Abstract

The Golconda allochthon of northern and central Nevada contains the Havallah sequence and correlative units of latest Devonian to early Late Permian age. The Havallah sequence is dominated by radiolarian ribbon chert and argillite associated with variable, but generally subordinate, siliciclastic, cal-carenitic, and volcaniclastic turbidites and slump deposits. Cherts of all ages (except Pennsylvanian?) locally rest depositionally on tholeiitic basalts. Some were altered and mineralized by ridge-type hydrothermal systems, suggesting deposition in an ocean basin containing active spreading center(s) for much of the upper Paleozoic.

The Havallah sequence was thrust eastward along the basal Golconda thrust onto coeval sediments of the western North American shelf during the latest Paleozoic-earliest Mesozoic Sonoma orogeny. Detailed studies of the Havallah sequence in the Sonoma and Tobin Ranges and in Battle Mountain indicate the presence of complex diagenetic and structural fabrics that developed prior to the obduction of the allochthon. A large number of thrusts, composite thrusts, and shear zones slice the Havallah into numerous tectonic packets of contrasting lithology and/or internal structural style. Internal structures in chert packets include bedding-parallel and bedding-normal solution cleavage, solution boudins, three or more sets of east-verging folds of variable geometry, and features associated with high pore-fluid pressures such as crack-seal fractures, dilation breccias, clastic dikes and sills, and clastic intrusions along thrust surfaces. The fabrics suggest a general progressive evolution from deformation characterized by bedding-normal compression and slight east-west extension (D1) to deformation dominated by bedding-parallel, east-directed shear (D2). Local chert packets may have suffered alternating episodes of high and low pore-fluid pressure. The degree of development and the styles and orientations of various structural elements vary, sometimes radically, from packet to packet.

The pre-Golconda thrust fabrics can be modeled as the result of imbrication and deformation of successive batches of ocean-floor sediments into the toe of an accretionary prism in front of an east-facing arc. Relatively undeformed Permian calcarenite units, interpreted as being trench-slope deposits, suggest that this prism was well developed by Permian time. Coarse slump deposits containing chert clasts with pre-depositional structural fabrics suggest a prism that was active long enough to recycle previously tectonized cherts. If we are correct, the classical Sonoma orogeny (D3) marked the culmination of a protracted structural evolution that may have spanned much of the upper Paleozoic.

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