TIMING AND TECTONIC SETTING OF THE LATE PALEOZOIC SOUTHWESTERN LAURENTIAN BORDERLAND: DIFFERENT AND DISTINCT FROM THE ANCESTRAL ROCKY MOUNTAIN OROGEN

Broadly coeval crustal deformation and basin formation across southwestern Laurentia, both adjacent to the western continental margin and within the intracratonic Ancestral Rocky Mountains province (ARM), have long been inferred to possess a common tectonic mechanism. Nevertheless, spatial patterns of deformation, foredeep development, and provenance of allochthonous eugeoclinal arenites across central Nevada, southern California, and Sonora, a region here termed the Southwestern Laurentian Borderland (SLAB), are well explained by regionally distributed Late Devonian-Middle Permian sinistral transpression.

Diachronous SLAB foredeep initiation ranged from Late Devonian-early Mississippian (Antler basin in central Nevada), to early Pennsylvanian-Early Permian (Bird Spring basin of southern Nevada), to Middle Permian (Mina México basin in Sonora). Adjacent thrust wedges consist of imbricated chert, uncommon basalt and Paleozoic turbidites. Lower Paleozoic deep-water sandstones from Nevada and Sonora contain a distinctive Paleoproterozoic Trans-Hudsonian (~1.8 Ga) zircon population derived from western Canada, indicating that these allochthons were likely transported tectonically southward relative to the continental margin. Grenville zircon ages (~1.0 Ga), ubiquitous in Paleozoic sandstones of Mexico and the United States south of the Transcontinental arch, are absent from the eugeoclinal sandstones and thus corroborate their Cordilleran affinity. Throughout the interval of diachronous SLAB foredeep subsidence, repeated times of sediment accumulation and unconformity development took place in central Nevada, indicating a protracted response to continental-margin tectonics. We infer that SLAB deformation and basin development took place during sinistral displacement of outboard basinal terranes and continental fragments severed from the continent. The borderland orogen thus constitutes a heterogeneous assemblage of lithospheric domains, a southern extension of the Northwest Passage orogenic system, which resulted from northward migration of Laurentia past the outboard terranes. In contrast, Pennsylvanian-Early Permian ARM deformation was a consequence of progressive development of the Ouachita-Marathon suture on the adjacent flank of Laurentia.