Structural evolution within the Luning-Fencemaker fold-thrust belt, Nevada: progression from back-arc basin closure to intra-arc shortening

The Luning-Fencemaker fold-thrust belt (LFTB) of central Nevada reflects ma jor Mesozoic shortening in the western US Cordillera, and involved contract ional deformation in Triassic and lower Jurassic back-are basinal strata. S tructural analyses in the Santa Rosa Range, in the northern LFTB, provide n ew insight into the evolution of this belt. Four phases of deformation are recognized in the Santa Rosa Range. D-1 involved tight to isoclinal folding , cleavage development under low-grade metamorphic conditions, and reverse faulting. This deformation phase reflects NW-SE shortening of greater than or equal to 55-70% in the Early and/or Middle Jurassic. D, structures are s imilar in orientation to D-1 but involved much less overall strain and are well developed only to the southeast. D-2 appears to be related to thrustin g along the eastern margin of the LFTB in the Middle and/or Late Jurassic. D-3 deformation reflects very minor shortening (<5%) in a subvertical direc tion, and is tentatively interpreted to reflect stresses generated during i nitial intrusion of mid-Cretaceous plutons in the area. D-4 deformation dem onstrably occurred synchronously with emplacement of Cretaceous granitoids dated at 102 Ma (U-Pb zircon) based on syntectonic relations between D-4 st ructures and thermal metamorphism associated with intrusion, and an upgrade in D-4 strain in the thermally softened metamorphic aureoles of the intrus ions. This last phase of deformation reflects minor regional NE-SW shorteni ng, coupled with localized strain associated with pluton emplacement. Formation of the LFTB structural province was accomplished during the D-1 a nd D-2 phases of deformation, and most shortening occurred during the D-1 e vent. This Jurassic deformation led to structural closure of the back-arc b asin by top-to-the-SE tectonic transport and development of a largely ducti le fold-thrust belt. Subsequent deformation (D-3 and D-4) is > 50 m.y. youn ger and unrelated to development of the LFTB. The younger deformation refle cts a combination of minor regional shortening, interpreted to be related t o the Cretaceous Sevier orogeny, plus localized shortening related to empla cement of Cretaceous intrusions. (C) 2001 Elsevier Science Ltd. All rights reserved.