Paces, 2000; Paces et al., 2004; Maher et al., 2007). Opal U–Th–Pb SHRIMP-RG analyses follow specific methods described elsewhere (Neymark et al., 2002; Maher et al., 2007; Neymark and Paces, 2013) with several lab specifications and modifications described below. Small centimeter-size fragments of fault-zone opal with reference materials BZVV-opal (Amelin and Back, 2006) and NIST611glass, were mounted on double-sided tape on glass slides, cast in a 25 mm diameter by 4 mm thick epoxy disc, ground and polished to a 1 micron finish. Samples were imaged with cathodoluminescence (CL) to identify different phases, inclusions, and detritus within opal phases. Sample surface was coated with ∼100 nm Au then mounts were stored under vacuum (10−7 Torr) for several hours before being moved into the source chamber of the SHRIMPRG to minimize degassing and isobaric hydride interferences. Specific settings for each session, acquisition routine, final age calculations, and data reduction of all spot analyses are given in Tables 1 to 4 in supplementary material, respectively. Secondary ions were sputtered from the target spot using O− or O− 2 primary ion beam, which was accelerated at 10 kV and had an intensity varying from 10–70 nA (Table 1). The primary ion beam spot had a diameter between 40–60 microns and a depth of 5–10 microns for the analyses performed in this study. All isotopes (see Table 2) were measured on a single ETP ® discrete-dynode electron multiplier operated in pulse counting mode and corrected for deadtime and background. Measurements were made at mass resolutions of M/M = 7,000–8,500 (10% peak height), which fully separates interfering molecular species from the isotopes of interest. Opal concentrations for U, Th, and Pb were standardized against well-characterized opal standard BZVV with reported values of 583–888 ppm U, 173–276 ppb Th, and 332–422 ppb Pb (Amelin and Back, 2006). The BZVV standards are repeatedly measured throughout the duration of the analytical sessions (after 3–5 unknown samples). The measured 238 U/208 Pb isotopic ratios of all spot analyses were corrected for instrument mass fractionation by applying a correction factor [equal to the ratio of isochron slopes (238 U/208 Pb vs. 206 Pb/208 Pb) obtained by TIMS and SHRIMP (Amelin and Back, 2006)]. Measured 234 U/238 U ratios are obtained by additional measurements with different settings to maximize measurement precision and the signal to noise ratio for 234 U (40 s integration time compared to 10 s in our routine runs). Measurements of the 234 U/238 U ratios were performed for 52 isochrons (Table 3). For all other isochrons (n = 22), the initial (234 U/238 U)i in nearby samples is used to correct ages for initial disequilibrium, assuming opal had precipitated by fluids from a similar local source. This assumption is based on stable isotopes (δ D/δ 18 O) and Rare Earth Element (REE) results of representative opal samples from this study, which indicate for the involvement of local fluids from meteoric origin and under near surface temperatures. For more details, see supplementary material. Tera-Wasserburg intercept ages (Tera and Wasserburg, 1972) are calculated based on multiple SHRIMP-RG spot analyses for each opal sample, with a minimum of 4 and maximum of 26 spot analyses. We anchored the common-lead 207 Pb/206 Pb value to 0.807 ± 0.015, calculated from the Y-intercept of 600 spot analyses in this study and close to the typical normal crustal values (0.8295; Stacey and Kramers, 1975). We corrected all ages for disequilibrium using initial (234 U/238 U)i values calculated from measured (234 U/238 U)Act and Tera-Wasserburg ages (we assumed negligible initial 230 Th since U concentrations are more than 1000 times greater than Th). Uncertainties of calculated model ages are reported as 2 standard errors (2SE). All calculations and plots are done with IsoplotR program (Vermeesch, 2018).
4. Results 4.1. Structural and microstructural results The opal occurrence is most common in late Cenozoic conglomerates (Fig. 3A–B), but also found in Early to Middle Miocene Barstow Formation (Fig. 3C; Byers, 1960). At several sites, kinematic indicators, such as horizontal slickenlines, indicate that opal precipitated in association with strike-slip faulting (Fig. 3C–D). Individual opal-bearing fault outcrops are 2 to 15 m long; exhibiting highly brecciated and sheared bands of opal (Fig. 3A–D). Opal material varies in thickness (1–150 mm) and texture and can form thin coatings, up to 10 mm thick, on fault surfaces (Fig. 3C–D). Microstructures of fault-filling material occur as a range of massive and undeformed to sheared or displaced material containing opal and various proportions of clay and other aggregates (Fig. 4A). The massive material may form layered morphology (Fig. 4B), but is mostly lacking any structure. Fragments and aggregates of massive opal are incorporated into the breccia zones of many samples (Fig. 4C–F). Observations with cathodoluminescence (CL) imaging highlight distinctly clear opal material that is found within small mm-scale syntectonic microstructures. Syntectonic structures occur as cementation of sheared sigmoidal structures (Fig. 4D–E), brecciated zones (Fig. 4A, C–E), or as sheared or striated coating layers along fault surfaces (Fig. 4F). Syntectonic opal is also found in deformed vein structures, with total displacement of several mm (Fig. 4G–H). Multiple deformation events can be detected on a single sample by cross-cut relationships or by distinct luminescence (CL) of the different opal generations (see Fig. 4D, F). These observations are important because they indicate that these phases could not form during a single event, and their ages must therefore represent distinct deformation events. More details on opal microstructures are available in supplementary material. We interpret the massive opal to form by passive infiltration of meteoric water that interacted with the host sediments (mostly silica-rich gravel units). Similar opal deposits are widespread in the western U.S. (Neymark and Paces, 2013; Maher et al., 2014; Oster et al., 2017) and in other semi-arid environments worldwide. Elevated uranium concentrations in massive opal (up to 900 ppm) can be explained by slow precipitation rates of opal, enhanced by large volume of fluid infiltration and/or evaporation processes within the deformation zone (Massey et al., 2014). Syntectonic opal appears to precipitate actively during deformation events as demonstrated by opal occurrences within microstructures such as breccia, veins, and sheared surfaces (Fig. 4A–B) and not elsewhere. During a deformation event, newly formed dilation sites (e.g., surrounding breccia fragments, dilation of fractures) are preferred paths for fluid infiltration and opal precipitation. The formation of cement-supported breccia and syn-deformation veins (displaced or sheared) further suggests temporal association between dilation (i.e. deformation) and precipitation processes. Similar microstructural observations within carbonate host units are found in the Dead Sea Fault and are attributed to the formation of pressurized solutions during sliding and enhanced friction along faults asperities (Nuriel et al., 2012, 2017). A similar mechanism of dissolution of pre-existing massive opal during deformation events may result in the formation of syntectonic opal. 4.2. U–Pb dating results Fault-related opal samples were investigated with almost 1000 spot analyses on more than 100 different opal phases (e.g. massive or syntectonic). The mean square weighted deviation (MSWD) is a measure of the scatter of the spot analyses about the TW isochrons relative to the size of the measurement uncertainties.
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Fig. 3. Examples of fault outcrops within the (A) Manix fault; (B) Camp Rock fault; (C) Cave Mt. fault; and (D) Cady fault systems; Insets show close-up of opal filling material. Kinematic indicators such as horizontal slickenlines indicate that opal precipitated in association with strike-slip faulting (dashed lines in C and D).
