Neogene Evolution of the Sonoran Margin: The Transition from Backarc Extension to Transtensional Rifting

控制大陆裂谷作用并最终导致大陆岩石圈破裂和海洋张开的基本过程仍然知之甚少。 加州的transtensionalGulf代表了一个极好的天然实验室来研究这些过程，因为它提供了沿走向和跨走向的裂谷过程，从分布式大陆伸展到北美岩石圈的最终破裂和太平洋板块捕获下加利福尼亚。 然而，这个裂谷的运动学和动力学模型关键取决于对邻近大陆边缘--特别是索诺兰边缘--的伸展和横向应变在空间和时间上如何分布的准确理解。索诺兰裂谷边缘新近纪变形和岩浆作用的类型，以评估从早期过渡的运动学（前1200万年前）分布后弧延伸到后1200万年前的扭张变形与下加利福尼亚向太平洋板块的俯冲和转移的停止有关。 这项多学科的研究汇集了来自美国和墨西哥的科学家和学生，旨在揭示加州湾附近最广泛、最完整、但了解最少的裂谷大陆边缘的伸展和走滑断层的历史。正在进行新的地质测绘，以确定重要的断层系统，并确定当地构造、火山、和沉积历史。 调查人员正在详细破译索诺拉西南部一个60乘100公里区域的结构演变，从瓜伊马斯附近的海岸到埃莫西约以东的马扎坦山脉变质核。 此外，正在对索诺拉东部两个有代表性的新近纪盆地进行结构和地层研究，以评估是否有重大的晚中新世变形影响了进一步向内的地区。 大约60个新的高精度Ar-Ar年龄测定正在获得关键的前，同，和后构造火山单位，以记录当地的火山和沉积历史，并括号的时间结构事件。 钾长石Ar-Ar多域扩散、磷灰石（U-Th）/He和裂变径迹分析被用来确定大型正断层系统下盘的低温冷却历史，以评估伸展滑动的时间、幅度和速率。

The fundamental processes that govern continental rifting and ultimately lead to rupturing of continental lithosphere and opening of an ocean are still poorly understood. The transtensional Gulf of California represents a superb natural laboratory to investigate these processes because it provides both along-strike and across-strike views of the rifting process, from distributed continental extension to the final rupturing of the North American lithosphere and the capture of Baja California by the Pacific Plate. However, models for the kinematics and dynamics of this rift depend critically on understanding exactly how extensional and transcurrent strains are distributed in space and time on the adjacent continental margins - particularly the Sonoran Margin.This project, supported by the Tectonics Program and the International Program, is quantifying the distribution, magnitude, timing, and style of Neogene deformation and magmatism across the Sonoran rifted margin in order to evaluate the kinematics of the transition from earlier (pre-12 million years ago) distributed backarc extension to post-12 million year old transtensional deformation associated with the cessation of subduction and transfer of Baja California to the Pacific Plate. This multi-disciplinary study has brings together scientists and students from the U.S. and Mexico to unravel the history of extension and strike-slip faulting across the widest and most completely exposed, but least well understood rifted continental margin adjacent to the Gulf of California.New geologic mapping is being carried out to identify the important fault systems and to work out the details of local structural, volcanic, and sedimentary histories. Investigators are deciphering in detail the structural evolution of a 60 by 100 km area in southwestern Sonora, from the coast near Guaymas to the Sierra Mazatan metamorphic core east of Hermosillo. In addition, structural and stratigraphic studies are being carried out on two representative Neogene basins in eastern Sonora to assess whether significant late Miocene deformation affected areas further inboard. Approximately 60 new high-precision Ar-Ar age determinations are being obtained on key pre-, syn-, and post-tectonic volcanic units to document local volcanic and sedimentation histories and to bracket the timing of structural events. K-feldspar Ar-Ar multi-domain diffusion, apatite (U-Th)/He, and fission track analyses are being employed to determine low temperature cooling histories in the footwalls of large normal fault systems to assess the timing, magnitude, and rates of extensional slip.