Cretaceous Extension and Tertiary Translation Along the S.W. Edge of North America? Evidence from the Valle Group: Vizcaino Terrane

The late Mesozoic Valle Group of western-central Baja California was deposited atop the leading edge of the North American plate, making it a sensitive recorder of tectonic activity deforming the edge of the plate. Recent stratigraphic, sedimentologic and paleomagnetic work on the Valle Group provides the following insights into the evolution of the plate margin. (1) The edge of North America experienced brittle extension in mid-Cretaceous time. That deformation is coincident with the timing of peak blueschist metamorphism in the lower plate of the convergent margin, and may be related to initial uplift of the high-pressure rocks that now lie faulted against unmetamorphosed Valle Group strata. (2) Data of various kinds taken from the Valle Group provide ambiguous or directly contradictory answers to the question of Vizcaino terrane motion. Paleomagnetic data robustly show that Valle Group strata and underlying basement rocks traveled significantly farther than that required to open the Gulf of California. Supporting terrane travel are U-Pb data from granitoid clasts in Valle conglomerates suggesting that the clasts were not likely derived from any nearby provenance. Faunal data from the Valle Group, weakly suggest that the paleomagnetic data are in error; in particular the presence of Collignonicerous woollgarl in the Valle Group may link the Valle Group to a more boreal, rather than Tethyan province. Likewise, isotopic data from quartzite clasts in Valle conglomerates seem to link the basin with nearby sources located in the ancient passive margin of southwestern North America.