Disturbance due to open system behavior is thought to produce increased scatter and MSWD’s a factor of 10–100 larger than produced by reasonable initial heterogeneity (Rasbury and Cole, 2009). We therefore excluded results with high scatter yielding MSWD above 10 (n = 26). The majority of the excluded results (n = 21) are from massive-opal, opal that is not deformed or sheared and generally contains more impurities. Only 5 syntectonic opal phases had MSWD >10 that could be related to either initial heterogeneity or open-system behavior. The results of all spot analyses and Tera-Wasserburg plots are given in supplementary material. Of the 100 opal phases investigated, 74 produced reliable Tera-Wasserburg dates. The disequilibrium-corrected 206 Pb∗ /238 U dates (n = 74) range from ∼11 to 0.2 Ma with standard errors between 3.3 and 36% (2SE); most are ≤20% (see Table 3 in supplementary material). The large errors (>20%, n = 6) are due to the relatively low U and radiogenic Pb in the opal phases. The oldest and youngest opal dates measured in each fault system are 4.9 ± 0.3 Ma/0.7 ± 0.1 Ma (CaF), 11 ± 1.2 Ma/0.57 ± 0.05 Ma (CMF), 7.3 ± 0.4 Ma/0.48 ± 0.06 Ma (CRF) and 2.3 ± 0.1 Ma/0.23 ± 0.03 Ma (MF). Fig. 5 shows Tera-Wasserburg plots for the oldest dates obtained in this study for each fault system, with locations of spot analysis within opal microstructures (for all other samples see supplementary material). Periods older than 2.5 Ma may be under-represented in our dataset because of exposure limitations. For example, syntectonic opal in the Manix Fault was sampled in ∼5 Ma deposits that are younger than the presumed ∼10 Ma fault initiation. Fig. 6 shows all dates we obtained that are younger than 2.5 Ma (n = 60) for each fault system. Interpreted periods of activity for all fault systems can be defined by grouping samples with overlapping dates and uncertainties (horizontal lines in Fig. 6). Note that the group-
ing is mainly restricted by dates with the lowest uncertainties, thus our ability to define events is restricted by the uncertainty on the dates, which is 3% at best. 5. Discussion We consider all the U–Pb dates on sheared and massive opal to represent ages of shallow high-magnitude earthquake events capable of generating continuous near-surface ground rupture [M > 6]. The slow erosion rates in the area (<10 mm/ka; Bierman, 1994) and the present-day surface exposure of faults suggest a maximum exhumation of 10 meters in the past 10 Ma. Empirical relationships among magnitude and surface rupture length or maximum displacement predict that even a few-meter long surface ruptures and small-scale displacement (1–100 cm) are almost always associated with M > 5.5 earthquakes (Wells and Coppersmith, 1994). Microstructural observations suggest that opal precipitated either following the formation of surface rupture (faultfill massive opal) or during additional deformation/shearing events (deformed or sheared syntectonic opal, with total displacements of several mm). Thus, in both cases the formation of opal is likely in association with high-magnitude events accommodated on secondary faults within the ECSZ (e.g. 1992 Landers and the 1999 Hector Mine earthquakes). We cannot completely exclude the possibility of opal precipitation in association with lower-magnitude events and we therefore interpret opal dates to reflect general periods of fault activity. The exact timing of fault initiation has major implications for the role of the ECSZ in the development of the Pacific-North American plates. As much as 9–23% of the total relative motion between the two plates has been attributed to the ECSZ, if shearing initiated at ∼10–6 Ma (Dokka and Travis, 1990b). Different initi-
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Fig. 4. Microstructures within fault-filling and fault-coating opal. (A) Sample contains both fault-wall breccia and fault-filling opal. Syntectonic opal is found within breccia cement of sigmoidal structure and surrounding angular fragments; (B) Cathodoluminescence image (CL) of massive opal, in layered structure, undeformed and relatively free of impurities, precipitated closest to fault wall; (C–D–E) CL image of opal cement in 3 breccia zones with distinct luminescence, suggesting formation by different fluids and/or precipitation conditions. In D, sigmoidal structure of pre-existing massive opal; (F) CL image of a typical opal coating layer (cutting perpendicular to fault surface) showing multiple deformation features such as the formation of initial striated fault wall, brecciation and cementation (C2–C5), shearing of carbonate material (C6), and coating of sheared surface (C1); (G–H) deformed and displaced opal vein-fill with up to 10 mm right-lateral displacement.
ation times for individual faults or changes in slip-rates are not considered in most kinematic models due to limited geochronological records of slip rates and fault activity in the ECSZ. The U–Pb ages measured in this study provide important temporal constraints on early fault activity, supplementing the limited existing records (e.g. Loomis and Burbank, 1988; Oskin and Stock, 2003; Bennett et al., 2004). Overall, our results support the assertion that faulting within the Mojave Desert block of the ECSZ initiated between 10 and 6 Ma (Dokka and Travis, 1990b; MacFadden et al., 1990; Oskin and Stock, 2003). The oldest ages of 11–10 Ma are from two different sites along the Cave Mountain Fault. One site is located near the main fault (CMF-14; 10 Ma) and the other is about 130 meters north of the main strand (site CMF-19; 11 and 10.4 Ma). The results suggest that brittle deformation events along secondary faults are associated with slip on the master fault. In addition, kinematic indicators from site CMF-14 show that opal was deposited in association with horizontal slickenlines (Fig. 3C). Thus, the age of 10 Ma represents the current kinematic deformation sense along the Cave Mt. Fault. We note that all left-lateral E-W
striking faults rotate and slip in unison over a geological time scale, and therefore all were probably initiated by the 10 Ma bound we found for the Cave Mountain fault. Right lateral NW striking faults coordinated with and bracketing the left-lateral domain must also be this old. The >7 Ma inception of the Camp Rock fault is in accordance with ages for the left-lateral faults we studied, indicating that some of the right-lateral faults west of the left-lateral domain are similar in age of inception. U–Pb ages of secondary opal also provide important constraints on the ages of alluvial sediments hosting the faults. Although many of these sediments are well-mapped, they are poorly dated and displacement rates are undetermined. For example, the laterally continuous basinal sediments within fault strands east of the CaF (Fig. 2D), are cut by structures as young as 0.7 Ma, but the inclusive sedimentary package must be older than the oldest crosscutting opaline material that exhibits Pliocene ages (∼4–5 Ma). The deposits are exposed in a broad, relatively low-relief graded plain with few, local bedrock outcrops. Younger overlying deposits express laterally continuous pedogenic horizonation consis-
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Fig. 5. Representative U–Pb SHRIMP-RG dating of fault-related opal. Tera-Wasserburg plots for the oldest ages measured in this study for samples from the (A) Cady Fault (CaF12-C1a); (B) Cave Mt. Fault (CMF19-C1); (C) Camp Rock Fault (CRF2-C2); and (D) Manix Fault (MF4b-C). Calculated linear fit (shaded black line) and intercept ages with 2SE are shown. Locations of spot analyses are shown on top of BS/CL images at the bottom of each TW plot.
