skip to main content


The NSF Public Access Repository (NSF-PAR) system and access will be unavailable from 11:00 PM ET on Friday, April 12 until 2:00 AM ET on Saturday, April 13 due to maintenance. We apologize for the inconvenience.

The topographic development of the Sierra Nevada, CA has been the topic of research for more than 100 years, yet disagreement remains as to whether 1) the Sierra Nevada records uplift in the late Mesozoic followed by no change or a decrease in elevation throughout the Cenozoic vs 2) uplift in the late Mesozoic followed by a decrease in elevation during the middle Cenozoic, and a second pulse of uplift in the late Cenozoic. The second pulse of uplift in the late Cenozoic is linked to late Cenozoic normal slip along the southern Sierra Nevada (SSN) range front normal fault (SSNF). To test this fault slip hypothesis, we report apatite (U-Th/He) (AHe) results from samples in the footwall of the SSNF collected along three vertical transects (from north to south, RV, MW, and MU) up the eastern escarpment of the SSN. Here, exposed bedrock fault planes and associated joints yield nearly identical strike-dip values of ~356°-69°NE. At the RV transect, 14 AHe samples record an elevation invariant mean age of 17.8 ± 5.3 Ma over a vertical distance of 802 m. At MW, 14 samples collected over a vertical distance of 1043 m yield an elevation invariant mean age of 26.6 ± 5.0 Ma. At MU, 8 samples record an elevation invariant mean age of 12.7 ± 3.7 Ma over a vertical distance of 501 m and 5 higher elevation samples record an elevation invariant mean age of 26.5 ± 3.3 Ma. At MU, the lowest elevation sample yielded an AFT age of 50 Ma and mean track length of 13.1 microns. Preliminary HeFTy modeling of the AHe and AFT ages from this sample yield accelerated cooling at ~22 Ma and ~10 Ma. Preliminary modeling (Pecube + landscape evolution) of the MU AHe results, elevation, and a prominent knickpoint yield an increase in fault slip rate at ~1-2 Ma. We interpret the elevation invariant ages and modeling results as indicating three periods—late Oligocene, middle Miocene, and Pliocene—of cooling and exhumation in the footwall of the SSNF due to normal fault slip. Our results are the first to document late Oligocene to Pliocene cooling and normal slip along the SSNF. Miocene and Pliocene age normal fault slip along the SSNF is contemporaneous with normal slip along range bounding faults across the Basin and Range, including the adjacent Inyo and White Mountains. Combined, these data indicate that since the late Oligocene the SSN defined the stable western limit of the Basin and Range.  more » « less
Award ID(s):
Author(s) / Creator(s):
; ; ;
Date Published:
Journal Name:
Geological Society of America Abstracts with Programs
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. The uplift history of the Sierra Nevada, California, is a topic of long-standing disagreement with much of it centered on the timing and nature of slip along the range-bounding normal fault along the east flank of the southern Sierra Nevada. The history of normal fault slip is important for characterizing the uplift history of the Sierra Nevada, as well as for characterizing the geologic and geodynamic factors that drove, and continue to drive, normal faulting. To address these issues, we completed new structural studies and extensive apatite (U-Th)/He (AHe) thermochronometry on samples collected from three vertical transects in the footwall to the east-dipping southern Sierra Nevada normal fault (SNNF). Our structural studies on bedrock fault planes show that the SNNF is a steeply (~70°) east-dipping normal fault. The new AHe data reveal two elevation-invariant AHe age arrays, indicative of two distinct periods of cooling and exhumation, which we interpret as initiation of normal faulting along the SNNF at ca. 28–27 Ma with a second phase of normal faulting at ca. 17–13 Ma. We argue that beginning in the late Oligocene, the SNNF marked the now long-standing stable western limit, or break-away zone, of the Basin and Range. Slip along SNNF, and the associated unloading of the footwall, likely resulted in two periods of uplift of Sierra Nevada during the late Cenozoic. Trench retreat, driven by