Transition from ductile to brittle conditions and out-of-sequence thrusting, Luning-Fencemaker fold-thrust belt, Paradise Range, western Great Basin

Abstract

The western Paradise Range of central Nevada exposes complexly deformed rocks comprising three imbricate nappes of the Pamlico allochthon, which is a part of the regionally extensive Luning-Fencemaker fold-thrust belt of mid-Jurassic to Early Cretaceous age. Mapping and structural analysis reveal a complex deformation history involving dynamothermal metamorphism, out-of-sequence thrusting, and transition from ductile to brittle conditions during progressive deformation. The deformation reflects a spectrum of ductile-to-brittle conditions: from a tectonic thickening stage involving large recumbent folds culminating in ductile conditions during greenschist-amphibolite dynamothermal metamorphism and the development of mylonites; to a progressive ductile-to-brittle transition resulting in the developments of kink bands, polyphase buckle folds and thrust emplacement at relatively shallow structural levels. Fold-thrust relations suggest the development of an out-of-sequence duplex with thrusts cutting the hanging-wall of older and lower thrusts. Four episodes of shortening are recognized on the basis of fold-thrust relations. Deformation commenced in the mid-Jurassic and was completed before intrusion of Late Cretaceous granitoid plutons. The first phase of deformation involves the local inversion of large tracts of Late Triassic and Early Jurassic carbonates, clastics, and volcanic rocks of the Pamlico assemblage in recumbent folds with ultimate tectonic burial to depths of 10-20 km. Ductile structures, consisting of penetrative fabrics, mylonitic shear zones, and major recumbent folds were generated at depth During progressive southeast transport, the ductily deformed rocks were carried to shallower structural levels by thrusts that truncate earlier structures. Buckle folds associated with the thrusts indicate a protracted history of motion parallel to the transport recorded in the ductile structures contained within nappes. Following emplacement the three nappes exposed in the western Paradise Range were folded in tight to open upright folds. The final phase of folds documents a shift in the regional shortening axis from northwest-southeast to southwest-northeast.