POTENTIAL FIELD INTERPRETATION ALONG THE SOUTHERN EDGE OF THE MINA DEFLECTION, NEVADA

East of the Sierra Nevada Block the Walker Lane/Eastern California Shear Zone (WL/ECSZ) currently accommodates about20-25% of the predominantly right-lateral shear associated with the relative motion between the Pacific and North American plates. The central WL/ECSZ is geologically dominated by a regional releasing bend known as the Mina deflection. This right-lateral stepover domain is structurally characterized by a series of curvilinear normal-faults and associated pull-apart basins as well and left-lateral strike-slip faults that together link the regional-scale right-lateral transcurrent shear zones of the WL/ECSZ. Recent potential field studies, along with mapping, and age dating show distinct differences in geological history of the basins along the southern edge of the Mina deflection, including structural style, faulting initiation age, and basin depth. Queen Valley began opening ~3 Ma and is surprisingly shallow, with an average depth of ~0.2 km and maximum apparent depth of ~0.5 km. There is a significant negative gravity anomaly in northern Queen Valley, but it is likely to be attributable to a low-density pluton. This late-Cretaceous pluton is exposed in the northern White Mountains and extends into the subsurface below southern Fish Lake Valley and the SW Silver Peak range. Northern Fish Lake Valley pull-apart began forming at ~6 Ma and is significantly deeper. Gravity data indicate that the main post-Miocene basin fills is on the order of ~1.5 km. It appears to be bounded by NE-trending normal faults along the northern and southern margins and is internally segmented by NNW-trending right-lateral strike-slip faults. Geophysical and structural data suggest that active oblique-slip faults along the eastern side of northern Fish lake Valley have been progressively back-stepping to the east. The active pull-apart basin is superimposed on an older (middle Miocene) structurally-controlled basin that is now largely covered by younger basin fill and by low-density Mio-Pliocene ash-flow tuffs to the north of the pull-apart basin. Farther to the north and east Columbus Salt Marsh and the northern Big Smokey Valley have very large gravity lows associated with them, with no apparent major contribution from low-density plutonic basement. Both valleys appear to be significantly deeper, perhaps up to 1.8 km in depth.