Structural and topographic evolution of the central Transverse Ranges, California, from apatite fission-track, (U-Th)/He and digital elevation model analyses

Abstract

Apatite fission-track (FT) and (U–Th)/He analyses are used to constrain the low-temperature thermal history of the San Gabriel and San Bernardino Mountains (SGM and SBM), which are part of the southern California Transverse Ranges. FT ages from 33 SGM samples range from 3 to 64 Ma. Helium ages, ranging from 3 to 43 Ma, were obtained from 13 of these samples: all of the He ages are the same or younger than their respective FT ages. FT ages from 10 SBM samples were older, ranging from 45 to 90 Ma. The FT and He data document at least three phases of cooling in the SGM, but only two in the SBM. Prior to ~7 Ma, the thermal history of the SGM appears to have been nearly identical to many of the core complexes in the Basin and Range of south-eastern California: a major phase of cooling is indicated from ~60 to 40 Ma, with a more recent phase beginning at ~23 Ma and continuing until ~10 Ma. The similarity of this timing to that of core complexes suggests that the SGM also originated as a core complex, when the rocks were adjacent to the Chocolate–Orocopia Mountains, and that some of the range-bounding faults were initially extensional. In the SBM, the two phases of cooling documented by the FT data occurred from ~65 to 55 Ma, and from ~18 Ma to the present. The timing on the second phase is very poorly constrained and, therefore, we do not speculate on the origin of the SBM. The most recent phase of cooling appears to have begun at ~7 Ma in the SGM, as the result of the onset of contractional deformation. A more accelerated phase of cooling may have begun at ~3 Ma. Distinct variations in the total amounts and rates of cooling between different fault-bounded blocks within the SGM are documented since 7 Ma. We use these variations in cooling rates to calculate denudation rates, which are then compared to topographic characteristics for each structural block. These comparisons suggest that more rapid bedrock uplift in the eastern and southern part of the range has strongly affected the present-day physiography. Despite a higher mean elevation, the SBM are much less dissected than the SGM, suggesting that the most recent phase of cooling and bedrock uplift began in the last 3 Myr, much later than the initiation of recent bedrock uplift in the SGM.

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