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1. Mesozoic Fabric Influence on the Formation and Geometry of the Quaternary Cucamonga Fault

   Robles, F.; Schwartz, J.; Miranda, E.; Klepeis, K.; and Mora-Klepeis, G. (October 2022, Meeting of the Southern California Earthquake Center (SCEC), LAX, Sept., 2022)

   Robles, F.; Schwartz, J.; Miranda, E.; Klepeis, K.; and Mora-Klepeis, G. (Ed.)

   Ancient basement rocks in Southern California contain mechanical anisotropies that may influence the architecture of Quaternary faulting. We study exposed basement rocks found within the southeastern San Gabriel lithotectonic block with the intention of reconciling the relationship between inherited ductile fabrics and the geometry of Quaternary faults that are part of the San Andreas Fault system. By focusing our study on the southeastern corner of the San Gabriel block we can study the exposed lower- to middle crustal shear zone fabrics near where the Cucamonga Fault and the San Jacinto Fault intersect. The brittle Quaternary Cucamonga Thrust Fault strikes E-W and dips to the north-northeast (35-25°) and is localized at the range front and cuts these older fabrics, however there is also brittle deformation distal from the fault that also affects the sequence of lower- to middle crustal (6-8 kbar) granulite- to upper amphibolite facies mylonite and granulite-facies metasedimentary rocks. Near the Cucamonga Fault, mylonitic fabrics strike E-W and dip northeast (40-50°). Quaternary brittle faults that strike E-W and dip northeast (30-40°) reactivate the mvlonites and slickenlines and record a sinistral, top-to-the-west sense of shear. Investigation of host rocks indicates that they formed in the roots of a continental arc which was active from the Middle Jurassic to Late Cretaceous (172-86 Ma) at 740-800°C. Ductile deformation was associated with granulite-facies metamorphism at approximately 30 km depth during the Late Cretaceous (88-74 Ma) at 730-800 °C. Our work shows that the exhumed Late Cretaceous mylonitic fabrics may have operated as stress guides during Quaternary faulting in the Cucamonga Fault zone. We conclude that these lower crustal fabrics influence the geometry and kinematics of late Cenozoic faulting of the Cucamonga and San Jacinto fault zones. 

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2. River patterns reveal two stages of landscape evolution at an oblique convergent margin, Marlborough Fault System, New Zealand

   https://doi.org/10.5194/esurf-8-177-2020  

   Duvall, Alison R.; Harbert, Sarah A.; Upton, Phaedra; Tucker, Gregory E.; Flowers, Rebecca M.; Collett, Camille (January 2020, Earth Surface Dynamics)

   Abstract. Here we examine the landscape of New Zealand'sMarlborough Fault System (MFS), where the Australian and Pacific plates obliquelycollide, in order to study landscape evolution and the controls on fluvialpatterns at a long-lived plate boundary. We present maps of drainageanomalies and channel steepness, as well as an analysis of the plan-vieworientations of rivers and faults, and we find abundant evidence ofstructurally controlled drainage that we relate to a history of drainagecapture and rearrangement in response to mountain-building and strike-slipfaulting. Despite clear evidence of recent rearrangement of the western MFSdrainage network, rivers in this region still flow parallel to older faults,rather than along orthogonal traces of younger, active strike-slip faults.Such drainage patterns emphasize the importance of river entrenchment,showing that once rivers establish themselves along a structural grain,their capture or avulsion becomes difficult, even when exposed to newweakening and tectonic strain. Continued flow along older faults may alsoindicate that the younger faults have not yet generated a fault damage zonewith the material weakening needed to focus erosion and reorient rivers.Channel steepness is highest in the eastern MFS, in a zone centered on theKaikōura ranges, including within the low-elevation valleys of main stemrivers and at tributaries near the coast. This pattern is consistent with anincrease in rock uplift rate toward a subduction front that is locked on itssouthern end. Based on these results and a wealth of previous geologicstudies, we propose two broad stages of landscape evolution over the last 25 million years of orogenesis. In the eastern MFS, Miocene folding above blindthrust faults generated prominent mountain peaks and formed major transverserivers early in the plate collision history. A transition to Pliocenedextral strike-slip faulting and widespread uplift led to cycles of riverchannel offset, deflection and capture of tributaries draining across activefaults, and headward erosion and captures by major transverse rivers withinthe western MFS. We predict a similar landscape will evolve south of theHope Fault, as the locus of plate boundary deformation migrates southwardinto this region with time. 

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3. CENOZOIC SLIP ALONG THE SOUTHERN SIERRA NEVADA RANGE FRONT NORMAL FAULT, CALIFORNIA: A LONG-LIVED STABLE WESTERN BOUNDARY OF THE BASIN AND RANGE

   https://doi.org/10.1130/abs/2021CD-363159  

   Lee, Jeffrey; Stockli, Daniel; Blythe, Ann; Fox, Matthew (April 2021, Geological Society of America Abstracts with Programs)

   null (Ed.)

   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. 

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4. Grooving in the midcontinent: A tectonic origin for the mysterious striations of L’Anse Bay, Michigan, USA

   https://doi.org/10.1130/GES02603.1  

   Alemu, Tadesse B; Hodgin, Eben B; Swanson-Hysell, Nicholas L (July 2023, Geosphere)

   A striated surface is present at an erosional unconformity between foliated Paleoproterozoic Michigamme Formation and fluvial conglomerate and sandstone of the Neoproterozoic Jacobsville Formation exposed at L’Anse Bay (Michigan, USA). These striations have been interpreted to be the result of ice flow in either the Proterozoic, the Pleistocene, or the modern. Recently, the glacial origin interpretation for this striated surface has been used to argue that it may be related to ca. 717–635 Ma Cryogenian snowball Earth glaciation. This interpretation would make the surface a rare example of a Neoproterozoic glacial pavement, with major chronostratigraphic implications that in turn impose constraints on the timing of intracratonic erosion related to the formation of the Great Unconformity. In this contribution, we present new observations showing that the surface is a tectonic slickenside caused by largely unconformity-parallel slip along the erosional unconformity. We document structural repetition of the Michigamme-Jacobsville contact with associated small-scale folding. The unconformity-parallel slip transitions into thrust faults that ramp up into the overlying Jacobsville Formation. We interpret that the surface records contractional deformation rather than ancient glaciation, recent ice movement, or recent mass wasting. The faulting likely occurred during the Rigolet phase of the Grenvillian orogeny, which also folded the Jacobsville Formation in the footwall of the Keweenaw fault. 

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5. Volcano-Tectonic Evolution of Central Baja California Peninsula, Mexico: Implications for Speciation and Barriers to Gene Flow

   Bennett, S; Darin, M; Hausback, B; Dorsey, R; Grandy, S; Gardner, K; Schmitt, A; Heizler, M; Sawlan, M; Dolby, G; et al (December 2021, American Geophysical Union)

   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. 

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