Paper No. 10
Presentation Time: 11:15 AM


FRIDRICH, Christopher J. and THOMPSON, R.a., U.S. Geological Survey, Box 25046, DFC, MS 980, Denver, CO 80225,

The Cenozoic (≥40 Ma to present) basin-fill strata of the Death Valley region contain seven regionally extensive stratigraphic boundaries, mainly angular unconformities. Radical changes are present across these boundaries in the distribution of basin-fill strata, provenance of sediments, and patterns of internal facies/depositional environments. Based on field relations, these changes record the creation of new basins and the major faults that bound them, and the abandonment or major structural modification of older basins and their master faults – in most cases, during rapid step-wise westward migrations in the locus of tectonism. The Cenozoic strata consist of three sequences: pre-basin-range, syn-basin-range, and post-basin-range. These are subdivided by the regional unconformities into seven assemblages and one lacuna that formed in seven distinct tectonic stages. Each of the four stages of basin-range tectonism (from ~16 to ~12 Ma, ~12 to ~7 Ma, ~7 to ~4 Ma, and ~4 Ma to present) have coincided spatially and temporally with a stage of extension-related volcanism, each distinct in the rates and compositions of magma erupted.

Six of the regional tectono-stratigraphic boundaries correspond in their timing to well-dated tectonic reorganizations documented at the western boundary of the North American plate, to the west and southwest (as documented by Mammerickx and Klitgord, 1982 and others), and one corresponds to the onset of Yellowstone hotspot volcanism at the northern limit of the Basin-and-Range province (at ~16 Ma). The strike-slip faults and zones of steep-axis rotation of the Eastern California shear zone, which includes the Death Valley region as its central and largest part, currently accommodate as much as 25 percent of the right-lateral divergence between the Pacific and North American plates. The exceptional degree to which the tectonic reorganizations at the nearby plate boundary are expressed in the tectonic evolution and resulting tectono-stratigraphy of the Death Valley region is thus easily rationalized, but is remarkable nonetheless. It strongly supports models that explain the widely distributed transtensional tectonism in southwestern North America since ~40 Ma as resulting from traction imposed by the adjacent, divergent Pacific plate.