Jason M. Dortch

Late Quaternary slip rate gradient defined using high‐resolution topography and 10Be dating of offset landforms on the southern San Jacinto Fault zone, California

[1]xa0Recent studies suggest the San Jacinto fault zone may be the dominant structure accommodating PA-NA relative plate motion. However, because the late Quaternary slip history of the southern San Andreas fault system is insufficiently understood, it is difficult to evaluate the partitioning of deformation across the plate boundary and its evolution. Landforms displaced by the Clark fault of the southern San Jacinto fault zone were mapped using high-resolution airborne laser-swath topography and selected offset landforms were dated using cosmogenic 10Be. Beheaded channels at Rockhouse Canyon, displaced by 500 ± 70 m and 220 ± 70 m, have been dated to 47 ± 8 ka and 28 ± 9 ka, respectively. Farther south, near the southern Santa Rosa Mountains, an alluvial deposit displaced by 51 ± 9 m has been dated to 35 ± 7 ka. From these sites, the slip rate of the Clark fault is determined to diminish southward from 8.9 ± 2.0 to 1.5 ± 0.4 mm/yr. This implies a slip-rate decrease along the Clark fault from Anza southeastward to its surface termination near the Salton Trough, where slip is transferred to the Coyote Creek fault, and additional deformation is compensated by folding and thrusting in the basin. These data suggest that since ∼30 to 50 ka, the slip rate along the southern San Jacinto fault zone has been lower than, or equivalent to, the rate along the southernmost San Andreas fault. Accordingly, either the slip rate of the San Jacinto fault has substantially decreased since fault initiation, or fault slip began earlier than previously suggested.

Episodic fluvial incision of rivers and rock uplift in the Himalaya and Transhimalaya

Abstract: Seventeen strath terraces in northern India were dated using 10Be surface exposure methods; ages range from c. 7 to c. 735 ka and provide fluvial incision rates of 0.02 ± 0.003 to 2.6 ± 1.9 mm a−1. On the northern side of the Ladakh Range, incision rates are c. 1 mm a−1; in the northern Zanskar Range they are ≤0.06 ± 0.005 mm a−1. New and published incision rates in southernmost Lahul range from 0.1 ± 0.02 to 13.2 ± 6.2 mm a−1; rates for ages >35 ka are ≤0.4 ± 0.2 mm a−1. Across the Himalaya and Transhimalaya, Holocene fluvial incision rates range from c. 0.02 to c. 26.0 mm a−1, whereas Pleistocene incision rates are ≤5 mm a−1. Many of the Holocene incision rates exceed exhumation rates, whereas Pleistocene incision rates are comparable with exhumation rates. This suggests that long-term fluvial incision is in dynamic steady state with exhumation. The temporal pattern for rates of fluvial incision is probably controlled by episodic incision linked to significant precipitation changes throughout the Quaternary, suggesting that strath terraces with ages >35 ka can be used for assessing long-term rates of rock uplift. In contrast, rates of fluvial incision based on Late Glacial and Holocene strath terraces reflect changes in monsoon intensity and deglaciation events. By determining ages for multiple samples on flights of strath terraces, it is possible to document changes in incision rate, assess whether post-abandonment transient shielding has occurred, and help elucidate tectonic v. climatic controls on their formation. Supplementary material: Tables (DS1–4) for previously published data and recalculated ages for strath terraces, and figures (DS1 and DS2) showing plots of incision rates against time for the Himalaya and Transhimalaya are available at http://www.geolsoc.org.uk/SUP18454.

Modelling glacial lake outburst flood impacts in the Bolivian Andes

The Bolivian Andes have experienced sustained and widespread glacier mass loss in recent decades. Glacier recession has been accompanied by the development of proglacial lakes, which pose a glacial lake outburst flood (GLOF) risk to downstream communities and infrastructure. Previous research has identified three potentially dangerous glacial lakes in the Bolivian Andes, but no attempt has yet been made to model GLOF inundation downstream from these lakes. We generated 2-m resolution DEMs from stereo and tri-stereo SPOT 6/7 satellite images to drive a hydrodynamic model of GLOF flow (HEC-RAS 5.0.3). The model was tested against field observations of a 2009 GLOF from Keara, in the Cordillera Apolobamba, and was shown to reproduce realistic flood depths and inundation. The model was then used to model GLOFs from Pelechuco lake (Cordillera Apolobamba) and Laguna Arkhata and Laguna Glaciar (Cordillera Real). In total, six villages could be affected by GLOFs if all three lakes burst. For sensitivity analysis, we ran the model for three scenarios (pessimistic, intermediate, optimistic), which give a range of ~u20091100 to ~u20092200 people affected by flooding; between ~u2009800 and ~u20092100 people could be exposed to floods with a flow depth ≥u20092xa0m, which could be life threatening and cause a significant damage to infrastructure. We suggest that Laguna Arkhata and Pelechuco lake represent the greatest risk due to the higher numbers of people who live in the potential flow paths, and hence, these two glacial lakes should be a priority for risk managers.

Spatially heterogeneous post-Caledonian burial and exhumation across the Scottish Highlands

The postassembly, postrift evolution of passive margins is an essential element of global continental tectonics. Thermal and exhumational histories of passive margins are commonly attributed to a number of drivers, including uplift and erosional retreat of a rift-flank escarpment, intraplate fault reactivation, mantle-driven uplift, and erosional disequilibrium, yet in many cases, a specific factor may appear to dominate the history of a given passive margin. Here, we investigate the complex evolution of passive margins by quantifying exhumation patterns in western Scotland. We build upon the well-studied thermal evolution of the Scottish North Atlantic passive margin to test the importance of spatially heterogeneous factors in driving postorogenic burial and exhumation. Independent investigations of the cooling history from seven different field sites across the western Scottish Highlands using radiogenic apatite helium thermochronometry ([U-Th]/He; n = 14; ca. 31–363 Ma) and thermal modeling confirm that post-Caledonian heating and burial, as well as cooling and exhumation, must have been variable across relatively short distances (i.e., tens of kilometers). Heating associated with Paleogene hotspot activity and rifting locally explains some of this spatial variation, but additional drivers, including margin tilting during rifting, vertical separation along reactivated faults, and nonuniform glacial erosion in the late Cenozoic, are also likely required to produce the observed heterogeneity. These results indicate that passive margins may experience variable burial, uplift, and erosion patterns and histories, without exhibiting a single, dominant driver for behavior before, during, and after rifting. LITHOSPHERE; v. 10; no. 3; p. 406–425; GSA Data Repository Item 2018139 | Published online 23 March 2018 https://doi.org/10.1130/L678.1