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

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 Moun tains (SGM and SBM), which are part of the southern California Transverse R anges. FT ages from 33 SGM samples range from 3 to 64 Ma. Helium ages, rang ing 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 docum ent at least three phases of cooling in the SGM, but only two in the SBM. P rior to similar to 7 Ma, the thermal history of the SGM appears to have bee n 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 simila r to 60 to 40 Ma, with a more recent phase beginning at similar to 23 Ma an d continuing until similar to 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, t he two phases of cooling documented by the FT data occurred from similar to 65 to 55 Ma, and from similar to 18 Ma to the present. The timing on the s econd 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 similar to 7 Ma i n the SGM, as the result of the onset of contractional deformation. A more accelerated phase of cooling may have begun at similar to 3 Ma. Distinct va riations in the total amounts and rates of cooling between different fault- bounded blocks within the SGM are documented since 7 Ma. We use these varia tions in cooling rates to calculate denudation rates, which are then compar ed to topographic characteristics for each structural block. These comparis ons 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, sugge sting 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.