Understanding the Sierran Uplift
Many observations have been made in an effort to understand the tectonic history of the. Many of these observations have conflicted and a few have been misleading. The following is a summary of observations made in the Sierra and conclusions that might be drawn from them.
The disciplines to be summarized are:
GEOMORPHOLOGY
GRAVITY
MAGNETOTELLURICS
SEISMOLOGY
GEOMORPHOLOGY
Boreal and Chagoopa geomorphic surfaces found in the southern Sierra are assumed to indicate a recent tilting of the Sierra Nevada Block [Jones, 1987].
Tilting and incision of sedimentary and volcanic rocks on the western slopes of the Sierra indicate ~2 km of elevation gain in the Neogene time [Savage et al., 1994]
Tilted beds in the Eastern San Joaquin Valley may indicate an uplift age of 5 MA [Jones et al., 1994].
Eocene cover in western foothills could indicate uplift at ~25 MA [Malin et al., 1995]
A paleosurface map of the Boreal and Chagoopa geomorphic surfaces is shown here.[Jones, 1987]
Conclusion: The uplift of the Sierra Nevada must have occurred between 25 and 5 MA.
If a large crustal root is assumed, paleogravity measured just before uplift would produce a -120 mGal anomaly over the center of the root with a +70 mGal anomaly on either side due to flexural bending. This is substantially larger than values measured today over eroded island arcs. [Chase and Wallace, 1988]
Gravity observed today in the southern Sierra shows a decreasing trend to the east with a low of -240 MGals near the high Sierra. This trend continues into the Basin and Range with a slight (50 mGal) rebound in the Eastern Sierra [Oliver, 1977]. It is also highly correlated with topography. This could indicate that this gravity anomaly is a recent feature created with the topography. [Jones, 1987]
Gravity anomaly only seen between Mono Lake and Owens Lake which would discount the idea that the anomaly was a feature created from batholith emplacement.
The limits of the low gravity anomaly correspond well to boundaries of the Death Valley sub-province suggesting that the source of the anomaly is in the mantle. [Jones, 1987]
A more detailed gravity map of the southern Sierra can be found here.
Conclusion: The gravity anomaly observed is most likely caused by an upper mantle feature that was created during the recent uplift.
Increased conductivity zones are observed beginning at depths of 10-20 km and continuing on to depths greater than 100 km. This anomaly extends west underneath the Sierran crest and then continues on into the Central Basin and Range.[ Park et al., 1996]
The anomaly is attributed to partial melt because of its large depth range [ Park et al., 1996].
Click here to see a pseudo-depth section underneath the Sierra [ Park et al., 1996].
Conclusion: The conductive feature could be a result of mantle upwelling leading to partial melt.
SEISMOLOGY
Receiver function analysis found a moho depth of 32-37 km in the Eastern southern Sierra. In the western southern Sierra, the depth to Moho varied dramatically with backazimuth. For western arrivals, the depth was 39-42 km while arrivals from the east produced depths of 31-33 km. [Jones and Phinney, 1998]
Receiver function analysis also detected a mid crustal feature in the eastern Sierra as well as from an array located in the Inyo and Argus ranges farther to the east . This feature was marked by a sharp negative pulse in the receiver function. The depth of the feature shallowed to the east from 22.6 km under the Sierra to 16.7 km farther east . This feature is most likely associated with a shear wave low velocity zone. [Jones and Phinney, 1998]
For a receiver function overview see Charles Ammon's Page.
Measurements of Pn velocities underneath the Sierra has produced an anamolously low velocity of 7.6-7.65 km/s [Jones et al., 1994].
The same study produced a Moho depth of 33+/- 5km underneath the high Sierra.
P wave "Pauper's tomography" has shown the presence of a high velocity body known as the Isabella anomaly underneath the Sierra that is coherent from 70 to 190 km depth[Jones et al., 1994]. To see these results click here.
Conclusion: Moho depths shallow to the east . This is possibly a result of displacement due to extension in the east . This would most likely lead to upwelling leading to heating of the mantle lithosphere and low Pn velocities. This upwelling could force the displacement of older mantle lithosphere possibly seen in the Isabella anomaly.
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