TECTONICS,VOL. 3, NO. 2, PAGES 179-200, APRIL 1984 PALEOMAGNETIC ASSESSMENT OF OROFLEXURAL DEFORMATION IN WEST-CENTRAL NEVADA AND SIGNIFICANCE FOR EMPLACEMENT OF ALLOCHTHONOUS ASSEMBLAGES John W. Geissman and James T. Callian Department of Geology, Colorado School of Mines John S. Oldow Department of Geology, Rice University Stephan E. Humphries Department of Geology, Colorado School of Mines Abstract. Abrupt westerly deflections of north-south trending facies boundaries, the isopleth of (87Sr/86Sr) ø = 0.076, and arcuate structural trends of Paleozoic, Mesozoic, and Cenozoic age in the west- central Basin and Range Province, have been explained by (1) post-Early Jurassic oroflexural folding and shear, (2) the existence of a pre-Late Jurassic resistant block along the western continental mar- gin, possibly representing late Precambrian breakup, and (3) regional flexure ("Mina deflection") prior to in- trusion of Late Cretaceous plutons which crosscut flexure-related structural trends. Paleomagnetic data gathered from Late Cretaceous plutons and remagnetized metasedimentary rocks along and north of the northern margin of the zone of east- west structural trends are not in support of significant oroflexural folding (and attending major clockwise rotation) since Late Cretaceous time. The data (Water Canyon pluton: D = 1 ø, I = 57 ø, •95 = 4o, k = 9, N = 132; Gunmetal pluton: D = 354, I = 60, •95 = 6, k = 21, N = 29; East Garfield Hills pluton: D = 349, I -- 74, e95 = 1!, k TM 14, N TM 12; La Panta pluton: D = 22, I = 66, e95 = 10, k = 16, N = 13; Gillis Canyon pluton: D = 354, I = 50, e95 = 4, k = 20, N = 50; and folded units of the Excelsior Mountains: D = 309, I = Copyright 1984 by the American Geophysical Union Paper number 4T0009. 0278-7407/84/004T-0009510.00 79, •5 = 7,k =12, N =37) are dis- perse from expected Cretaceous direc- tions for the Mina deflection region, but they do not conform to a model of major systematic regional clockwise rotation. The generally small deflections of unit means from expected directions could be explained by irregular components of Late Cenozoic tilting and/or rotation. The paleomagnetic data do not deny the possi- bility of pre-Late Cretaceous oroclinal bending. Nevertheless, regional struc- tural analysis complimented by the paleo- magnetic results indicate that the Mina deflection is in all likelihood a primary feature whose geometry reflects the late Precambrian fragmentation of western North America. Structures related to the accretion of allochthonous assemblages between Middle Devonian and Cretaceous time in this part of the Cordillera also reflect the primary nature of this feature. INTRODUCTION In west-central Nevada, Late Cenozoic extensional structures are superimposed upon a complex assemblage of deformed pre- Tertiary rocks. Understandings of the Mesozoic paleogeography of this region, the kinematics of regional folding and thrusting, the ages of deformation, and regional structural/stratigraphic correla- tions are all critical to deciphering the tectonic history of the western United States. Much work has addressed these subjects [Albers, 1967; Ferguson and