2014 – Continental Extension Old and
New at the Edge of the Mojave
CONTINENTAL EXTENSION OLD AND NEW AT THE EDGE OF THE MOJAVE
November 8-9, 2014
FIELD TRIP OVERVIEW
This year’s Pacific Section SEPM Fall Field trip will look at an excellently exposed section of the Meso-Neoproterozoic Pahrump Group (pre-Cryogenian Crystal Spring, Horse Thief Springs and Beck Spring Formation, Cryogenian Kingston Peak Formation and Ediacaran Noonday and Ibex Formations) in the Silurian Hills, as well as newly-described Miocene rock avalanche megabreccias in the northern Salt Spring Hills, with the goal of understanding the challenges of how to fit the deposits into a broader basin analysis context. Detailed correlations with regions outside the Mojave are challenged by complex faulting, magmatic overprinting, and a poor understanding of original basin geometry.
The Silurian Hills and Salt Spring Hills are found at the northern border of the Mojave Desert, at the eastern terminus of the Garlock Fault and south of Death Valley. During the Meso-NeoProterozoic, this region shifted from one of intracratonic platform to the leading edge of a new passive continental margin (Laurentia, formed from rifting of the older Rodinian supercontinent) .
North and northeast of the Silurian and Salt Spring Hills, , Neogene extension has exposed large swaths of time-correlative Proterozoic strata in such well-studied locales as the Panamint Range, Saddle Peak Hills, Kingston Range, and Alexander Hills. To the east lies the Shadow Valley, the location of spectacular Neogene extensional basins with thick sedimentary deposits, including enormous megabreccia sheets representing far- traveled rock avalanches. South and southeast, significant exposures of Mesoproterozoic (and rare Paleoproterozoic) basement complexes of the Mojave Province are found in low relief hills. Recent crustal thinning in the area enabled extensive basaltic volcanism and young cinder cones and flows found throughout the area. Directly south and west of the area, late Permian through Cretaceous granitoids formed during the arc magmatism which obliterated the older rock record, although greenschist-grade windows of metasediments are preserved.
In contrast to the extensive normal-faulting within the Basin and Range province, the Mojave Desert is dominated by strike-slip faults and lateral translations which make direct correlations difficult. Similarly, understanding the geologic setting during major Miocene unroofing and extension is challenged by lateral translation, magmatism and uplift, and lack of appreciation for long run-out rock avalanches. . The Salt Spring Hills are the westernmost expression of the megabreccia deposits well-described from the nearby Shadow Valley area and Valjean Hills.