Animation of delamination cartoon ...of line downwelling ...of point downwelling
Fig. 1. Cartoons highlighting some of the potential differences between removing dense lithosphere (purple) through (a) delamination and (b) convective instability. Subsidence of the surface (top surface) is blue, uplift yellow. The bottom panels (c) illustrate a kinematic concept where the downwelling focuses into a cylinder as upwelling remains linear. Such a possibility has not yet been simulated numerically. Note that (c) has an asymmetric removal of material, which we suspect is likely in view of the plutonic origin of eclogite, but again is untested to our knowledge. In all cases shear between the continent and underlying asthenosphere could skew the geometry of the downwelling material.
Fig. 2. Two time slices taken from an animation of Cenozoic magmatism in California and Nevada, prepared from the NAVDAT dataset. The full animation is available at http://navdat.geongrid.org/Navweb/MOVIE10.MOV. The area of interest for volcanic studies in this project lies within the blue ellipse; the lower edge of the ellipse is approximately at latitude 38N. The 9.6 Ma panel shows intensive volcanism in the Sonora Pass area, north of Yosemite National Park, and little in the Sierra south of there. The 3.4 Ma panel shows the Pliocene outburst in the southern Sierra and eastern California, an event that has been attributed to delamination (Manley et al., 2000). At this time there was apparently little volcanism in the northern Sierra (area of blue ellipse), but those rocks are scarcely known and the Pliocene event could be present there (see text).
Fig. 3. Stratigraphy of volcanic units in selected parts of California. Generalized stratigraphic columns for the Mojave Desert, southern Sierra Nevada, and proposed study area. Tan swath give the approximate time of passage of the Mendocino fracture zone under the various regions, based on the reconstruction of Atwater and Stock (1998). There is little correlation between these plate-boundary events and volcanism (see text). .
Fig. 5: Comparison of east-west geophysical profiles through the southern Sierra by Park (2004) and Boyd et al. (2004). Profiles at left are more typical geophysical profiles (P wavespeed and resistivity); at the right are more unusual sections (vp/vs and attenuation, resistivity-derived temperature and melt). Outlines of bodies from Boyd et al superimposed on all profiles; solid line is inferred garnet peridotite, dashed line outlines possible eclogites, dash-dot is likely warm spinel peridotite. In light of the resistivity structure, it is possible that the seismologically inferred eclogite is in fact melt-rich peridotite and the eclogite is in fact richer in garnet and present between the dashed and solid line bodies.
Fig. 6. Two views of Sierran geodynamics (a) eclogite and lithosphere only removed in the Kings-San Joaquin area and (b) eclogite and lithosphere removed from nearly the full length of the Sierra.
Fig. 7. A spinel peridotite (a) and a garnet amphibolite (b) from Jackson Butte. Peridotite is in cross polars, amphibolite in in plane view.
Fig. 8. Wt. %K2O, and Wt. % Fe2O3t/MgO vs. wt. % SiO2 for late Cenozoic igneous rocks at ~38°N in the northern Sierra Nevada. Data from Brem (1977), Priest (1979), Farmer et al. (2002), Roelofs (2004) and Farmer (unpublished).
Fig. 9 Initial eNd vs. age for northern Sierra Nevada volcanic rocks. Data from Farmer et al. (2002), Farmer (unpublished), and Roelofs (2004).
Fig. 10.
Fig. 11. Map of the southern Sierra Nevada and western Walker Lane belt showing results of kinematic analyses of background seismicity (modified from Unruh and Hauksson, 2004). Focal mechanisms from individual groups of earthquakes (numbered) have been inverted for components of a reduced strain rate tensor (Twiss and Unruh, 1998). Values of the vertical deformation parameter V, defined as the vertical component of the strain rate tensor normalized by the maximum extensional principal strain rate, are contoured to determine areas characterized by strike-slip faulting (V = 0), transtensional shearing (-0.7 < V < 0), horizontal extension (-1.0 < V < -0.7), and oblate flattening or "pancaking" of the crust (V < -1.0). Note that horizontal crustal extension in the southern high Sierra occurs in an approximately 25 km wide, north-south trending region directly east of the Isabella anomaly. There is a 20-30 km wide "transtensional" domain between the extensional southern High Sierra and the dominantly strike-slip Walker Lane belt. ALF = Airport Lake fault; LLF = Little Lake fault; OVFZ = Owens Valley fault zone.
Mihai text related to this: Insights into the deep lithospheric structure of the Sierra can also be gained from studying the exhumation history of the Sierra. Basement rocks from the southern Sierra have cooled through 70C (the closure temperature for the apatite U-Th/He thermochronometer) some 40-80 Ma (House et al., 2001 for a review) suggesting slow exhumation of the range since the early Cenozoic. In order to compare the geologic of the southern and northern Sierra throughout the Cenozoic, we will carry out an apatite U-Th/He study on basement rocks from the northern Sierra Nevada – where no exhumation data are published. Preliminary data obtained on a transect crossing the Sierra Nevada at the latitude of Sacramento overall shows old ages (60-100 Ma) with younger values in the core of the range (above). This suggests a steady state evolution of the topography and that the modern range mimics in shape a late Cretaceous relief.
Figure 3. Cave-derived river incision rates in South Fork Kings River canyon. A: Topographic profile across South Fork Kings River canyon in vicinity of Boyden Cave. Note ~2 km local relief. B: Inner gorge of South Fork Kings River canyon, containing suite of dated caves preserved by exceptionally steep canyon walls. These caves reveal order of magnitude decline in incision rate toward present. While oldest cave demonstrates 400 m of canyon cutting in past 2.7 m.y., larger context shown in A shows that this represents only ~20% of present local relief. (From Stock et al., 2004)
Figure 4. Response of South Fork Kings River to late Cenozoic
tectonic and climatic events. A: Conceptual model of late Cenozoic uplift.
Westward tilting steepens pre-uplift surface (dark gray) and river profile
(dashed red); surface uplift increases orographic precipitation on western
slope of range and enhances rain shadow to east. Thin crust beneath range crest
(Wernicke et al., 1996) likely reflects delamination of batholithic root (Ducea
and Saleeby, 1998). B: Example of stream power-based numerical simulation.
Steady river profile (dashed red) with steps corresponding to quartzite in
two metamorphic belts is subjected to ~1.5 km of crestal uplift. 1 m.y. profiles
(blue) show that over next 9 m.y., wave of rapid incision begins at hinge line
and propagates up profile. Inset shows 6 m.y. incision history at cave site;
wave of rapid incision passes between ca. 5 and 2 Ma, followed by return to
low pre-uplift rates (dashed curve after 2 Ma). Further reduction in late Quaternary
rates (solid
curve after 2 Ma) reflects sediment mantling of bed associated with large glaciers
in headwaters. Final modeled river profile (purple) fits modern profile (red)
to just upstream of cave site, above which glacial erosion, not represented
in our river incision rule, has dominated past few million years. (Stock et
al., 2004)