tent with Middle Pleistocene aged-surface stability. Some of these opaline-cored faults, including deformation zones up to 0.35 m thick, displaced Stage III Bk horizons near the present ground surface. These same faults produced back-facing topographic scarps evident as local topographic highs of fan surfaces and likely represent horst-like structures associated with east-vergent shortening. Given the age distribution that includes U–Pb ages younger than 1 Ma (Fig. 6), the accommodation zone east of the primary Cady fault may still be tectonically active. The limiting ages provided by U–Pb on opal in faults cutting undated deposits can thus place limits on meaningful tectonic variables, including slip rates and inception dates. The longevity of faults determines whether a given segment is likely to be active today. For example, the Lockhart-Lenwood fault system (Fig. 1) has many segments that have not ruptured in the past 0.7 Ma, and may indicate inactive fault segments (Herbert et al., 2014). The longevity of individual structures (i.e. the difference between the maximum and minimum ages at each site) ranges from 11 to 1.9 Myr, and reactivation can occur 6 times or more during this period (Fig. 6). In the period between 2.5 Ma to 0.5 Ma there are 10 well-defined episodes of activity, occurring
every 0.7–0.1 Myr (arrows in Fig. 6). Opal ages from this study suggest that secondary faults are long-lived features that reactivate on longer timescales compared to the earthquake cycle (Rockwell et al., 2000; Dolan et al., 2007). The measurement of U–Pb opal as a record of paleo-earthquake events is most likely not complete, and periods of activity may be under-represented in our dataset. For example, between 10–2 Ma in the CMF, and 5.7–2.3 Ma in the CRF, there are no opal ages. Favorable conditions that influence the formation and preservation of syntectonic opal, such as high fluid conditions prevailing during wet periods, or geothermal heat flow accelerating formation rate, may contribute to uncertainty in the established record of fault activity. Nevertheless, this incomplete record provides insight into brittle deformation events along secondary faults that may be associated with slip on the master fault. To provide insight into the potential distribution of timing of events, we produced a synthetic record of 6500 events of fault activity during a 2 Ma period (Fig. 7 inset). The synthetic record includes a cluster of 0.5 Ma period of high activity when fault rupture events occur every 100 yr. The remainder of the time was considered to have low activity with events every 1000 yr (his-
Fig. 6. U–Pb dates and their 2S.E. uncertainties of all dated opal samples <2.5 Ma in the Cady (CaF, n = 19), Cave Mt. (CMF, n = 14), Camp Rock (CRF, n = 14), and the Manix (MF, n = 13) fault systems. Grey intervals indicate interpreted episodes of activity, restricted by ages with the lowest uncertainties (black-filled marks).
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Fig. 7. Fault activity pattern from simulation and from opal record for the ECSZ. Inset shows histogram with synthetic record of 6500 events of fault activity during a 2 Ma period, including a cluster of high activity between 1.25–0.75 Ma (events occur every 100 yr). Curve line shows the prediction by 100 different scenarios of randomly selected events (n = 60); Histogram showing U–Pb ages of fault-zone opal from this study for all fault systems (n = 74). Data-defined probability density function is shown by the curve line. Bottom X-axis shows the U–Pb data.
togram in Fig. 7 inset). We then randomly select 60 events from those 6500 events of the synthetic record and repeat 100 times (100 scenarios). The results indicate that 99% of the scenarios will predict enhanced activity during the 1.25–0.75 Ma period (curve in Fig. 7 inset). The synthetic record experiment suggests that a partial record (<1%), could statistically provide meaningful information on fault behavior characteristics. The distribution of U–Pb ages, represented by a relative probability density plot in Fig. 7 (n = 74) suggests a period of enhanced activity in the ECSZ, starting at 2 Ma and peaking at 0.9 Ma. Measured ages younger than 0.5 Ma may be under-represented in our data because of limitations of the U–Pb dating technique (e.g. low concentrations of radiogenic lead). In more detail, in the period between 2.5 Ma to 0.5 Ma there are 10 well-defined episodes of activity (Fig. 6). This post-2 Ma period correlates with an observed decrease in slip-rates in the San Andreas Fault region (Bennett et al., 2004), supporting paleoseismic observations from the past 12 ka that indicated fault switching behavior between the San Andreas and the ECSZ (Dolan et al., 2007). 6. Conclusions In-situ U–Pb ages measured from fault-related opal provide temporal constraints on the activity of secondary faults within the ECSZ. We conclude that ECSZ activity commenced at or before 10 Ma, supporting previously published research. Multiple deformation events, dated within a single structure, demonstrate that fault longevity and reactivation occurred over 105 yr timescales. The distribution of deformation events indicates enhanced fault activity starting at 2 Ma and peaked around 1 Ma. Implementation of this method at other sites in the ECSZ, and other systems worldwide, will advance our understanding of how strain is distributed over geological timescales. Acknowledgements We thank A.K. Ault, L. Neymark, T. Rasbury, and G. Axen for constructive reviews of this paper. We thank J. Vazquez and M. J. Grove for technical help during SHRIMP-RG analyses, and Takuya Iwamura for the help with simulation and TW plots in R program. This work was supported by the U.S. National Science Foundation [EAR-1321511, 2013]. Appendix A. Supplementary material Supplementary material related to this article can be found online at https://doi.org/10.1016/j.epsl.2019.05.047.
