Retroarc foreland basins record the timing and magnitude of uplift and erosion within the interiors of mountain belts, and the nature of the sediments filling such basins can be used to investigate the nature of deformation at continental margins. This project is evaluating the timing of foreland deformation and deposition in retroarc foreland basins in the southwestern North American Cordillera to constrain the timing of magmatism in the continental margin magmatic arc and shortening in the foreland. High precision dates from plutons in the Cordilleran arc suggest high volume batholith emplacement was a relatively short-lived phenomenon superimposed on a much less voluminous background magmatic flux. Precise geochronologic control on evolution of the foreland fold and thrust belt is necessary to better establish the nature of orogen-wide stress field over long time scales, and thus clarify whether foreland shortening is a cause or consequence of arc magmatism.
In this study, of the McCoy basin of southeastern California/southwestern Arizona and in equivalent basins in Sonora Mexico are being studied. The age and source area of these sediments is being determined by dating detrital zircons using a combination of ion microprobe and inductively coupled plasma-mass spectrometer techniques. Previous application of ion microprobe dating to the lowermost and uppermost parts of the McCoy Mountains Formation has demonstrated that most of the formation is Cretaceous in age, in contrast to previous studies, which have suggested that much or most of the unit was deposited during the Jurassic. Furthermore, dating of granitic rocks that intrude the McCoy Mountains Formation in southeastern California has shown that the most of the formation is older than about 75 million years. However, studies in southwestern Arizona by previous workers have implied that a significant fraction of the unit must be younger than 78 million years. Unless rates of sedimentation in Arizona were anomalously high, this requires either that the age estimates in Arizona are incorrect or that the McCoy Mountains formation must young significantly to the east, which would also imply that deformation migrated eastward with time. These two alternatives are being discriminated using the zircon dating techniques used in this study.
Deposition of the McCoy Mountains Formation is believed to be largely the result of deformation in the Maria fold and thrust belt. Thus, brackets on the timing of the Maria deformation will be provided by the dating of McCoy sediments. In addition, various isotopic techniques are being utilized to directly date structures within the Maria belt. These include potassium-argon dating of minerals that grew in the McCoy Mountains formation during metamorphism of the Maria belt and uranium-lead determination of the igneous age of granitic rocks that intruded synchronously with deformation.
Preliminary data from this study suggest that lag time for foreland basin detrital zircon deposition is very short; thus this retroarc foreland basin sequence can yield a record of whether lithospheric thickening by foreland shortening may have contributed to high volume batholith magmatism, or whether shortening served as a space-making mechanism or gravitational response to batholith emplacement and lithospheric thickening in the arc.
