ROLES OF PLUTONISM, MIDCRUSTAL FLOW, TECTONIC RAFTING, AND HORIZONTAL COLLAPSE IN SHAPING THE MIOCENE STRAIN FIELD OF THE LAKE MEAD AREA, NEVADA AND ARIZONA

The Lake Mead area contains a well-exposed record of highly contrasting Neogene structural development that includes major magmatism and extension in its southern part and large-magnitude lateral translations (to 65 km) of amagmatic structural blocks in its northern part. These highly contrasting parts are joined at the Hamblin Bay fault, a major crustal boundary that forms the south margin of the Lake Mead fault system and the north margin of the lower Colorado River extensional corridor. Intrusive rocks adjacent to this crustal boundary on the south record major crustal rifting normal to an 350 degrees axis. They also display steep-axis disharmonic folds that formed by postmagmatic extension-related horizontal collapse of highly weakened crust. The rocks directly north of the crustal boundary not only display large lateral translations, but they also are cut by fault systems suggestive of diverse paleostress conditions and formed into diversely oriented coeval folds collectively producing a complex Neogene deformation field that defies interpretation in terms of a uniform system of remote stresses. Because structures forming this complex deformation field range from outcrop scale to 65 km of lateral displacement and can not be uniformly partitioned according to relative age, we prefer tectonic models based on a protracted uniform dynamic process or superposed dynamic processes to models based on time-varying states of stress. We suggest that the large lateral displacements and extreme structural complexity record synextensional rafting of structural blocks atop a flowing undermass and concomitant contraction of the zone of flowage. Structural blocks north of the main zone of flowage (the Great Basin sector of the Basin and Range) moved southward and occluded to the block south of the zone (the Colorado River extensional corridor) as a large westward widening wedge of upper crust was carried to the west-southwest toward California on the flowing undermass. Where occlusion is almost complete in the northern Black Mountains, north-south shortening is estimated at 55 km. The narrow zone of strong rifting, plutonism, crustal softening, and horizontal collapse that developed at the north margin of the Colorado River extensional corridor is interpreted to result from contact with the zone of crustal flowage.