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Oster, J.L., Kitajima, K., Valley, J.W., Rogers, B., Maher, K., 2017. An evaluation of paired δ 18 O and (234 U/238 U) 0 in opal as a tool for paleoclimate reconstruction in semi-arid environments. Chem. Geol. 449, 236–252. Paces, J.B., Neymark, L.A., Whelan, J.F., Wooden, J.L., Lund, S.P., Marshall, B.D., 2010. Limited hydrologic response to Pleistocene climate change in deep vadose zones — Yucca Mountain, Nevada. Earth Planet. Sci. Lett. 300, 287–298. Paces, J.B., Neymark, L.A., Wooden, J.L., Persing, H.M., 2004. Improved spatial resolution for U-series dating of opal at Yucca Mountain, Nevada, USA, using ion-microprobe and microdigestion methods. Geochim. Cosmochim. Acta 68, 1591–1606. Parrish, R.R., Parrish, C.M., Lasalle, S., 2018. Vein calcite dating reveals Pyrenean orogen as cause of Paleogene deformation in southern England. J. Geol. Soc. Pluhar, C.J., Kirschvink, J.L., Adams, R.W., 1991. Magnetostratigraphy and clockwise rotation of the Plio-Pleistocene Mojave River formation, central Mojave desert, California. Rasbury, E.T., Cole, J.M., 2009. Directly dating geologic events: U–Pb dating of carbonates. Rev. Geophys. 47, 3. Reheis, M.C., Redwine, J.R., Wan, E., McGeehin, J.P., Van Sistine, D.P., 2014. Surficial Geology and Stratigraphy of Pleistocene Lake Manix, San Bernardino County, California. Scientific Investigations Map SIM-3312. US Geological Survey. Ring, U., Gerdes, A., 2016. Kinematics of the Alpenrhein-Bodensee graben system in the Central Alps: Oligocene/Miocene transtension due to formation of the Western Alps arc. Tectonics 35, 1367–1391. Roberts, N.M., Walker, R.J., 2016. U-Pb geochronology of calcite-mineralized faults: absolute timing of rift-related fault events on the northeast Atlantic margin. Geology 44 (7), 531–534. Rockwell, T.K., Lindvall, S., Herzberg, M., Murbach, D., Dawson, T., Berger, G., 2000. Paleoseismology of the Johnson Valley, Kickapoo, and Homestead Valley faults: clustering of earthquakes in the Eastern California Shear Zone. Bull. Seismol. Soc. Am. 90, 1200–1236. Schermer, E.R., Luyendyk, B.P., Cisowski, S., 1996. Late Cenozoic structure and tectonics of the northern Mojave Desert. Tectonics 15, 905–932. Schmidt, K., Mahan, S., Langenheim, V., 2012. Constraining Quaternary offset of the Cady fault, Eastern California Shear Zone, southern California, with geologic mapping, luminescence dating, and geophysics. In: AGU Fall Meeting Abstracts. Schmidt, K.M., Langenheim, V.E., 2012. Quaternary offset of the Cady fault, Eastern California Shear Zone, southern California. In: Reynolds, E.R. (Ed.), Searching for the Pliocene Southern Exposure, The 2012 Desert Research Symposium. California State University Desert Studies Center. Stacey, J.S., Kramers, J.D., 1975. Approximation of terrestrial lead isotope evolution by a two-stage model. Earth Planet. Sci. Lett. 26, 207–221. Tera, F., Wasserburg, G.J., 1972. U–Th–Pb systematics in three Apollo 14 basalts and the problem of initial Pb in lunar rocks. Earth Planet. Sci. Lett. 14, 281–304. Treiman, J.A., Kendrick, K.J., Bryant, W.A., Rockwell, T.K., McGill, S.F., 2002. Primary surface rupture associated with the Mw 7.1 16 October 1999 Hector Mine earthquake, San Bernardino County, California. Bull. Seismol. Soc. Am. 92, 1171–1191. Unruh, J.R., Lettis, W.R., Sowers, J.M., 1994. Kinematic interpretation of the 1992 Landers earthquake. Bull. Seismol. Soc. Am. 84, 537–546. Vermeesch, P., 2018. IsoplotR: a free and open toolbox for geochronology. Geosci. Front. Wells, D.L., Coppersmith, K.J., 1994. New empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement. Bull. Seismol. Soc. Am. 84, 974–1002.
Contents lists available at ScienceDirect
Earth and Planetary Science Letters www.elsevier.com/locate/epsl
Ten-million years of activity within the Eastern California Shear Zone from U–Pb dating of fault-zone opal Perach Nuriel a,b,∗ , David M. Miller c , Kevin M. Schmidt c , Matthew A. Coble a , Kate Maher a a b c
Department of Geological Sciences, Stanford University, Stanford, CA 94305, USA Geological Survey of Israel, 32 Yeshayahu Leibowitz St., Jerusalem, 9692100, Israel U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, USA
a r t i c l e
i n f o
Article history: Received 28 November 2018 Received in revised form 28 May 2019 Accepted 31 May 2019 Available online 18 June 2019 Editor: A. Yin Keywords: U–Pb SHRIMP opal fault dating Eastern California Shear Zone
a b s t r a c t Reconstructions of long-term fault activity are essential for understanding both the mechanisms controlling fault behavior and accurate earthquake hazard assessments. Increasing evidence for temporal variations in strain accumulation suggests non-uniform strain rates over a range of historic to geologic timescales. The paucity of long-term records of fault activity has limited our ability to resolve these variations. We present a method for constraining long-term fault activity based on U–Pb dating of fault-related opal from secondary fault segments within the Eastern California Shear Zone (ECSZ). The presence of sheared and breccia-cemented opaline silica within well-exposed faults at near-surface conditions suggest that opal formation is associated with high-magnitude earthquakes capable of surface rupture (>6 M). Temporal constraints from massive and sheared syntectonic opal (n = 74) on related secondary faults from this study provide new insights on the timing of fault initiation, reactivation, and longevity. The oldest dates obtained indicate that ECSZ activity commenced at or before 10 Ma. Multiple deformation events dated within a single structure on episodically deposited and sheared opal (up to six generations), demonstrate that fault reactivation occurred over 105 yr timescales (0.7–0.1 Myr). Relative probabilities of dated deformation events can be used to evaluate changes in fault activity in the past 2.5 Ma (n = 60). This analysis indicates enhanced fault activity starting at 2 Ma and peaking around 1 Ma, possibly due to fault-interactions and distribution of deformation between the ECSZ and the San Andreas Fault. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Knowledge of the early phases of brittle fault activity is restricted by available paleoseismic records, which mainly span the last 50 ka. Furthermore, for long-lived active fault systems, such as the San Andreas Fault (SAF) and the Eastern California Shear Zone (ECSZ), only the recent fault history is represented by paleoseismic records (e.g. Rockwell et al., 2000; Dolan et al., 2007; Oskin et al., 2008). Consequently, slip history of complex fault systems, including fault initiation, tectonic quiescence, and reactivation, is not well established. Seismic hazard assessments within the ECSZ rely on such knowledge for the evaluation of longterm displacement rates, identification of active faults, and changes in deformation style through time (Dokka and Travis, 1990b; Friedrich et al., 2003; Bennett et al., 2004). The spatial activity and timing of fault initiation in the ECSZ is currently debated. Schermer et al. (1996) argue for ∼10 Ma initi-
*
Corresponding author. E-mail address: [email protected] (P. Nuriel).
https://doi.org/10.1016/j.epsl.2019.05.047 0012-821X/© 2019 Elsevier B.V. All rights reserved.
ation although others suggested younger onset of deformation. For example, Langenheim and Powell (2009) argue for initiation before ∼7 Ma; Oskin and Stock (2003) suggested ∼5–6 Ma, coincident with opening of the Gulf of California; and more recently, ∼3.8 Ma is suggested for the initiation of the west part of the ECSZ (Andrew and Walker, 2017). Schermer et al. (1996) established that locally slip began in the northeast sector of the ECSZ after an ∼12 Ma tuff bed was deposited. They noted that if vertical-axis rotation of blocks bounded by left-lateral faults is extrapolated from shortduration records [∼4◦ /Ma; (Pluhar et al., 1991)], the observed full rotations are consistent with ∼10 Ma fault initiation. Average slip rates for faults differ depending on assumed inception ages and variation in inception ages may inform on progressive evolution of the ECSZ. In addition, large discrepancies between geodetic and geologic strain-accumulation rates in the ECSZ (Gan et al., 2000; Oskin et al., 2007, 2008) suggest that strain rates likely varied over geological timescales. Some of the observed discrepancies may be due to earthquake clustering patterns from the past 20 ka that indicate fault-network switching behavior, whereby seismicity al-
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10 Ma) record of fault activity, including age constraints on (1) early phases of activity; (2) reactivation and longevity of specific structures; and (3) relative probability of activity of secondary faults associated with larger regional block-boundary faults. The results of this study and following studies can help to better understand fault reactivation behavior over geological timescales. 2. Geologic setting and sampling
Fig. 1. Fault-zone opal in the ECSZ. Neotectonic map of the Mojave Desert block ECSZ (https://earthquake.usgs.gov/hazards/qfaults/), showing locations of sampling sites within the Cave Mountain (CMF), Cady (CaF), Manix (MF), and Camp Rock (CRF) fault zones. Lockhart-Lenwood (LWF), Pisgah (PF), Calico (CF), and Helendale (HF) faults are also shown. Surface ruptures associated with the 1992 Landers (Unruh et al., 1994) and the 1999 Hector Mine (Treiman et al., 2002) earthquakes are shown with yellow and light blue lines, respectively. Opal sampling sites within secondary faults in the Cady, Camp Rock, Manix, and Cave Mt. faults are shown in red circles. White rectangles indicate location of Fig. 2A–D. (For interpretation of the colors in the figure(s), the reader is referred to the web version of this article.)
