Collaborative Research: Timing and Geodynamic Significance of Mesozoic Basement Tectonism and Basin Evolution in the Southwestern North American Cordillera

弧后前陆盆地记录了山带内部隆升和侵蚀的时间和幅度，沉积盆地的性质可用于研究大陆边缘的变形性质。本项目旨在评价北美科迪勒拉西南弧后前陆盆地的前陆变形和沉积时间，以约束大陆边缘岩浆弧的岩浆活动时间和前陆的缩短时间。从科迪勒拉弧的岩体中获得的高精度数据表明，大体积的岩基侵位是一个相对短暂的现象，叠加在体积小得多的背景岩浆流上。为了更好地确定长时间尺度的全造山带应力场性质，阐明前陆缩短是弧岩浆作用的原因还是结果，有必要对前陆褶皱和冲断带的演化进行精确的年代学控制。在这项研究中，正在研究加利福尼亚州东南部/亚利桑那州西南部的McCoy盆地和墨西哥索诺拉的等效盆地。通过结合离子探针和电感耦合等离子体质谱技术测定碎屑锆石的年代，确定了这些沉积物的年龄和来源区域。先前对McCoy山组最下部和最上部进行的离子探针测年表明，大部分地层的年龄是白垩纪，与之前的研究相反，之前的研究表明，该单元的大部分或大部分沉积在侏罗纪。此外，对侵入加州东南部麦考伊山脉地层的花岗岩进行的年代测定表明，大部分地层的年龄超过7500万年。然而，以前的工作人员在亚利桑那州西南部进行的研究表明，该单元的很大一部分必须小于7800万年。除非亚利桑那州的沉积速率异常高，否则这就要求要么亚利桑那州的年龄估计是不正确的，要么麦考伊山脉的地层必须在向东明显年轻，这也意味着变形随着时间的推移向东迁移。这两种选择是通过本研究中使用的锆石定年技术加以区分的。McCoy山组的沉积被认为主要是由于Maria褶皱和冲断带的变形。因此，通过对McCoy沉积物的测年，可以提供关于玛丽亚变形时间的托架。此外，正在利用各种同位素技术来直接确定玛丽亚带内的结构日期。这些方法包括对玛丽亚带变质时期McCoy山脉地层中生长的矿物进行钾氩测年，以及对与变形同步侵入的花岗质岩石进行铀铅测年。本研究初步资料表明，前陆盆地碎屑锆石沉积滞后时间很短；因此，这一弧后前陆盆地序列可以提供记录，说明由前陆缩短引起的岩石圈增厚是否可能导致了大体积的岩基岩浆活动，或者岩石圈的缩短是否作为一种造空机制或对弧内岩基侵位和岩石圈增厚的重力响应。

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.