ANATOMY OF THE SOUTHERN CORDILLERAN HINGELINE, UTAH AND NEVADA, FROM DEEP ELECTRICAL-RESISTIVITY PROFILING

To address outstanding questions in Mesozoic-Cenozoic structure and pr esent-day deep physicochemical state in the region of the southern Cor dilleran hingeline, a detailed, east-west profile of magnetotelluric ( MT) soundings 155 km in length was acquired, From these soundings, a r esistivity interpretation was produced using an inversion algorithm ba sed on a structural parameterization. In the upper ten kilometers of t he transect, the interpretation shows two segments of low resistivity lying beneath allochthonous rocks of the Late Mesozoic, Sevier thrust sheet. Subsequent industry drilling motivated in part by our surveying confirms the existence and position of the eastern subthrust conducto r and, more spectacularly, identifies the presence of yet deeper, auto chthonous Mesozoic rocks. The conductors cannot be specified uniquely with present public data, because their electrical characteristics app ear consistent with Paleozoic, pyrolized graphitic strata of either La te Devonian-Mississippian or Middle Ordovician age, However, the drill ing results show that Late Paleozoic and younger rocks lie underthrust much farther west than recognized previously, and perhaps as far west as the Utah-Nevada border. A simple structural interpretation is offe red where one underthrust segment of low-resistivity sediments was cre ated originally, but this segment was broken later into two major ones during higher-angle Tertiary extension. For the middle and lower crus t, the MT data imply a nearly 1-D resistivity structure of remarkable uniformity across the entire transect. In particular, there occurs a d eep low-resistivity layer most pronounced (about 8 ohm-m) in the nomin al depth interval of 17.5 to 40 km, The MT data indicate that the laye r cannot be confined to a single thin layer in the lower crust but ins tead represents vertically distributed low resistivity, With temperatu res estimated from surface heat flow to range from 550 degrees C to 10 50 degrees C with depth in the layer, and with a metaigneous mineralog y of high metamorphic grade assumed, mechanisms to produce the low res istivity can be constrained, The deep layer is thus consistent with H2 O-rich brines at its upper levels, fluids of lower H2O activity toward middle levels, and H2O-deficient melting below about 30 km, The marke d uniformity of the deep conductive layer across the transect suggests a similar uniformity of deep physicochemical state. However, this is not at odds with recent analyses of heat flow Curie depth, Quaternary extension, and basaltic volcanism. Pre-existing structural fabrics hav e had no measureable influence on localizing regions of high temperatu re, fluids and melting in the lower crust, at least averaged over the scale of tens of kilometers. Given its uniformity over a distance of 1 55 km or more, the depth to the regional deep conductor does not appea r related to the distribution of high-temperature geothermal resources .