No abstract

Magmatism and volcanism have evolved the Martian lithosphere, surface, and climate throughout the history of Mars. Constraining the rates of magma generation and timing of volcanism on the surface clarifies the ways in which magma

The Agua Blanca fault (ABF) is a west-northwest-trending oblique dextralnormal fault that defines the southern boundary of the Big Bend domain (BBD) of the Pacific-North American plate margin and the northern limit to the rigid Baja California microplate. Our geologic and geodetic studies demonstrate that finite slip on the ABF reaches a maximum of ~11 km of nearly pure dextral strike slip in central portions of the fault, whereas the magnitude of displacement decreases and the proportion of extension increases in the sections to both the east and west. To the east, the ABF appears to die out before crossing into the San Pedro Mártir fault, with slip transferred onto a series of more northerly-trending, dip-slip faults. To the west, the ABF bifurcates to form the Santo Tomás fault (STF) in the Valle Santo Tomás section, where we measure ~5 km of dextral offset for the ABF and ~3 km estimated offset on the STF. And we report a measurement of ~7 km of dextral offset on the ABF in the Punta Banda section. Small offset faults proximal to the ABF likely accommodate additional dextral shear in the western sections of the fault. The STF in the Valle Santo Tomás section and the ABF in the Punta Banda section exhibit 0.58 and 0.65 km of extensional heave, or ~7% and 10% of the total displacement in each section, respectively. Block modeling based on geodetic data agrees well with geologic determinations of slip direction and reveals near perfect alignment of the central ABF with the relative block motion vectors and increased proportions of fault-perpendicular extension to both the east (3%-10%) and west (5%-13%). Based on our new estimates of the total offsets combined with existing slip rates, the ABF likely initiated between 3.3 and 1.5 Ma. This age range overlaps with those reported for other faults within the area of the BBD southwest of the San Andreas fault. The ABF has a more westerly orientation than the transpressional restraining-bend segment of the San Andreas, yet it accommodates transtensional shearing. This requires a reevaluation of the processes that control transpression and transtension within the BBD. GEOSPHERE GEOSPHERE; v. 15, no. 1

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Aeromagnetic surveys over the Amargosa Desert, Nevada, have revealed the presence of several magnetic anomalies that have been interpreted to be caused by buried volcanoes; many of these anomalies have been confirmed by drilling. We present data collected from a high-resolution, ground-based magnetic survey over Anomaly B, the largest of these anomalies, that reveal details about a buried crater and its associated lava flow, not observed in the aeromagnetic surveys. These details provide insight into the nature of the eruption and volume of this buried volcano. Results from non-linear inversion demarcate a crater with a diameter of approximately 700 m and a base approximately 150 m below the ground surface. Coupled with well log data, the inversion results suggest a total volume for the Anomaly B crater area and associated lava flows of approximately 1.0 ± 0.4 km 3 , based on an estimated lava flow field area of 24 km 2 and a lava thickness of 42 ± 15 m. A workflow is presented for processing such large ground-based magnetic data sets with attendant GPS data, filtering these data and constructing maps and models using the provided PERL scripts.