ternates between the ECSZ and Los Angeles regions (Dolan et al., 2007). However, resolving spatial-temporal deformation processes requires longer records of fault activity, from initiation to present. The deformation within the ECSZ is also distributed between two main strike-slip fault domains (Fig. 1). North- to northweststriking right-lateral faults (e.g., Calico, Camp Rock, Lenwood, Helendale, and Paradise faults), and east-striking, left-lateral faults (e.g., Cady, Manix and Cave Mountain faults and faults farther north and south) both indicate activity over long time periods (Dokka and Travis, 1990b; Miller et al., 2007; Miller, 2012), however, the spatial-temporal evolution of the two contrasting fault sets is not well understood. Fault-network switching of the SanAndreas and ECSZ systems, and of the different strike-slip domains within the ECSZ may thus contribute to the observed discrepancies between geodetic and geologic records. Geochronology of authigenic and syntectonic material within fault zones can potentially extend the record of activity within fault systems into the Pliocene and Miocene. This can significantly improve constraints on duration and fluctuations in fault activity and how strain is distributed spatially over geological timescales. For example, U–Th and U–Pb ages of syntectonic calcite have been recently used to constrain the activity of many fault systems around the world (Nuriel et al., 2012, 2017, 2019; Ring and Gerdes, 2016; Roberts and Walker, 2016; Hansman et al., 2018; Parrish et al., 2018). Others have used (U–Th)/He dating of synkinematic hematite to date pulses of fault activity in the southern SAF and the Wasatch fault in Utah (Ault et al., 2015; McDermott et al., 2017). Previous studies on opal-filling fractures from Yucca Mt. NV, using secondary ion mass spectrometry (SIMS) U–Pb dating, helped to constrain the timing of deformation (Neymark et al., 2002; Dublyansky et al., 2010; Paces et al., 2010). These studies demonstrated the potential of using ages of syntectonic opal deposition in order to constrain the timing of brittle fault activity. Despite the potential to obtain long-term constraints using authigenic minerals, these approaches have not yet been attempted for the ECSZ. In this study we combine microstructural observations with high spatial resolution U–Pb geochronology of fault-related opal from secondary faults in the ECSZ to provide a long-term (ca.
We sampled opaline silica from more than 20 individual outcrops along the Camp Rock (CRF), Cady (CaF), Manix (MF), and Cave Mountain (CMF) fault systems (Fig. 1). The faults investigated are both right-lateral and left-lateral strike-slip offset, and span ∼60 km west to east, encompassing much of the ECSZ (Fig. 1). Each fault in detail is composed of one to three primary strands that are part of a fault set that is enveloped by a zone of less pronounced faults as wide as 600 m (Fig. 2). These <600 m wide fault rupture zones are comparable to surface damaged zones associated with the 1992 Landers (Unruh et al., 1994) and the 1999 Hector Mine (Treiman et al., 2002) earthquakes (Fig. 1). The northernmost surface rupture from the Landers earthquake terminated just south and west of the CaF. We observe that cores of primary fault strands typically consist of complex breccia and clay gouge with no through-going opal-filled fractures that can be sampled for geochronology. Adjacent strands of secondary faults with shorter segment lengths and varying orientations, however, commonly host calcite or opal bands that are both sheared and undeformed, in many cases in complex stacked sequences. These secondary fault strands are kinematically coordinated with the primary strands as indicated by anastomosing map patterns and sub-horizontal slickenlines. In the following section we provide descriptions of the fault segments, supplementing recent mapping in the area with detailed studies of secondary faults and associated opal deposits. The Cave Mountain fault (CMF) strikes ∼east and is left lateral (Fig. 2A). It is a recently recognized fault (Miller et al., 2007) with estimated 4–6 km of total offset (Miller, 2017). Minor strands on either side of the primary strand exhibit sheared opal and calcite. Strands north of the primary strand cut sandstone thought to be middle Miocene in age (Byers, 1960) whereas strands south of the primary strand cut conglomerate of probable Pliocene age (Miller et al., 2014). The Manix fault (MF) is a left-lateral fault south of the Cave Mountain fault (Fig. 1). It branches near its east end into two or more major strands (Reheis et al., 2014) as well as into minor strands that we have mapped as splays with NNE strikes (Fig. 2B). The strand we sampled cuts two gravel units of presumed Pliocene or early Quaternary age as well as middle Quaternary deposits of Lake Manix (Reheis et al., 2014). The lowest of the units, which hosts the opal we sampled, recently yielded a tephrochronology age from ash recovered nearby. The ash is one of the Alturas sources and 4.8–5.0 Ma in age (E. Wan written commun, 2018). The Manix fault has greater than 5.2 km total offset (Miller, 2017 and references cited therein). The Camp Rock fault (CRF) is a right-lateral, northwest-striking fault, part of which ruptured during the Landers earthquake (Fig. 1; Unruh et al., 1994). It terminates to the northwest in a complex compressive left step that involves Mesozoic granite and its overlying Early Miocene strata (Fig. 2C; Dibblee, 1964; Dokka and Travis, 1990a). Younger gravels overlying the Miocene strata are folded and faulted, with main fault sets striking NNE and SE (Fig. 2C). Several of the small faults are mineralized with opal, offering opportunities to date fault materials hosted by Early Miocene rocks as well as Pliocene and early Quaternary materials. Maximum total offset on the fault is about 4.6 km (Miller, 2017).
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Fig. 2. Geologic maps of the four fault systems we sampled. (A) Cave Mountain fault area. Samples are from opal-bearing faults cutting Miocene and Pliocene materials. (B) Manix fault area near Afton Canyon. Unit Qi includes lake deposits as well as alluvium. Samples from undated early Pleistocene or Pliocene gravel. (C) Camp Rock fault area, Daggett Ridge. Samples are from Miocene and probable Pliocene deposits. (D) Cady fault area, Cady Mountains. Samples are from old gravels underlying intermediate alluvium (Qi), exposed in deep arroyos. All geologic mapping conducted by the authors except map B, Manix fault, modified from Reheis et al. (2014). Sample labels are abbreviated and pertain to individual fault systems, see Table 3 in supplementary material.
