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

支配大陆裂谷并最终导致大陆岩石圈破裂和海洋开放的基本过程仍然知之甚少。加州湾的伸展是研究这些过程的一个极好的天然实验室，因为它提供了从分布的大陆伸展到北美岩石圈的最终破裂和太平洋板块俘获下加利福尼亚的沿走向和跨走向的裂谷过程的图像。然而，这个裂谷的运动学和动力学模型关键取决于准确地了解伸展应变和横流应变在邻近大陆边缘-特别是索诺兰边缘-在空间和时间上的分布情况。该项目由构造计划和国际计划支持，正在量化索诺拉裂谷边缘新近纪变形和岩浆活动的分布、规模、时间和风格，以评估从早期(1200万年前)分布的弧后伸展到1200万年后与下加利福尼亚停止俯冲和向太平洋板块转移有关的转换的运动学。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，以揭示加利福尼亚州海湾附近最宽、最完全暴露、但最不为人所知的裂谷大陆边缘的伸展和走滑断层的历史。正在进行新的地质制图，以确定重要的断层系统，并确定当地构造、火山和沉积史的细节。调查人员正在详细破译索诺拉西南部从瓜伊马斯附近海岸到埃尔莫西洛以东的马扎坦山脉变质核60×100公里区域的构造演化。此外，正在对索诺拉东部的两个具有代表性的新近纪盆地进行构造和地层学研究，以评估晚中新世重大变形是否会进一步影响到内陆区。目前正在对关键的构造前、构造同期和构造后的火山单元进行大约60个新的高精度Ar-Ar年龄测定，以记录当地的火山和沉积历史，并确定构造事件的时间。K-长石-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、强度和速率。

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.