INTERACTION OF TECTONIC AND IGNEOUS PROCESSES AT THE WEST-TRENDING VOLCANIC BELTS OF THE GREAT BASIN

Relations between continental extension and magmatism vary in time and space and the ways in which these processes interact remain uncertain. In the Great Basin, voluminous Cenozoic volcanism migrated southward from ca. 40 Ma near the Idaho-Nevada border to ca. 12 Ma in southern Nevada, whereas Basin-Range extension began at ~17-16 Ma across much of the province. Extension thus post-dated the main pulse of volcanism in many areas, but overlapped or predated it in others. Volcanic centers are concentrated in W-trending belts spaced 100-200 km apart and, apart from pyroclastics erupted from the W-trending belts, Cenozoic igneous rocks are sparse or absent elsewhere. Nonvolcanic areas have classic Basin-Range topography governed by broadly north-striking normal faults that terminate at intersections with W-trending volcanic belts. W-trending volcanic belts have three-dimensional fault structures and complex topography. Ignimbrites erupted from the W-trending volcanic belts range in volume up to > 3000 km3; the typical 10:1 volume ratio of intrusive rocks to associated volcanic rocks implies that large volumes of plutonic rock were emplaced in the W-trending belts. These observations pose several important questions about 3-D crustal and lithospheric structure that may be addressed as part of EarthScope. (1) How does the volcanic pattern reflect the distribution of magmatism in general? Are the crust and lithosphere of nonvolcanic areas largely unaffected by Cenozoic magmatism—in which case we expect pronounced contrasts in seismic velocity structure between volcanic and nonvolcanic areas—or were the lithosphere and lower crust pervasively intruded by mantle-derived magma, and volcanism only was focused into discrete belts near the surface, e.g., by pre-existing structures? (2) To what extent did Cenozoic intrusion modify the structure and rheology of the crust and upper mantle, over what depth range, and how is this expressed in contemporary tectonics and regional geomorphology? (3) Why do the W-trending belts segment Basin-Range faults? Were crustal mechanical properties modified by mid-Tertiary magmatism, or it is that the W-trending volcanic belts follow older structures (e.g., the Uinta graben system) that may have been more mechanically significant than effects of Cenozoic magmatism?