The Cady fault (CaF) is an assemblage of dominantly eaststriking left-lateral faults (Fig. 1). Quaternary surficial deposits mantle the underlying Pre-Tertiary granitic basement as well as Miocene volcanic and sedimentary rocks (Fig. 2D). The faults are flanked to the south by higher topography of the bedrock dominated Cady Mountains. One fault displays evidence of Holocene activity where young alluvial fan apexes (Qy in Fig. 2D), dated by infrared-stimulated luminescence (IRSL) to 10.7 ka, are offset by roughly 7 m from their upslope watersheds (Schmidt et al., 2012). Schmidt and Langenheim (2012) estimate a total cumulative offset of ∼6 km over a long slip history but with no direct constraint on the timing of initiation. At its eastern terminus, the CaF intersects or merges with an unnamed east-vergent, westdipping, N-NE striking thrust. Immediately east of this thrust fault lies an extensive, deeply incised, alluvial sequence composed of sediments with a clast composition consistent with a local source from Cady Mountains. This alluvial sequence is displaced by numerous discontinuous, steeply west dipping (65–85◦ ), low-strain faults ranging in structural orientation from N-NW to N-NE, situated a distance of ∼1 to >2 km east of the eastern terminus of the CaF. Abundant, conspicuous opaline precipitates display evidence of multiple-generations of syntectonic precipitation. These
same faults produced back-facing scarps evident on local topographic highs of fan surfaces and likely represent horst-like structures associated with east-vergent shortening of this alluvial fill. Deformation features in this accommodation zone are consistent with shortening associated with a right step in a broader leftlateral fault system. 3. Methods In order to resolve the ages of sub-millimeter features within the fault-filling opal we used a micron-scale spatial resolution geochronological approach. In the past decade U–Pb and U-series techniques have been successfully applied to date opaline silica (Neymark et al., 2000; Maher et al., 2007; Neymark and Paces, 2013), and amorphous silica coating associated with glacial polish (Blackburn et al., 2019) providing important temporal constraints on paleoenvironments. In this study, we apply in-situ U–Pb geochronology to opals using SHRIMP-RG (Sensitive High-Resolution Ion Microprobe – Reverse Geometry). Unlike bulk solution analyses, the method can resolve age-averaging and age-mixing problems that arise from slow-growth or complex textures at the sub-millimeter scale
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(Neymark et al., 2000; Neymark and Paces, 2000; Paces et al., 2004; Maher et al., 2007). Opal U–Th–Pb SHRIMP-RG analyses follow specific methods described elsewhere (Neymark et al., 2002; Maher et al., 2007; Neymark and Paces, 2013) with several lab specifications and modifications described below. Small centimeter-size fragments of fault-zone opal with reference materials BZVV-opal (Amelin and Back, 2006) and NIST611glass, were mounted on double-sided tape on glass slides, cast in a 25 mm diameter by 4 mm thick epoxy disc, ground and polished to a 1 micron finish. Samples were imaged with cathodoluminescence (CL) to identify different phases, inclusions, and detritus within opal phases. Sample surface was coated with ∼100 nm Au then mounts were stored under vacuum (10−7 Torr) for several hours before being moved into the source chamber of the SHRIMPRG to minimize degassing and isobaric hydride interferences. Specific settings for each session, acquisition routine, final age calculations, and data reduction of all spot analyses are given in Tables 1 to 4 in supplementary material, respectively. Secondary ions were sputtered from the target spot using O− or O− 2 primary ion beam, which was accelerated at 10 kV and had an intensity varying from 10–70 nA (Table 1). The primary ion beam spot had a diameter between 40–60 microns and a depth of 5–10 microns for the analyses performed in this study. All isotopes (see Table 2) were measured on a single ETP ® discrete-dynode electron multiplier operated in pulse counting mode and corrected for deadtime and background. Measurements were made at mass resolutions of M/M = 7,000–8,500 (10% peak height), which fully separates interfering molecular species from the isotopes of interest. Opal concentrations for U, Th, and Pb were standardized against well-characterized opal standard BZVV with reported values of 583–888 ppm U, 173–276 ppb Th, and 332–422 ppb Pb (Amelin and Back, 2006). The BZVV standards are repeatedly measured throughout the duration of the analytical sessions (after 3–5 unknown samples). The measured 238 U/208 Pb isotopic ratios of all spot analyses were corrected for instrument mass fractionation by applying a correction factor [equal to the ratio of isochron slopes (238 U/208 Pb vs. 206 Pb/208 Pb) obtained by TIMS and SHRIMP (Amelin and Back, 2006)]. Measured 234 U/238 U ratios are obtained by additional measurements with different settings to maximize measurement precision and the signal to noise ratio for 234 U (40 s integration time compared to 10 s in our routine runs). Measurements of the 234 U/238 U ratios were performed for 52 isochrons (Table 3). For all other isochrons (n = 22), the initial (234 U/238 U)i in nearby samples is used to correct ages for initial disequilibrium, assuming opal had precipitated by fluids from a similar local source. This assumption is based on stable isotopes (δ D/δ 18 O) and Rare Earth Element (REE) results of representative opal samples from this study, which indicate for the involvement of local fluids from meteoric origin and under near surface temperatures. For more details, see supplementary material. Tera-Wasserburg intercept ages (Tera and Wasserburg, 1972) are calculated based on multiple SHRIMP-RG spot analyses for each opal sample, with a minimum of 4 and maximum of 26 spot analyses. We anchored the common-lead 207 Pb/206 Pb value to 0.807 ± 0.015, calculated from the Y-intercept of 600 spot analyses in this study and close to the typical normal crustal values (0.8295; Stacey and Kramers, 1975). We corrected all ages for disequilibrium using initial (234 U/238 U)i values calculated from measured (234 U/238 U)Act and Tera-Wasserburg ages (we assumed negligible initial 230 Th since U concentrations are more than 1000 times greater than Th). Uncertainties of calculated model ages are reported as 2 standard errors (2SE). All calculations and plots are done with IsoplotR program (Vermeesch, 2018).
