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Spatial and Temporal Patterns of Uplift and Erosion in the San Gabriel Mountains, CA: Clues from Topography and Geochronology

DiBiase, Roman A.; Whipple, Kelin X.; Heimsath, Arjun M.; Blythe, Ann E.
[email protected]

Earth’s surface topography encodes the rich story of the competition between uplift and erosion. However, extracting quantitative measurements of the patterns and timing of differential rock uplift requires untangling a complicated suite of surface processes whose erosion rates depend not only on tectonically driven topographic relief, but also on climate and rock strength. Furthermore, spatial patterns in topography may either reflect steady, but spatially variable forcing or be indicative of a transient landscape responding to a temporal change in relative base level.

Here we investigate the long-term (103-106 yr) pattern of uplift and erosion in the San Gabriel Mountains, CA using a combination of topographic analysis and results from new and published erosion rates (apatite fission track and detrital cosmogenic 10Be). We analyze channel long profiles to identify knickpoints that separate relict, slowly eroding landscapes from adjusting, rapidly incising landscapes; we estimate the timing of changes in rates of base level fall by projecting relict stream profiles and either directly measuring or inferring erosion rates for each channel segment; and we use new and published low temperature thermochronometry data to constrain temporal changes in exhumation rate.

In the western San Gabriel Mountains, the 300 km2 catchment of Big Tujunga Creek shows evidence of a two-staged increase in uplift rate in the form of main stem and tributary channel knickpoints that split the landscape into three physiographic domains. The upper (earlier) transient in the Big Tujunga catchment has nearly propagated through the entire fluvial network, with only isolated patches of an elevated low-relief weathering surface remaining. The lower transient is characterized by a dramatic inner gorge with over-steepened valley walls and fluvial hanging valleys. Using projected relict stream profiles as dynamic geomorphic markers, we find the age of the upper transient to be ca. 7-9 Ma, which likely corresponds to the initiation of uplift in the San Gabriel Mountains associated with the transfer of slip from the San Gabriel Fault to the current trace of the San Andreas Fault. The younger transient dates to ca. 1 Ma, which overlaps with estimates of activation of the San Jacinto Fault Zone, whose inception has been linked to increases in rock uplift rate in the nearby San Bernardino Mountains. In the eastern San Gabriel Mountains, only the younger knickpoint remains, due to more rapid rates of uplift and erosion.

 

AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013