ORIGIN OF W-DIRECTED SHEAR ZONES IN THE NORTHERN SNAKE RANGE DECOLLEMENT

The Northern Snake Range Decollement (NSRD) in eastern Nevada is generally accepted to be an E-directed low-angle normal fault that places unmetamorphosed Paleozoic carbonates above medium-grade Precambrian to Cambrian metasediments. Previous workers recognized evidence for both E and W shear senses in mylonitic rocks below the NSRD, and interpreted them as either (a) conjugate shear bands formed during coaxial extension or (b) localized W-dipping shear bands in the wake of boudins in a predominantly E-directed shear zone.

Our observations in calc-mylonites below the NSRD in the NE Snake Range provide new constraints on the evolution of the detachment. (1) Mafic dikes caught up in the mylonite zone provide useful markers for the amount and sense of shear strain. (2) The detachment zone includes low-strain domains in which an early (pre-detachment) foliation is preserved in coarsely crystalline marble. The dikes cut at high angles across this fabric. (3) E-directed shear is indicated by E-dipping shear bands on 1 mm to 20 cm scale in marble, intensifying into detachment-parallel zones more than 10 m thick of calc-mylonite containing attenuated E-vergent folds, and asymmetric dolomite and mica porphyroclasts. Dikes in these high-strain zones form E-asymmetric boudins, and folds have highly attenuated limbs with hinges that roll over into geometries resembling delta-type porphyroclasts with a top-E sense of shear. (4) All these structures are cut and overprinted by 1 cm to 10 m scale W-dipping W-directed shear-bands, which deform the dikes, form W-directed porphyroclasts within the main fabric, and are seen to cut and overprint the E-directed shear indicators. Westward shear is transferred onto detachment-parallel shear zones; where these coincide with earlier E-directed shear zones, highly complicated structures representing both E and W directed shear are produced, including sheath folds and dikes that have been both folded and boudinaged.

Possible explanations consistent with these observations include: (i) The W-directed shear is related to deformation associated with passage of the footwall through an upper rolling-hinge in an overall E-directed shear zone; and (ii) displacement has been transferred from W-directed normal faults in the upper plate at a late stage in the evolution of the E-directed NSRD.