KINEMATICS OF UNDERTHRUSTING IN THE PALEOZOIC ANTLER FORELAND BASIN

Field mapping and structural analysis of Paleozoic rocks in the northe rn Roberts Mountains in central Nevada reveal that fine-grained silici clastic strata deposited early in the history of the Antler foreland b asin underthrust and underplated as two discrete fault-bound packets t o the Roberts Mountains allochthon as it encroached upon the North Ame rican continental shelf. Underplating also resulted in the intercalati on of North American outer shelf/slope strata between the two foreland basin packets as the basal detachment of the Roberts Mountains alloch thon stepped downward in the sedimentary section. Deformation of forel and basin and outer shelf/slope strata was in response to ESE in-layer shortening accompanied by top ESE layer-parallel shear within 10 m of fault surfaces along which underplating and underthrusting occurred. Stratal disruption, fold tightness and asymmetry, and cleavage develop ment near the lower and upper-bounding faults are greater than in the center of underthrust packages, indicating higher strain along packet margins. Furthermore, maximum strains occurred at the base of packets as evidenced by greater fold asymmetry and cleavage intensity along th e lower than along the upper bounding faults of packets. Strata in the basal 25 m of packets, therefore, continued to deform after underplat ing. Better and later cleavage development at the edges than in the ce nters of packets may have been facilitated by local rapid dewatering o f sedimentary strata along faults during underthrusting, particularly the fault along the base of the overridden foreland basin and outer sh elf/slope sequence. Faults within the indurated Roberts Mountains allo chthon also may have provided fluid migration pathways, further aiding dewatering during underthrusting.