Collaborative Research: Testing the Role of Transtension in Continental Rupture: An Integrative Study of the Sonoran Margin and Tiburón Basin, Northern Gulf of California

张性裂谷中应变分配的发展可能在确定大陆破裂的位置上起重要作用。晚中新世（1200 ~ 600万年前）加州原海湾伸展和右旋剪切的划分仍然是一个基本的未解决的问题，阻碍了对洋盆形成初始条件的理解。来自沿海索诺拉的初步证据支持一个区域模型，该模型在大约600万年前加州湾的板块边界运动本地化之前，在一个宽度不确定的区域内促进右旋transtension。尚不清楚晚中新世右旋剪切是否足够快或集中，以改变流变条件，并改变裂谷模式从分布式盆岭式伸展到局部岩石圈颈缩、沉降和大陆断裂。在这个项目中，美国和墨西哥科学家的目标是在板块边界局部化期间限制右旋剪切随时间的结构分布。具体来说，他们将确定是否晚中新世transtension在原加州湾是由弥漫的三维应变在一个广泛的区域同时代走滑和正常故障或发生在一个狭窄的区域离散右旋故障和/或旋转块嵌入一个更广泛的区域内的东西延伸。为了区分这些模型，该小组正在调查索诺拉大陆边缘的地质结构和盆地发展，包括邻近的蒂布隆岛和近海索诺拉大陆架：（1）通过详细的结构测绘和古地磁分析记录右旋剪切的分布;（2）利用构造重建、火山岩40 Ar/39 Ar定年、同构造沉积盆地和火山盆地沉降分析等方法定量研究张扭应变速率;（3）通过对近海Tiburon盆地多道地震资料的解释，将陆上地质记录与近海构造和盆地相结合，加州湾为研究大陆裂解过程提供了一个极好的天然实验室。以往的研究表明，温暖的大陆地壳倾向于抵抗局部变形，这是大陆岩石圈破裂所必需的。这项研究的数据将提高对北美西部温暖地壳如何以及为什么破裂形成加州湾的理解，该湾自大约600万年前以来一直在开放。该项目由美国国家科学基金会国际科学与工程办公室（NSF Office of International Science and Engineering）和NSF/OCE MARGINS项目支持。 这项研究涉及研究生和本科生的大量参与，与美洲原住民的合作，以及与墨西哥研究机构的强有力的国际合作伙伴关系。

Development of strain partitioning in transtensional rifts may play a significant role in localizing continental rupture. Partitioning of extension and dextral shear in the proto-Gulf of California during Late Miocene time (12 to 6 million years ago) remains a fundamental unresolved problem that hinders understanding of the initial conditions for ocean basin formation. Preliminary evidence from coastal Sonora is supportive of a regional model that promotes dextral transtension in a zone of uncertain width prior to localization of plate-boundary motion in the Gulf of California approximately 6 million years ago. It is not known whether Late Miocene dextral shear was sufficiently fast or focused to alter rheological conditions and change the mode of rifting from distributed Basin-and-Range-style extension to localized lithospheric necking, subsidence, and continental rupture. In this project U.S. and Mexican scientists aim to constrain the structural distribution of dextral shear over time during localization of the plate boundary. Specifically, they will determine whether late Miocene transtension in the proto-Gulf of California was accommodated by diffuse 3-dimensional strain in a wide zone of coeval strike-slip and normal faults or occurred in a narrow zone of discrete dextral faults and/or rotating blocks embedded within a broader zone of East-West extension. To discriminate between these models, the team is investigating the geologic structure and basin development of the Sonoran continental margin, including adjacent Isla Tiburon and the offshore Sonora shelf by: (1) documenting the distribution of dextral shear through detailed structural mapping and paleomagnetic analysis; (2) quantifying transtensional strain rates using structural reconstructions, 40Ar/39Ar dating of intercalated volcanic rocks, and subsidence analysis of syntectonic sedimentary and volcanic basins; and (3) integrating the onshore geologic record with offshore structures and basins by interpretation of existing multi-channel seismic data from the offshore Tiburon basin.The Gulf of California provides an excellent natural laboratory for studying processes of continental break-up. Previous studies have shown that warm continental crust tends to resist localized deformation, which is required for rupture of continental lithosphere. Data from this study will improve understanding of how and why the warm crust of western North America ruptured to form the Gulf of California, which has been opening since about 6 million years ago. This project is supported by the EAR Tectonics Program, the NSF Office of international Science and Engineering, and the NSF/OCE MARGINS program. The research involves significant participation of graduate and undergraduate students, collaboration with Native Americans, and strong international partnerships with research institutions in Mexico.