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

Abstract

The Luning–Fencemaker fold-thrust belt (LFTB) of central Nevada reflects major Mesozoic shortening in the western US Cordillera, and involved contractional deformation in Triassic and lower Jurassic back-arc basinal strata. Structural analyses in the Santa Rosa Range, in the northern LFTB, provide new insight into the evolution of this belt. Four phases of deformation are recognized in the Santa Rosa Range. D1 involved tight to isoclinal folding, cleavage development under low-grade metamorphic conditions, and reverse faulting. This deformation phase reflects NW–SE shortening of ≧55–70% in the Early and/or Middle Jurassic. D2 structures are similar in orientation to D1 but involved much less overall strain and are well developed only to the southeast. D2 appears to be related to thrusting along the eastern margin of the LFTB in the Middle and/or Late Jurassic. D3 deformation reflects very minor shortening (<5%) in a subvertical direction, and is tentatively interpreted to reflect stresses generated during initial intrusion of mid-Cretaceous plutons in the area. D4 deformation demonstrably occurred synchronously with emplacement of Cretaceous granitoids dated at 102Ma (U–Pb zircon) based on syntectonic relations between D4 structures and thermal metamorphism associated with intrusion, and an upgrade in D4 strain in the thermally softened metamorphic aureoles of the intrusions. This last phase of deformation reflects minor regional NE–SW shortening, coupled with localized strain associated with pluton emplacement.Formation of the LFTB structural province was accomplished during the D1 and D2 phases of deformation, and most shortening occurred during the D1 event. This Jurassic deformation led to structural closure of the back-arc basin by top-to-the-SE tectonic transport and development of a largely ductile fold-thrust belt. Subsequent deformation (D3 and D4) is >50m.y. younger and unrelated to development of the LFTB. The younger deformation reflects a combination of minor regional shortening, interpreted to be related to the Cretaceous Sevier orogeny, plus localized shortening related to emplacement of Cretaceous intrusions.