No abstract

Pliocene (ca. 3.5 Ma) removal of dense eclogitic material under the Sierra Nevada has been proposed from variations in the petrology and geochemistry of Neogene volcanic rocks and their entrained xenoliths from the southern Sierra. The replacement of eclogite by buoyant, warm asthenosphere is consistent with present-day seismologic and magnetotelluric observations made in the southern Sierra. A necessary consequence of replacing eclogite with peridotite is that mean surface elevations and gravitational potential energy both increase. An increase in potential energy should increase extensional strain rates in the area. If these forces are insuffi cient to signifi cantly alter Pacifi c-North American plate motion, then increased extensional strain rates in the vicinity of the Sierra must be accompanied by changes in the rate and style of deformation elsewhere. Changes in deformation in California and westernmost Nevada agree well with these predictions. Existing geologic evidence indicates that a period of rapid uplift along the Sierran crest of more than ~1 km occurred between 8 and 3 Ma, most likely as a consequence of removal of lower lithosphere. About this same time, extensional deformation was initiated within ~50 km of the eastern side of the Sierra (5-3 Ma), and regional shortening began to produce the California Coast Ranges (5-3 Ma). We suggest that these events were induced by the >1.2 × 10 12 N/m increase of gravitational potential energy generated by the Sierran uplift. Evidence for Pliocene uplift, adjoining crustal extension, and shortening in directly opposing parts of the Coast Ranges is found along nearly the entire length of the Sierra Nevada and implies that lithosphere was removed beneath all of the presentday mountain range. The uplifted area lies between two large, upper-mantle, high-Pwave-velocity bodies under the south end of the San Joaquin Valley and the north end of the Sacramento Valley. These high-velocity bodies plausibly represent the present position of material removed from the base of the crust. Lithospheric removal may also be responsible for shifting of the distribution of transform slip from the San Andreas Table 1) where estimates have been made of the timing of initial extension, the western edge of extension at ca. 5 Ma (thick purple line) and ca. 3 Ma (thick red line), the location of fl oras showing possible uplift (blue dots: T-Table Mountain and W-Webber Lake localities of Wolfe et al. [1998, 1997]), and the extent of tilted Miocene sedimentary rocks along the Sierra/ Great Valley margin (hatched area). The geology in the Sierra is shown in B (after Wakabayashi and Sawyer, 2000) along with the location of the ancestral Yuba River channel plotted in Figure 3 (thin red line) and the positions (bold letters) of other paleochannels plotted in Figure 3: M-Mokelumne River; S-Stanislaus River; and T-Tuolumne River. The position of the Gorda plate's southern edge relative to the Sierra at different times in the past lies along the green lines (from Atwater and Stock, 1998) assum...

▪ Abstract  Cenozoic extension in the western United States presents a complex interrelation of extension, volcanism, and plate boundary tectonics that defeats simple notions of “active” or “passive” rifting. Forces driving extension can originate at plate boundaries, through basal traction, basal normal forces, or from buoyancy forces internal to the crust and lithospheric mantle. The latter two are most responsible for driving extension where it is observed in the Basin and Range. The complex evolution of the northern Basin and Range probably represents removal or alteration of mantle lithosphere interacting with buoyancy stored in the crust. In contrast, crustal buoyancy forces combined with a divergent plate boundary between about 28 and 16 Ma to drive extension in the southern Basin and Range. The central Basin and Range most likely extended as a result of boundary forces external to itself but arising from buoyancy forces elsewhere in the western United States.

Dynamic topography is commonly understood to be deflection of the Earth's surface that results from convection of the mantle. Because different authors use the words "dynamic topography" differently, topography designated as dynamic may amount to only a few hundred meters or may exceed 2000 m. For most regions, however, surface heights computed on the assumption that the lithospheric column is in isostatic equilibrium provide good approximations to observed topography. The small free-air gravity anomalies and still smaller isostatic anomalies (<~30-50 mGal) associated with long-wavelength topography suggest that deflections of the Earth's surface induced by flow-induced normal tractions applied to the base of the lithosphere do not exceed~300 m. Little evidence exists to show that such tractions support more than~100 m of high terrain in regions like southern Africa, the Rocky Mountains and Colorado Plateau of the USA, eastern Asia, or the Aegean-Anatolian region, where some have argued for many hundreds to >2000 m of dynamic topography. Moreover, simple examples show that if flow-induced stresses maintain a given density distribution, then the resultant surface deflections can be smaller than those that would exist in isostatic equilibrium. In light of confusion over the term "dynamic topography," we offer readers simple tools that we hope will enable them to diagnose the component of topography that results from flow-induced stresses and to distinguish it from topography that is compensated isostatically.