A NEW 3-DIMENSIONAL VIEW OF THE MID-MIOCENE VOLCANIC AND PLUTONIC ROCKS OF THE RIVER MOUNTAINS, SOUTHERN NEVADA

Cooperative scientific and engineering studies in the River Mountains (RM) related to major tunneling projects have provided a new 3-D view of the volcanic field. Projects by the Southern Nevada Water Authority's Intake #3 project and the Clean Water Coalition's System Conveyance and Operations Program (SCOP) have resulted in new field studies and over 25 borings with recoverable core. We highlight three of the most significant observations: (1) The RM were separated from the Wilson Ridge Pluton by westward motion along the Saddle Island (SI) detachment fault. The RM were thought to represent the volcanic upper plate without exposed plutonic or hypabyssal roots (except for the RM stock). Recent studies indicate that fine-grained intrusive rock is more abundant at the surface and in the shallow subsurface and that the upper plate of the SI fault contains a considerable volume of the hypabyssal part of the subvolcanic pluton. Evidence includes: porphyritic dacite (D2), originally mapped with fault contacts, is now thought to be intrusive. D2 is characterized by basalt enclaves, andesite inclusions, and diorite inclusions and enclaves. Brecciated D2 also contains dacite inclusions with diorite enclaves. Along the SCOP tunnel alignment, D2 was encountered in boring RMT3-107 at a depth of 345 ft. Furthermore, coarse-grained dacite containing enclaves of diorite occurs in boring RMT3-113 at a depth of 637 ft beneath a section of dacite flows. Based on these observations, a subvolcanic pluton may underlie much of the central RM at a shallow depth. (2) A new vent for basalt was identified in the eastern RM. The vent contains scoria, agglomerate, dikes varying from plagioclase-rich to plagioclase-poor basalt, and volcanic bombs (to 1 m in size). Basalt flows related to the vent have abundant plagioclase. The vent may be the source of basalt in the northern and central RM. (3) The surface trace of the SI detachment fault is less than 200 m long, but borings related to the Intake #3 project intersected the fault 0.3 km west of the exposed fault on SI, and penetrated the altered volcanic section of the upper plate beneath the floor of Lake Mead nearly 3 km to the east of SI. The observed length of the SI fault has been increased by over an order of magnitude by the study of core, thus further supporting the role of the SI fault as a major crustal structure.