2 From Contraction to Extension: The Mesozoic to Cenozoic Tectonic Evolution of the Northern Great Basin I

Wednesday, 18 May 2011
8:00 AM - 12:00 PM
Willow

LATE MESOZOIC AND CENOZOIC EVOLUTION OF EAST-CENTRAL NEVADA: HINTERLAND TECTONICS AND THE ORIGIN OF METAMORPHIC CORE COMPLEXES

Paper number: 2-3
Program: Theme Sessions
Time: 8:30 AM – 8:55 AM

Authors

Abstract

Structural and geochronologic studies in the White Pine, Egan, Schell Creek, and Snake/Kern/Deep Creek ranges of east-central Nevada reveal a complex late Mesozoic and Cenozoic history of contractional and extensional deformation and magmatism. Local magmatic activity is recorded by Jurassic and mid Cretaceous granitoids (~160 and 110 Ma), late Cretaceous “S-type” leuocogranites (75-90 Ma), widespread 40-30 Ma (mainly ~35 Ma) intermediate to silicic volcanic rocks, and rare Miocene dikes and tuffs (22-20 Ma). Late Cretaceous contractional deformation was highly localized at supracrustal levels as discontinuous large-scale non-cylindrical folds – best exposed in the Snake-Deep Creek and northern White Pine ranges. Episodic Cenozoic extension is recorded by distinct generations of normal faults, half-graben basins, and rapid cooling of midcrustal rocks. At least two distinct Tertiary extensional events are well documented: 1) Eocene-Early Oligocene (40-30? Ma) with both E-W and N-S extension, and 2) Early to Mid Miocene (20-15 Ma)

The northern Snake Range is widely recognized as an archetypical metamorphic core complex and has spawned a myriad of models. These models do not adequately address the fundamentally composite, polyphase origin of this and other core complexes. Localized late Cretaceous thickening resulted in deep burial, high grade metamorphism, and melting in a portion of the footwall (but not all of it). Early Tertiary (mainly Eocene) extensional exhumation brought the footwall back to depths of 10-15 km, along a proto NSRD shear zone/normal fault and was when most footwall mylonitic fabrics were developed. After prolonged quiescence, rapid slip initiated along the mainly brittle Snake Range detachment at ~20 Ma - perhaps reactivating the Eocene shear zone. By this time, the western part of the footwall had cooled to temperatures of ≤150°C, whereas the eastern part of the footwall was still >350°C, implying that the brittle Miocene fault originated as a moderately to steeply east-dipping normal fault. An important question that is posed by this composite history is “ How did earlier tectonic events and structures influence the geometry and kinematics of what happened next?”