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1. 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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2. Little plutons between big plutons: probing transitions of magma flux in the Sierra Nevada batholith

   https://doi.org/10.1130/abs/2022AM-383726  

   Lackey, Jade Star; Brady, Graham; McCarty, Kyle (January 2022, Geological Society of America Abstracts with Programs)

   Cretaceous plutons in the Sierra Nevada provide a ~35 million year record of magma production in the Cordillera. Some periods of Sierran magmatism were exceptionally productive: the 95-85 Ma Sierra Crest event and the 101-98 Ma period in the axial Sierra Nevada batholith yielded some of the largest plutons; however, the transitions into and out of these intervals of building of big plutons and high net magma flux periods are not well-studied. To better understand the the transitions between high levels of magma flux in the Sierra arc, we have focused on a suite of small, granite to granodiorite plutons in the Kings River and Monarch Divide regions of Kings Canyon National Park. Notable among them are the Tehipite Dome, White Divide, Kennedy Lakes, Dougherty Peak, Cartridge Pass, Arrow, and Pyramid plutons. These plutons lie between the large middle Cretaceous plutonic suites of the axial Sierra Nevada (e.g. Mitchell Suite) and the Mount Whitney and John Muir suites of the Sierra Crest. New U-Pb zircon of ~97-92 million years old among the plutons confirms their transitional placement and era. Geochemically, the granite suites show distinct geochemical arrays from the coeval granodiorite plutons, suggesting that the two are not related by fractionation or degree of magma mixing. Radiogenic and stable isotopes also point to these plutons as distinct from each other. It follows that the mixed bulk chemistry and isotopic character of the Kings River and Monarch intrusions likely reflects a switchover in source during diminished magma flux. Thus, they appear to be recording heterogeneous source and plumbing systems in the transition that may be otherwise erased during more efficient, high-flux magmatism. 

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3. Incision history of the San Juan River, USA, in the past 1.2 million years

   https://doi.org/10.1130/GES02834.1  

   Albonico, Micael T; Karlstrom, Karl E; Heizler, Matthew T; Gillam, Mary L; Aslan, Andres; Crossey, Laura J; Reed, Cameron; Klema, Nathaniel T; Karlstrom, Leif; Granger, Darryl E (May 2025, Geosphere)

   40Ar/39Ar detrital sanidine (DS) dating of river terraces provides new insights into the evolution and bedrock incision history of the San Juan River, a major tributary of the Colorado River, USA, at the million-year time scale. We dated terrace flights from the San Juan−Colorado River confluence to the San Juan Rocky Mountains. We report >5700 40Ar/ 39Ar dates on single DS grains from axial river facies within several meters above the straths of 30 individual terraces; these yielded ∼2.5% young (<2 Ma) grains that constrain maximum depositional ages (MDAs) and minimum incision rates. The most common young grains were from known caldera eruptions: 0.63 Ma grains derived from the Yellowstone Lava Creek B eruption, and 1.23 Ma and 1.62 Ma grains derived from two Jemez Mountains eruptions in New Mexico. Agreement of a DS-derived MDA age with a refined cosmogenic burial age from Bluff, Utah, indicates that the DS MDA closely approximates the true depositional age in some cases. In a given reach, terraces with ca. 0.6 Ma grains are commonly about half as high above the river as those with ca. 1.2 Ma grains, suggesting that the formation of the terrace flights likely tracks near-steady bedrock incision over the past 1.2 Ma. Longitudinal profile analysis of the San Juan River system shows variation in area-normalized along-stream gradients: a steeper (ksn = 150) reach near the confluence with the Colorado River, a shallower gradient (ksn = 70) in the central Colorado Plateau, and steeper (ksn = 150) channels in the upper Animas River basin. These reaches all show steady bedrock incision, but rates vary by >100 m/Ma, with 247 m/Ma at the San Juan−Colorado River confluence, 120−164 m/Ma across the core of the Colorado Plateau, and 263 m/Ma in the upper Animas River area of the San Juan Mountains. The combined dataset suggests that the San Juan River system is actively adjusting to base-level fall at the Colorado River confluence and to the uplift of the San Juan Mountains headwaters relative to the core of the Colorado Plateau. These fluvial adjustments are attributed to ongoing mantle-driven differential epeirogenic uplift that is shaping the San Juan River system as well as rivers and landscapes elsewhere in the western United States. 

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4. 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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5. Cenozoic slip along the southern Sierra Nevada normal fault, California (USA): A long-lived stable western boundary of the Basin and Range

   https://doi.org/10.1130/GES02574.1  

   Lee, Jeffrey; Stockli, Daniel F.; Blythe, Ann E. (April 2023, Geosphere)

   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. 

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