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起和侵蚀的时间和幅度，填充此类盆地的沉积物的性质可用于研究大陆边缘变形的性质。该项目正在评估北美科迪勒拉山脉西南部弧后前陆盆地的前陆变形和沉积时间，以限制大陆边缘岩浆弧的岩浆作用和前陆缩短的时间。科迪勒拉岛弧中的岩体的高精度年代表明，大量岩基侵位是一种相对短暂的现象，叠加在体积小得多的背景岩浆通量上。对前陆褶皱和冲断带演化进行精确的年代学控制对于更好地确定长期尺度上造山带应力场的性质是必要的，从而阐明前陆缩短是弧岩浆作用的原因还是结果。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麦科伊盆地以及墨西哥索诺拉州的类似盆地。这些沉积物的年龄和来源区域是通过结合离子微探针和电感耦合等离子体质谱技术对碎屑锆石进行测年来确定的。之前对麦科伊山地层最低和最高部分进行离子探针测年的应用表明，大部分地层的年龄是白垩纪，这与之前的研究形成鲜明对比，之前的研究表明该单元的大部分或大部分是在侏罗纪沉积的。此外，对侵入加利福尼亚州东南部麦考伊山地层的花岗岩进行的年代测定表明，该地层的大部分年龄超过约 7500 万年。然而，先前的研究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，否则要么亚利桑那州的年龄估计不正确，要么麦考伊山脉地层必须明显向东年轻，这也意味着变形随着时间向东迁移。使用本研究中使用的锆石测年技术来区分这两种替代方案。麦科伊山地层的沉积被认为主要是玛丽亚褶皱和冲断带变形的结果。因此，麦考伊沉积物的年代测定将提供玛丽亚变形时间的括号。此外，各种同位素技术也被用来直接确定玛丽亚带内结构的年代。其中包括对玛利亚带变质作用期间麦科伊山地层中生长的矿物进行钾-氩年代测定，以及对与变形同步侵入的花岗岩火成岩年龄进行铀-铅测定。这项研究的初步数据表明，前陆盆地碎屑锆石沉积的滞后时间很短；因此，这个弧后前陆盆地序列可以记录前陆缩短导致的岩石圈增厚是否可能导致大量岩基岩浆活动，或者缩短是否充当了弧后岩基侵位和岩石圈增厚的空间形成机制或重力响应。

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.