TEMPORAL CONSTRAINTS ON THE LANDSCAPE RESPONSE TO RIFTING IN THE LORETO REGION OF BAJA CALIFORNIA SUR, MEXICO
We utilise apatite fission track (AFT) and apatite (U-Th)/He (AHe) low-temperature thermochronometry performed on samples collected from the basement piedmont to show that footwall denudation and escarpment exhumation occurred at ~5.5 Ma. A closely overlapping 40Ar/39Ar age obtained from a lava overlying the piedmont surface corroborates this timing and indicates that escarpment exhumation occurred rapidly. West of the escarpment crest, we obtain 40Ar/39Ar ages both from canyon-incised and canyon-filling lavas, in order to constrain the timing of canyon incision, which we use as an independent proxy for the timing of rift flank uplift. These ages closely bracket the timing of canyon incision with the timing of Loreto fault footwall denudation. In the known absence of a shift to a wetter climate at this time, attributing widespread canyon incision to surface uplift is reasonable.
These data require rift flank uplift and canyon incision to occur concurrently with denudation of the Loreto fault footwall and exhumation of the Main Gulf Escarpment; the close temporal association suggests a causal relationship, with rift flank uplift likely occurring as a result of flexural footwall uplift. This indicates that rift flank uplift did not occur due to regional doming prior to rifting, which has important implications for the history of the Gulf of California rift.