westward motion of the North American plate, along the Farallon–North American subduction zone boundary, as well as the gravitationally unstable northern and southern Basin and Range pushing on the cold Sierra Nevada, likely drove the late Oligocene- aged normal slip along the SNNF and the similar-aged but generally local and minor extension within the Basin and Range. We posit that the thick proto–Basin and Range lithosphere was primed for late Oligocene extension by replacement of the steepening Farallon slab with hot and buoyant asthenosphere. While steepening of the Farallon slab had not yet reached the southern Sierra Nevada by late Oligocene time, we speculate that late Oligocene slip along the SNNF reactivated a late Cretaceous dextral shear zone as the Sierra Nevada block was pulled and pushed westward in response to trench retreat and gravitational potential energy. The dominant middle Miocene normal fault-slip history along the SNNF is contemporaneous with high-magnitude slip recorded along range-bounding normal faults across the Basin and Range, including the east-adjacent Inyo and White mountains, indicating that this period of extension was a major regional tectonic event. We infer that a combination of slab-driven trench retreat along the Juan de Fuca–North America subduction zone boundary and clockwise rotation of the southern ancestral Cascade Range superimposed on continental lithosphere pre-conditioned for extension drove this episode of middle Miocene normal slip along the SNNF and extension to the east across the Basin and Range. Transtensional plate motion along the Pacific–North America plate boundary, and likely a growing slab window, continued to drive extension along the SNNF and the western Basin and Range, but not until ca. 11 Ma when the Mendocino triple junction reached the latitude of our northernmost (U-Th)/He transect. 
    more » « less
  2. We interpret the kinematics of the Tangra Yumco (TYC) rift by evaluating spatiotemporal trends in fault displacement, extension onset, and exhumation rates. We present new geologic mapping, U-Pb geochronology, zircon (U-Th)/He (ZHe) thermochronology, and HeFTy thermal modeling results that are critical to testing dynamic models of extension in Tibet. The TYC rift is bounded by two NNE striking (~N10°E-N35°E) high angle (~45-70°) active normal faults that alternate dominance along strike. Footwall granodiorites show foliation, slip lineation, and fault plane striation measurements indicative of northeast directed oblique sinistral-normal slip. In North and South TYC, hanging wall deposits are cut by a series of active high-angle normal faults which likely sole into a master fault at depth, while in central TYC, hanging wall deposits display synthetic graben structures potentially indicative of low-angle faulting. Analysis of ~50 samples collected across key structural relationships in and around TYC yield 14 mean U-Pb dates between ~59-49 Ma and ~190 single-grain ZHe dates between ~60-4 Ma with spatial trends in ZHe data correlating strongly with latitude. Samples from Gangdese latitudes show a concentration of ~28-15 Ma ages, while those north of ~29.8° latitude yield both younger (~9-4 Ma) and older (~59-45 Ma) ages. We interpret (1) Gangdese Range samples reflect exhumation during contraction and uplift along the GCT peaking at ~21-20 Ma, (2) ~9-4 Ma ages reveal extension timing along fault segments experiencing significant rift-related exhumation, and (3) ~59-45 Ma ages represent un-reset or partially-reset samples from fault segments that have experienced lesser magnitudes of rift exhumation. HeFTy thermal models indicate a two-stage cooling history with initial slow cooling followed by accelerated cooling rates in Late Miocene-Pliocene time (~13-4 Ma) consistent with prior results from TYC and other Tibetan rifts. Our data are consistent with a segment linkage fault evolution model for the TYC rift, with underthrusting of Indian lithosphere likely related to the northward acceleration of rifting. Future work will utilize advanced HeFTy modeling including U-Pb and apatite fission track data to further constrain the exhumation history of TYC and test dynamic models of extension for southern Tibet. 