4. Results 4.1. Structural and microstructural results The opal occurrence is most common in late Cenozoic conglomerates (Fig. 3A–B), but also found in Early to Middle Miocene Barstow Formation (Fig. 3C; Byers, 1960). At several sites, kinematic indicators, such as horizontal slickenlines, indicate that opal precipitated in association with strike-slip faulting (Fig. 3C–D). Individual opal-bearing fault outcrops are 2 to 15 m long; exhibiting highly brecciated and sheared bands of opal (Fig. 3A–D). Opal material varies in thickness (1–150 mm) and texture and can form thin coatings, up to 10 mm thick, on fault surfaces (Fig. 3C–D). Microstructures of fault-filling material occur as a range of massive and undeformed to sheared or displaced material containing opal and various proportions of clay and other aggregates (Fig. 4A). The massive material may form layered morphology (Fig. 4B), but is mostly lacking any structure. Fragments and aggregates of massive opal are incorporated into the breccia zones of many samples (Fig. 4C–F). Observations with cathodoluminescence (CL) imaging highlight distinctly clear opal material that is found within small mm-scale syntectonic microstructures. Syntectonic structures occur as cementation of sheared sigmoidal structures (Fig. 4D–E), brecciated zones (Fig. 4A, C–E), or as sheared or striated coating layers along fault surfaces (Fig. 4F). Syntectonic opal is also found in deformed vein structures, with total displacement of several mm (Fig. 4G–H). Multiple deformation events can be detected on a single sample by cross-cut relationships or by distinct luminescence (CL) of the different opal generations (see Fig. 4D, F). These observations are important because they indicate that these phases could not form during a single event, and their ages must therefore represent distinct deformation events. More details on opal microstructures are available in supplementary material. We interpret the massive opal to form by passive infiltration of meteoric water that interacted with the host sediments (mostly silica-rich gravel units). Similar opal deposits are widespread in the western U.S. (Neymark and Paces, 2013; Maher et al., 2014; Oster et al., 2017) and in other semi-arid environments worldwide. Elevated uranium concentrations in massive opal (up to 900 ppm) can be explained by slow precipitation rates of opal, enhanced by large volume of fluid infiltration and/or evaporation processes within the deformation zone (Massey et al., 2014). Syntectonic opal appears to precipitate actively during deformation events as demonstrated by opal occurrences within microstructures such as breccia, veins, and sheared surfaces (Fig. 4A–B) and not elsewhere. During a deformation event, newly formed dilation sites (e.g., surrounding breccia fragments, dilation of fractures) are preferred paths for fluid infiltration and opal precipitation. The formation of cement-supported breccia and syn-deformation veins (displaced or sheared) further suggests temporal association between dilation (i.e. deformation) and precipitation processes. Similar microstructural observations within carbonate host units are found in the Dead Sea Fault and are attributed to the formation of pressurized solutions during sliding and enhanced friction along faults asperities (Nuriel et al., 2012, 2017). A similar mechanism of dissolution of pre-existing massive opal during deformation events may result in the formation of syntectonic opal. 4.2. U–Pb dating results Fault-related opal samples were investigated with almost 1000 spot analyses on more than 100 different opal phases (e.g. massive or syntectonic). The mean square weighted deviation (MSWD) is a measure of the scatter of the spot analyses about the TW isochrons relative to the size of the measurement uncertainties.
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Fig. 3. Examples of fault outcrops within the (A) Manix fault; (B) Camp Rock fault; (C) Cave Mt. fault; and (D) Cady fault systems; Insets show close-up of opal filling material. Kinematic indicators such as horizontal slickenlines indicate that opal precipitated in association with strike-slip faulting (dashed lines in C and D).
Disturbance due to open system behavior is thought to produce increased scatter and MSWD’s a factor of 10–100 larger than produced by reasonable initial heterogeneity (Rasbury and Cole, 2009). We therefore excluded results with high scatter yielding MSWD above 10 (n = 26). The majority of the excluded results (n = 21) are from massive-opal, opal that is not deformed or sheared and generally contains more impurities. Only 5 syntectonic opal phases had MSWD >10 that could be related to either initial heterogeneity or open-system behavior. The results of all spot analyses and Tera-Wasserburg plots are given in supplementary material. Of the 100 opal phases investigated, 74 produced reliable Tera-Wasserburg dates. The disequilibrium-corrected 206 Pb∗ /238 U dates (n = 74) range from ∼11 to 0.2 Ma with standard errors between 3.3 and 36% (2SE); most are ≤20% (see Table 3 in supplementary material). The large errors (>20%, n = 6) are due to the relatively low U and radiogenic Pb in the opal phases. The oldest and youngest opal dates measured in each fault system are 4.9 ± 0.3 Ma/0.7 ± 0.1 Ma (CaF), 11 ± 1.2 Ma/0.57 ± 0.05 Ma (CMF), 7.3 ± 0.4 Ma/0.48 ± 0.06 Ma (CRF) and 2.3 ± 0.1 Ma/0.23 ± 0.03 Ma (MF). Fig. 5 shows Tera-Wasserburg plots for the oldest dates obtained in this study for each fault system, with locations of spot analysis within opal microstructures (for all other samples see supplementary material). Periods older than 2.5 Ma may be under-represented in our dataset because of exposure limitations. For example, syntectonic opal in the Manix Fault was sampled in ∼5 Ma deposits that are younger than the presumed ∼10 Ma fault initiation. Fig. 6 shows all dates we obtained that are younger than 2.5 Ma (n = 60) for each fault system. Interpreted periods of activity for all fault systems can be defined by grouping samples with overlapping dates and uncertainties (horizontal lines in Fig. 6). Note that the group-
ing is mainly restricted by dates with the lowest uncertainties, thus our ability to define events is restricted by the uncertainty on the dates, which is 3% at best. 5. Discussion We consider all the U–Pb dates on sheared and massive opal to represent ages of shallow high-magnitude earthquake events capable of generating continuous near-surface ground rupture [M > 6]. The slow erosion rates in the area (<10 mm/ka; Bierman, 1994) and the present-day surface exposure of faults suggest a maximum exhumation of 10 meters in the past 10 Ma. Empirical relationships among magnitude and surface rupture length or maximum displacement predict that even a few-meter long surface ruptures and small-scale displacement (1–100 cm) are almost always associated with M > 5.5 earthquakes (Wells and Coppersmith, 1994). Microstructural observations suggest that opal precipitated either following the formation of surface rupture (faultfill massive opal) or during additional deformation/shearing events (deformed or sheared syntectonic opal, with total displacements of several mm). Thus, in both cases the formation of opal is likely in association with high-magnitude events accommodated on secondary faults within the ECSZ (e.g. 1992 Landers and the 1999 Hector Mine earthquakes). We cannot completely exclude the possibility of opal precipitation in association with lower-magnitude events and we therefore interpret opal dates to reflect general periods of fault activity. The exact timing of fault initiation has major implications for the role of the ECSZ in the development of the Pacific-North American plates. As much as 9–23% of the total relative motion between the two plates has been attributed to the ECSZ, if shearing initiated at ∼10–6 Ma (Dokka and Travis, 1990b). Different initi-
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Fig. 4. Microstructures within fault-filling and fault-coating opal. (A) Sample contains both fault-wall breccia and fault-filling opal. Syntectonic opal is found within breccia cement of sigmoidal structure and surrounding angular fragments; (B) Cathodoluminescence image (CL) of massive opal, in layered structure, undeformed and relatively free of impurities, precipitated closest to fault wall; (C–D–E) CL image of opal cement in 3 breccia zones with distinct luminescence, suggesting formation by different fluids and/or precipitation conditions. In D, sigmoidal structure of pre-existing massive opal; (F) CL image of a typical opal coating layer (cutting perpendicular to fault surface) showing multiple deformation features such as the formation of initial striated fault wall, brecciation and cementation (C2–C5), shearing of carbonate material (C6), and coating of sheared surface (C1); (G–H) deformed and displaced opal vein-fill with up to 10 mm right-lateral displacement.
