3 Lithosphere Stretching and Magmatism Leading to the Gulf of California

Thursday, 29 March 2012
8:30 AM - 1:00 PM
Siglo XVIII

TEMPORAL CONSTRAINTS ON THE LANDSCAPE RESPONSE TO RIFTING IN THE LORETO REGION OF BAJA CALIFORNIA SUR, MEXICO

Paper number: 3-11
Program: Theme Sessions
Time: 11:55 AM – 12:15 PM

Authors

Abstract

The Gulf of California is a young, highly oblique rift thought to have initiated in the Middle to Late Miocene. However, the precise chronology of the rifting and the development of rift-related structural and landscape features continues to be controversial. In this study, we examine the landscape response to rifting in the Loreto region of Baja California Sur. Here, the rift-bounding structure is the east-dipping dextral-normal Loreto fault. To the west of the fault, an eroded basement piedmont 3 – 5 km wide abuts the Main Gulf Escarpment, which attains elevations of 800 – 1200 m asl. West of the escarpment crest, the uplifted rift flank has been incised by a network of west-draining canyons, 300 – 700 m deep; many of the canyons are beheaded at the escarpment crest, implying a palaeo-drainage divide east of the modern escarpment crest. These canyons both incise, and are filled by, a series of lava flows which were erupted continuously from ~14 Ma onwards.

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.