    more » « less
  3. Late Cenozoic evolution of the Baja California (BC) peninsula governs its species diversity, with changes to terrestrial habitats and shorelines driven by volcanic and tectonic processes. New geologic mapping and geochronology in central BC help assess if recent landscape evolution created a barrier to gene flow. The NW-trending topographic divide of the BC peninsula near San Ignacio-Santa Rosalia (27.4N) is a low (400500 m asl), broad (2030 km-wide) pass. At the pass, ~2022-Ma volcaniclastic strata, mafic lavas, fluvial conglomerate, cross-bedded eolian sandstone, and a felsic tuff dip ~515 SW. Similar lithology and chronology suggest these strata correlate to the lower Comondu Group (CG). They are overlain by middle Miocene (~1114 Ma) mafic lavas with similar SW dips that overlap in age with the upper CG. NW of the pass, upper Miocene (~9.511 Ma) post-CG volcaniclastic strata and mafic lava flows are exposed in the Sierra San Francisco and dip ~10 SE on its SE flank, inclined differently than older SW-dipping CG at the pass. The basalt of Esperanza (~10 Ma) unconformably overlies the CG at and west of the pass. Its ~1 regional dip suggests that ~515 of SW tilting occurred prior to ~10 Ma in the footwall of the NW-striking Campamento fault, located at the base of the ~150 m-high rift escarpment. The N-striking Arroyo Yaqui fault, ~10 km E of the Campamento fault in a low-relief region capped by Quaternary marine strata, exposes crystalline basement in its footwall and may be a major rift margin structure. Thus the location, orientation, and age of the divide may be controlled by rift-related faulting and tilting plus beveling and lateral retreat of the escarpment. Pliocene tidal sediments occur up to ~200 m asl ~20 km west of the low pass similar to Pliocene marine strata east of the pass at ~300 m asl, indicating late Miocene to Pliocene subsidence was followed by >200 m of post-4 Ma uplift. Uplift was likely driven by transtensional faulting and possibly magmatic inflation by ~7090 km-wavelength domes. Further mapping will constrain the timing of vertical crustal motions and test whether the tidal embayment crossed the peninsula through this low pass, isolated species, and prevented terrestrial gene flow. Integration of geologic and genetic data will determine how volcano- tectonic processes shaped genetic diversity. 
    more » « less
  4. Abstract Tectonic interpretation of the central Sierra Nevada—whether the crest of the Sierra Nevada (California, USA) was uplifted in the late Cenozoic or whether the range has undergone continuous down-wearing since the Late Cretaceous—is controversial, since there is no obvious tectonic explanation for renewed uplift. The strongest direct evidence for late Cenozoic uplift of the central Sierra Nevada comes from study of the Trachyandesite of Kennedy Table, which followed the course of the Miocene San Joaquin River but has a steeper gradient than the modern river. Early workers attributed this steeper gradient to tilting of the Sierra Nevada block since the late Miocene, resulting in 2 km of range-crest uplift. However, this interpretation has been contested on grounds that the Miocene river gradient had to be assumed and that the Sierran Batholith could have warped during tilting, thus failing to uplift the range crest. The objective of this study was to obtain quantitative data that test these criticisms. The Trachyandesite of Kennedy Table is a chain of 33 remnants of a single lava flow as thick as 65 m, preserved for 21 km from Squaw Leap to Little Dry Creek, close to the modern San Joaquin River in the foothills of the Sierra Nevada. Several remnants lie on fluvial gravel of the late Miocene San Joaquin River. Early workers speculated that the lava concealed its own (unrecognized) vent, but in 2011, we identified the vent on the Middle Fork of the San Joaquin River, 13.5 km south of Deadman Pass and 70 km northeast of Kennedy Table. The vent complex intrudes Cretaceous granite, has 285 m relief, and is an intricately jointed intrusion that grades up into a glassy lava flow. Composition (58% SiO2) and 40Ar/39Ar age (9.3 Ma) are identical at the vent and downstream. Basal elevations of remnants were recorded, and the