The Deep Crust in Convergent and Divergent Terranes: Laramide Uplifts and Basin-Range Rifts
- Muawia Barazangi and
- Larry Brown
Published Online: 15 MAR 2013
Copyright 1986 by the American Geophysical Union.
Reflection Seismology: The Continental Crust
How to Cite
Thompson, G. A. and Hill, J. L. (2013) The Deep Crust in Convergent and Divergent Terranes: Laramide Uplifts and Basin-Range Rifts, in Reflection Seismology: The Continental Crust (eds M. Barazangi and L. Brown), American Geophysical Union, Washington, D. C.. doi: 10.1029/GD014p0243
- Published Online: 15 MAR 2013
- Published Print: 1 JAN 1986
Print ISBN: 9780875905143
Online ISBN: 9781118670118
- Seismic reflection method—Congresses
Seismic profiles across the Pacific Creek anticline, the Wind River Range, and the Casper arch in western Wyoming provide a coherent, nearly continuous section deep into the crystalline crust through these Laramide basement uplifts. Anticlines or monoclines in the sedimentary section overlie reflective thrust faults in the Precambrian crystalline basement. Structural relief on the folds and displacements on the faults are quantitatively coupled. The thrust faults tend to splay and/or decrease in dip with depth until they disappear as distinct reflectors at mid-crustal levels. The reflection data clearly reveal transitions downward from folding to brittle thrust faulting to distributed or ductile compression in the deep crust.
Seismic sections in the rifted Basin and Range province (including the Rio Grande rift) also reveal striking transitions with depth. The near-surface, high-angle normal faults commonly merge with, and do not displace, subhorizontal detachment faults at depths of only a few kilometers. The abruptness and shallowness of the change suggest to us that it is not governed solely by rock softening due to increased temperature with depth. Instead we suggest that conditions of open hydrothermal circulation in the broken upper crust change abruptly at depth to conditions of high pore-water pressure in rocks self-sealed by mineral deposition or metamorphic processes.
Zones of subhorizontal reflectors at midcrustal depths are much better developed in extensional regions such as the Basin and Range provinces than in the compressional Wyoming province. On reflection sections from both regions, however, the basal crust appears laminated. A tectonic explanation is suggested for the subhorizontal reflectors by the flattening of both thrusts and normal faults in the deep crust, but an origin coupled also to metamorphic and magmatic events is likely.