ation times for individual faults or changes in slip-rates are not considered in most kinematic models due to limited geochronological records of slip rates and fault activity in the ECSZ. The U–Pb ages measured in this study provide important temporal constraints on early fault activity, supplementing the limited existing records (e.g. Loomis and Burbank, 1988; Oskin and Stock, 2003; Bennett et al., 2004). Overall, our results support the assertion that faulting within the Mojave Desert block of the ECSZ initiated between 10 and 6 Ma (Dokka and Travis, 1990b; MacFadden et al., 1990; Oskin and Stock, 2003). The oldest ages of 11–10 Ma are from two different sites along the Cave Mountain Fault. One site is located near the main fault (CMF-14; 10 Ma) and the other is about 130 meters north of the main strand (site CMF-19; 11 and 10.4 Ma). The results suggest that brittle deformation events along secondary faults are associated with slip on the master fault. In addition, kinematic indicators from site CMF-14 show that opal was deposited in association with horizontal slickenlines (Fig. 3C). Thus, the age of 10 Ma represents the current kinematic deformation sense along the Cave Mt. Fault. We note that all left-lateral E-W
striking faults rotate and slip in unison over a geological time scale, and therefore all were probably initiated by the 10 Ma bound we found for the Cave Mountain fault. Right lateral NW striking faults coordinated with and bracketing the left-lateral domain must also be this old. The >7 Ma inception of the Camp Rock fault is in accordance with ages for the left-lateral faults we studied, indicating that some of the right-lateral faults west of the left-lateral domain are similar in age of inception. U–Pb ages of secondary opal also provide important constraints on the ages of alluvial sediments hosting the faults. Although many of these sediments are well-mapped, they are poorly dated and displacement rates are undetermined. For example, the laterally continuous basinal sediments within fault strands east of the CaF (Fig. 2D), are cut by structures as young as 0.7 Ma, but the inclusive sedimentary package must be older than the oldest crosscutting opaline material that exhibits Pliocene ages (∼4–5 Ma). The deposits are exposed in a broad, relatively low-relief graded plain with few, local bedrock outcrops. Younger overlying deposits express laterally continuous pedogenic horizonation consis-
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Fig. 5. Representative U–Pb SHRIMP-RG dating of fault-related opal. Tera-Wasserburg plots for the oldest ages measured in this study for samples from the (A) Cady Fault (CaF12-C1a); (B) Cave Mt. Fault (CMF19-C1); (C) Camp Rock Fault (CRF2-C2); and (D) Manix Fault (MF4b-C). Calculated linear fit (shaded black line) and intercept ages with 2SE are shown. Locations of spot analyses are shown on top of BS/CL images at the bottom of each TW plot.
tent with Middle Pleistocene aged-surface stability. Some of these opaline-cored faults, including deformation zones up to 0.35 m thick, displaced Stage III Bk horizons near the present ground surface. These same faults produced back-facing topographic scarps evident as local topographic highs of fan surfaces and likely represent horst-like structures associated with east-vergent shortening. Given the age distribution that includes U–Pb ages younger than 1 Ma (Fig. 6), the accommodation zone east of the primary Cady fault may still be tectonically active. The limiting ages provided by U–Pb on opal in faults cutting undated deposits can thus place limits on meaningful tectonic variables, including slip rates and inception dates. The longevity of faults determines whether a given segment is likely to be active today. For example, the Lockhart-Lenwood fault system (Fig. 1) has many segments that have not ruptured in the past 0.7 Ma, and may indicate inactive fault segments (Herbert et al., 2014). The longevity of individual structures (i.e. the difference between the maximum and minimum ages at each site) ranges from 11 to 1.9 Myr, and reactivation can occur 6 times or more during this period (Fig. 6). In the period between 2.5 Ma to 0.5 Ma there are 10 well-defined episodes of activity, occurring
every 0.7–0.1 Myr (arrows in Fig. 6). Opal ages from this study suggest that secondary faults are long-lived features that reactivate on longer timescales compared to the earthquake cycle (Rockwell et al., 2000; Dolan et al., 2007). The measurement of U–Pb opal as a record of paleo-earthquake events is most likely not complete, and periods of activity may be under-represented in our dataset. For example, between 10–2 Ma in the CMF, and 5.7–2.3 Ma in the CRF, there are no opal ages. Favorable conditions that influence the formation and preservation of syntectonic opal, such as high fluid conditions prevailing during wet periods, or geothermal heat flow accelerating formation rate, may contribute to uncertainty in the established record of fault activity. Nevertheless, this incomplete record provides insight into brittle deformation events along secondary faults that may be associated with slip on the master fault. To provide insight into the potential distribution of timing of events, we produced a synthetic record of 6500 events of fault activity during a 2 Ma period (Fig. 7 inset). The synthetic record includes a cluster of 0.5 Ma period of high activity when fault rupture events occur every 100 yr. The remainder of the time was considered to have low activity with events every 1000 yr (his-
Fig. 6. U–Pb dates and their 2S.E. uncertainties of all dated opal samples <2.5 Ma in the Cady (CaF, n = 19), Cave Mt. (CMF, n = 14), Camp Rock (CRF, n = 14), and the Manix (MF, n = 13) fault systems. Grey intervals indicate interpreted episodes of activity, restricted by ages with the lowest uncertainties (black-filled marks).
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Fig. 7. Fault activity pattern from simulation and from opal record for the ECSZ. Inset shows histogram with synthetic record of 6500 events of fault activity during a 2 Ma period, including a cluster of high activity between 1.25–0.75 Ma (events occur every 100 yr). Curve line shows the prediction by 100 different scenarios of randomly selected events (n = 60); Histogram showing U–Pb ages of fault-zone opal from this study for all fault systems (n = 74). Data-defined probability density function is shown by the curve line. Bottom X-axis shows the U–Pb data.
togram in Fig. 7 inset). We then randomly select 60 events from those 6500 events of the synthetic record and repeat 100 times (100 scenarios). The results indicate that 99% of the scenarios will predict enhanced activity during the 1.25–0.75 Ma period (curve in Fig. 7 inset). The synthetic record experiment suggests that a partial record (<1%), could statistically provide meaningful information on fault behavior characteristics. The distribution of U–Pb ages, represented by a relative probability density plot in Fig. 7 (n = 74) suggests a period of enhanced activity in the ECSZ, starting at 2 Ma and peaking at 0.9 Ma. Measured ages younger than 0.5 Ma may be under-represented in our data because of limitations of the U–Pb dating technique (e.g. low concentrations of radiogenic lead). In more detail, in the period between 2.5 Ma to 0.5 Ma there are 10 well-defined episodes of activity (Fig. 6). This post-2 Ma period correlates with an observed decrease in slip-rates in the San Andreas Fault region (Bennett et al., 2004), supporting paleoseismic observations from the past 12 ka that indicated fault switching behavior between the San Andreas and the ECSZ (Dolan et al., 2007). 6. Conclusions In-situ U–Pb ages measured from fault-related opal provide temporal constraints on the activity of secondary faults within the ECSZ. We conclude that ECSZ activity commenced at or before 10 Ma, supporting previously published research. Multiple deformation events, dated within a single structure, demonstrate that fault longevity and reactivation occurred over 105 yr timescales. The distribution of deformation events indicates enhanced fault activity starting at 2 Ma and peaked around 1 Ma. Implementation of this method at other sites in the ECSZ, and other systems worldwide, will advance our understanding of how strain is distributed over geological timescales. Acknowledgements We thank A.K. Ault, L. Neymark, T. Rasbury, and G. Axen for constructive reviews of this paper. We thank J. Vazquez and M. J. Grove for technical help during SHRIMP-RG analyses, and Takuya Iwamura for the help with simulation and TW plots in R program. This work was supported by the U.S. National Science Foundation [EAR-1321511, 2013]. Appendix A. Supplementary material Supplementary material related to this article can be found online at https://doi.org/10.1016/j.epsl.2019.05.047.
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