STRATIGRAPHY AND STRUCTURE OF THE NORTHERN HIGHLAND RANGE, COLORADO RIVER EXTENSIONAL CORRIDOR, SOUTHERN NEVADA
The basal strata in the northern Highland Range are a thin (240 m) series of aphyric trachyandesites and crystal-rich trachydacite flows and breccia domes that sit nonconformably on Precambrian basement. Above this are 3000 m of monotonous intercalated trachyandesites and trachyandesite breccias, with a lithic tuff sequence (640 m) occurring in the middle part of the sequence. The regionally extensive Tuff of Bridge Spring (15.2 Ma) and Tuff of Mt. Davis (30-40 m) ignimbrites cap the trachyandesite sequence; boulder conglomerates comprise the remaining strata above the ignimbrites. Mainly N to NE-striking intermediate dikes variably cut the basement and overlying strata. Petrographic similarities between dikes and some flows suggest they likely served as feeders.
The strata are predominantly east-tilted 20° to 40°, and cut by a series of NW- to NE-trending, mostly W-dipping (40°-90°) normal faults with variable displacements (10’s to 1000+ m). In the southern part of the map area, which is at the northern boundary of the accommodation zone (a NW-plunging anticline), N-trending faults are cut by NW-trending faults. North of the accommodation zone boundary, faults mostly trend N or NE and a small basement horst is present. Dikes in the south are consistently N-striking but also change to either NE or NW-strikes in the north. Stratal tilts indicate the northern Highland Range is part of the east-tilted Lake Mead structural domain but the changing orientations of faults and dikes suggest a complicated strain field, perhaps a result of the western CREC breakaway fault (McCullough Range Fault) dying out at depth to the north beneath the mapping area.