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Abstract: Relations among geothermal system dynamics, evolution of an 8-m thick biomorphic mound, and requirements to protect critical habitat of Gila boraxobius, a fish protected under the Endangered Species Act of 1973, were explored at Borax Lake, a shallow thermal lake in the tectonically active northwestern Basin-and-Range Province. Neutral pH, thermal water at ~105°C discharges within a carrot-shaped vent at a depth of 29m and rapidly mixes within the vent and lake which is mostly <1m deep. The 14hectare thermal lake lies at the crest of a biomorphic mound composed of diatoms, silt-size opal-A silica particles, silica encrusted and permineralized vascular plant fragments, sinter fragments, and discontinuous carbon-rich plant and charcoal layers. Siliceous sinter lines the vent, part of the lake bed, and forms a discharge apron that caps the mound and gently dips (<3°) away from the shore. The hydrothermal deposit unconformably overlies lacustrine deposits of Pleistocene Lake Alvord and began to accumulate prior to BC 3775 to 3655, the age of a charcoal layer near the mound crest. Slope degradation occurs primarily from rilling, gullying, and solifluction as the result of seasonal seepage, overland flow and freeze/thaw cycles. Winter seepage on the flanks of the mound ceases as the water table drops (~1.5m) during the summer dry season. Piping from an upward seepage gradient also degrades slopes. Stability analysis indicates that the mound is stable under current static stress conditions, however, high water table and cohesionless diatomaceous silt with shear wave velocities less than 164ms−1 and proximity of seismic sources indicates the mound may be susceptible to liquefaction and slope failure during dynamic earthquake loading. Borax Lake is the unique product of the geothermal system and the biological communities it supports. Portions of the mound may have failed in the past during seismic loading, eroded, and been rebuilt by processes similar to those active today. [Copyright &y& Elsevier]

Abstract: This work constitutes the first attempt to understand the seismic hazard in northeast Mexico. We present a compilation of regional seismicity in northeast Mexico (24–31°N, 97–106°W), finding 148 earthquakes for the 1787–2006 period. The study area lies within three morphotectonic provinces: Basin, Range, and Rio Grande rift; Sierra Madre Oriental; and Gulf coastal plain. Peak ground acceleration (PGA) maps were computed for three different scenarios: 1928 Parral, Chihuahua (M W =6.5); 1931 Valentine, Texas (M W =6.4); and a hypothetical earthquake located in central Coahuila. Ground motion values were computed using attenuation relations developed for central and eastern North America and the Basin and Range provinces. The earthquake in central Coahuila is considered a critical scenario for the main cities of northeast Mexico. The damage associated with this hypothetical earthquake could be severe because most buildings were constructed without seismic criteria. The expected PGA values in Monterrey, Saltillo, and Monclova are between 30 and 70cm/s2. This earthquake might also produce or trigger significant landslides and rock falls in the Sierra Madre Oriental, where several cities are located on the mountain range. [Copyright &y& Elsevier]

New (U–Th)/He in apatite ages suggest rapid slip rates on the Tertiary Buckskin–Rawhide detachment fault, which crops out in the Buckskin and Rawhide Mountains of western Arizona. Single-crystal (U–Th)/He in apatite ages were determined for a suite of crystalline rock samples from the footwall to this detachment. The samples were collected from a transect parallel to the fault slip direction, and proximal to the fault zone; they show a linear decrease in age with distance, from ∼14 to ∼11 Ma over ∼15 km. The age versus distance relationship is interpreted to show a slip rate of 4.2 (+1.2/−0.8) mm/yr, continuing for a period of >3 Myr. This rate is more-or-less in agreement with, but more precise than, previously published slip rates from this and other large-offset extensional detachments. Such slip rates are alarmingly high, being roughly 5–10 times faster than rates commonly observed on active structures at the Earth’s surface. These rapid slip rates suggest that significant seismic hazard might be posed by some extensional structures, and they raise questions regarding the relationship between such rapidly slipping, large-offset structures and the slower, smaller offset faults that are more commonly observed at the Earth’s surface. [Copyright &y& Elsevier]

The Salina area of central Utah contains an unusually complete Tertiary stratigraphic record, affording unique opportunities to study details of the Tertiary geologic history. Stratigraphic and structural studies reveal Neogene ages for tectonically significant parts of all major structures such as the 80-km-long Sanpete-Sevier Valley anticline (SSVA) and its flanking structures, the Wasatch monocline and Sevier Valley syncline. Newly identified critical components of the Neogene history in the SSVA include: (1) a protracted Eocene through early Miocene period of quasi-continuous sedimentation and relative structural quiescence that buried an early phase of the anticline; (2) east-trending folds in rocks of the anticlinal core; (3) widespread plutons in the southern part of the core; (4) strike-slip faults; (5) major thinning of strata (vertical structural convergence) between uplifted Jurassic rocks of the anticlinal core and downdropped superjacent Tertiary rocks; and (6) a widespread zone of altered rock at the contact between core rocks and overlying Tertiary rocks. We interpret the thinning and widespread alteration (5 and 6) to be linked through protracted processes of extensional faulting (possibly including detachment faulting), fluid flow, massive dissolution, and dissolution collapse at the contact between the Jurassic core rocks and overlying Tertiary rocks. We suggest that the SSVA, one of several diapir-like elongate uplifts in central Utah, is somewhat analogous to flank ridges that develop at landslide margins by outward and upward flow of mechanically weak clay-rich rock or gouge. By analogy, the SSVA results from outward and upward movement of the relatively incompetent Arapien Shale from beneath the adjacent Sevier Valley. If the structures formed during these newly recognized components of Neogene history are palinspastically restored, much of the evidence for contractional deformation at shallow structural levels is eliminated; this affects... [ABSTRACT FROM AUTHOR]