Reconstruction of Normal Fault Blocks in the Ann-Mason and Blue Hill Areas, Yerington District, Lyon County, Western Nevada
The Yerington porphyry copper-skarn-iron oxide-copper-gold district is a classic area of continental extension, having been extended more than 150% by multiple generations of Cenozoic east-dipping normal faults that penetrated a minimum of 8 km depth into the crust, initiated at high angles, and rotated to shallower dips until being cut by younger sets of faults. This study examines the Cenozoic normal faults in the vicinity of the Ann-Mason and Blue Hill areas through detailed mapping of two major faults, logging intervals of drill core containing the fault damage zones, and constructing fault surface maps, i.e., geologic maps of the proximal footwall and hanging wall of faults superimposed on structural contour maps of the fault planes. Six normal faults, representing four geometric sets or temporal generations of faults numbered from oldest to youngest are described and analyzed, with particular emphasis on the oldest two generations. Fault surface maps constrain the slip vectors of the faults and their variability along strike. Faults of the latest three generations strike northerly and dip easterly. Faults of the second generation, which are of middle Miocene age, include the Blue Hill (~2.8 km slip with increasing displacement to the north) and Singatse (3.7 km slip) faults. These second-generation faults have damage zones that persist ~15 m on either side of the fault and have hanging-wall splays that merge into the main fault surface. The first generation faults are represented by the 1A fault, which is one of a series of sub-parallel, generally small-offset faults that presently strike southeast and dip steeply to the southwest. Analysis of drill hole data indicates that the 1A fault, which might have the most amount of slip of any fault in the set, has ~230 m of apparent sinistral separation. The incremental untilting of the three later generations of faults restores the 1A fault to a steeply south-dipping fault with normal, dip-slip displacement. The 1A fault appears to connect with a fault that is exposed at the surface west of the area of drill hole constraints and cuts the Weed Heights member of the Mickey Pass Tuff (27 Ma) and is cut by the middle Miocene Singatse fault, bracketing its timing.
The structural contour maps reveal new insights into the subsurface geometry of several faults. The Blue Hill fault presently dips ~22° southeast along the northwestern side of a large mullion in the fault plane. A stepwise reconstruction indicates that all faults had initial dips of ≥ 60°, with the Singatse initiating at 73°. The significance of the first generation of faults at Yerington, though poorly understood, may have counterparts in eastern Nevada, where sets of easterly striking normal faults also formed prior to periods of extreme extension. The restorations demonstrate that mineralization in the Ann-Mason and Blue Hill areas originated in the same dike swarm, and thus are genetically related, and that another dike swarm to the south passing near the Casting Copper-Ludwig septum could have porphyry mineralization to the northwest at depth on one or both sides of the Blue Hill fault.