present-day basal gradients of several were adjusted for apparent dip and projected along a vertical plane at 220° (the estimated tilt azimuth). The basal gradients are far steeper than that of the modern river, but they differ slightly from reach to reach and are thus inconsistent measures of the post-Miocene tilt. Likewise, relief eroded atop most remnants renders modeling of upper surfaces suspect. At Little Dry Creek, however, a chain of nine remnants rests on fluvial floodplain sand and gravel; this chain trends 230°, and its smooth basal contact now dips 1.36° (adjusted at 220°). Projection of this dip 89 km from the 207 m base of the most distal remnant at Little Dry Creek to the vent intrusion falls far below the 2760 m intrusion-to-lava-flow transition near the Sierran crest, showing that the Sierran block has not undergone pronounced convex warping. Using elevation data on paleoriver meanders preserved by the lava flow, we show that the paleogradient has a cosine dependence on meander-section azimuth, indicating tilting. Subtraction of 1.07° of dip restores the data to an azimuth-independent configuration, indicating total tilting since 9.3 Ma of 1.07° and an original large-scale gradient of 0.46°, similar to the published value of 0.33° at Squaw Leap, but larger than the previously obtained value of 0.057° at Little Dry Creek. Subtraction of those Miocene estimates from the observable 1.643° tilt along the section from Little Dry Creek to the vent yields vent uplift of 2464 m (for 0.057°), 1835 m (for 0.46°), and 2040 m (for 0.33°). Confirmation of earlier assumptions regarding Miocene river gradient and block rigidity greatly strengthens the case for ~2 km of late Cenozoic uplift of the central Sierra Nevada crest. 
    more » « less
  5. null (Ed.)
    Abstract Terrane accretion forms lithospheric-scale fault systems that commonly experience long and complex slip histories. Unraveling the evolution of these suture zone fault systems yields valuable information regarding the relative importance of various upper crustal structures and their linkage through the lithosphere. We present new bedrock geologic mapping and geochronology data documenting the geologic evolution of reactivated shortening structures and adjacent metamorphic rocks in the Alaska Range suture zone at the inboard margin of the Wrangellia composite terrane in the eastern Alaska Range, Alaska, USA. Detrital zircon uranium-lead (U-Pb) age spectra from metamorphic rocks in our study area reveal two distinct metasedimentary belts. The Maclaren schist occupies the inboard (northern) belt, which was derived from terranes along the western margin of North America during the mid- to Late Cretaceous. In contrast, the Clearwater metasediments occupy the outboard (southern) belt, which was derived from arcs built on the Wrangellia composite terrane during the Late Jurassic to Early Cretaceous. A newly discovered locality of Alaska-type zoned ultramafic bodies within the Clearwater metasediments provides an additional link to the Wrangellia composite terrane. The Maclaren and Clearwater metasedimentary belts are presently juxtaposed by the newly identified Valdez Creek fault, which is an upper crustal reactivation of the Valdez Creek shear zone, the Late Cretaceous plate boundary that initially brought them together. 40Ar/39Ar mica ages reveal independent post-collisional thermal histories of hanging wall and footwall rocks until reactivation localized on the Valdez Creek fault after ca. 32 Ma. Slip on the Valdez Creek fault expanded into a thrust system that progressed southward to the Broxson Gulch fault at the southern margin of the suture zone and eventually into the Wrangellia terrane. Detrital zircon U-Pb age spectra and clast assemblages from fault-bounded Cenozoic gravel deposits indicate that the thrust system was active during the Oligocene and into the Pliocene, likely as a far-field result of ongoing flat-slab subduction and accretion of the Yakutat microplate. The Valdez Creek fault was the primary reactivated structure in the suture zone, likely due to its linkage with the reactivated boundary zone between the Wrangellia composite terrane and North America in the lithospheric mantle. 
    more » « less