Clement G. Chase - Geodynamics Research

Uplift of the Sierra Nevada of California
with M. Robinson Cecil and J. A. Wolfe

Stratigraphic analyses of paleochannel deposits located on the northwestern slopes of the Sierra Nevada suggest that early Cenozoic landsculpting processes were highly variable and that the Eocene river systems were capable of moving large amounts of coarse material. Such analyses contradict the conventional idea that Eocene Sierra Nevadan landscape was dominated by low-elevation, low-relief features formed by broad, low-gradient river systems prior to Neogene uplift. In fact, the nature of Eocene fluvial deposits in the northern Sierra Nevada is consistent with a landscape of higher elevation and paleostream systems that varied with climate.

Gravels from the deepest canyons of the ancestral Yuba River are dominated by large (cobble-sized), well-rounded clasts, suggesting that Eocene river systems had gradients sufficient to move large clasts appreciable distances. In at least four locations, rounded boulders with diameters of 1 - 3 m were present in these deposits. Additionally, paleostream deposits of the ancestral Yuba River reveal multiple episodes of downcutting and backfilling. Such variation in sedimentological regime requires a variation in stream power, something which is generally caused by either surface tilting or a change in discharge. Clast-size analyses of gravels in the pebble-sized upper gravels, on the other hand, are more consistent with decreased stream gradients.

Changing climatic regimes may be capable of explaining some of the episodic erosional and depositional features preserved in the Eocene paleovalleys independent of tectonic uplift or tilting, but the paleogradient calculations present more intense challenges. Leaves preserved in some of the finer-grain Eocene gravels were suitable for studies of paleoaltimetry using leaf morphology. It was while making extensive collections that Jack Wolfe died in a fall in the field.

Geoid Anomalies and Lithospheric Structure:  The Aspen Anomaly and Colorado Plateau
with D. Coblentz, A. Sussman, and J. Libarkin

Aspen Geoid Anomaly

The present-day lithosphere of the Central Rockies is a composite of structures that formed during lithosphere assembly and during later modification, including active tectonism.  The Aspen Anomaly (an upper-mantle low-velocity feature similar in horizontal scale and velocity contrast to the Yellowstone hotspot, but of lower amplitude) underlies the highest topography in this region and appears to coincide with Proterozoic lithospheric structures within the Colorado mineral belt. Deep incision is taking place in river gorges (Colorado, Gunnison, Arkansas) that drain the area, due to either rapid and recent rock uplift or marked climate change.

Because geoid anomalies in isostatically compensated regions can be directly related to the local dipole moment of the density-depth distribution, they can be used to evaluate the subsurface distribution of mass associated with various surface features.  Geoid anomalies caused by long-wavelength continental topography are proportional to the elevation multiplied by the mean depth of compensation. Thus, for a particular elevation, the greater the average depth of the isostatic "root," the larger the geoid anomaly.

The main complication in analyzing geoid data for this kind of problem is separating lithospheric geoid anomalies from those with deeper sources, especially those that arise in the lower mantle. The ratio of geoid anomaly amplitude to elevation is a useful indicator of the regional depth of isostatic compensation. At very long wavelengths, the geoid/elevation value can be interpreted as a one-dimensional problem.  For example, evaluation of the Colorado Plateau (with an average elevation of 2 km and an average geoid/elevation ratio of  ~ 7 m/km) indicates a Moho depth of 50 km, which implies that the isostatic compensation of the Plateau can be entirely crustal. 

Because it is inappropriate to treat the geoid/elevation relationship in the Central Rockies as one-dimensional, we have divided a NASA-provided geoid and topography into 0.25 arc degree blocks and analyzed the results at various wavelengths in order to evaluate the contribution to the geoid from mass sources at various ascending depth limits between 700 and 90 km.  The geoid/elevation ratio for the Central Rockies is significantly less than for the Colorado Plateau, ranging between 3 and 4 m/km suggesting a large degree of the compensation must reside in the upper mantle.

Flexure and Geoid Anomalies of the Southern American Plate
with K. Connelly

Calculated flexure of the     South American lithosphere	Filtered geoid anomalies	Filtered geoid in color, flexure    in dashed lines

Correlating the three-dimension flexure of a uniform elastic plate with geoid anomalies:

Using the mass of the topographic load of the Andes mountains (elevations above 800 m) and an elastic plate thickness of 50 km, the flexural response of the lithosphere is shown in the upper left-hand panel. The predicted flexure has a maximum amplitude of ~550 m and is located well out in the Amazon jungle. Any flexural moat would be buried in the alluvial fans to the west. Note that the curvature of the Andes affects the amplitude and location of the forebulge.

Geoid anomalies require careful filtering to remove more global deeper-mantle density distributions. Using a filter that passes wavelengths of ~4000 km and less, the upper center panel shows the result. Most of what we see is expected (30 m over the high parts of the range), but not the high values out in the Amazon region (~5 m). However, note the strong correlation between the outer geoid anomaly and the forebulge (upper right-hand panel, geoid in color, flexure in red dashed lines). Many features of both the flexure and the geoid are explained by these simple models.