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A Newly Recognized Shear Zone in Western Idaho: Implications for Late Cretaceous Thick and Thin-Skinned Shortening in the Rocky Mountain Foreland

Payne, Jonathan D. 1, William C. McClelland2 (1) Devon Energy Production Company, Oklahoma City, OK (2) University of Idaho, Moscow, ID

 

In west-central Idaho crustal-scale shear zones marking the arc-continent boundary are expressed by highly deformed igneous rocks and steep isotopic gradients. At its northern extent, this Nstriking boundary bends sharply NW. Structural analysis and U/Pb Geochronology indicates that the bend results from overprinting of mid-Cretaceous N-striking structures by fabrics of the newly recognized Orofino shear zone(OSZ). Latest Cretaceous and younger modification of the arc-continent boundary occurred above a lowangle, lower crustal detachment zone linking the Rocky Mountain foreland to the east and the active plate boundary to the west. The OSZ coincides with a pronounced north to south increase in depth to the basal decollemént of the Rocky Mountain fold and thrust belt. To the north, thin-skinned contraction in the foreland of the southern Canadian Rockies steps westward into thick-skinned contraction of the Shushwap complex. To the south, basement-involved uplifts of the Laramide foreland project westward beneath thin-skinned structures of the Sevier fold and thrust belt. Coeval displacements recognized north and south of the OSZ require kinematic coordination between the two foreland structural domains. Accordingly, the basal decollemént must step down from a depth of 10 - 15 km beneath the Canadian foreland to 35 - 40 km beneath the Laramide foreland. This transition is likely accommodated by a SW-dipping oblique basement ramp, extending from thick-skinned Laramide uplifts in Wyoming to the eastern margin of the Shushwap duplex in southeastern British Columbia. In this context, the Orofino shear zone behaves as a deepseated backthrust operating above the basement ramp.

 

AAPG Search and Discovery Article #90055©2006 AAPG Rocky Mountain Section Meeting, Billings, Montana