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

张拉裂谷中应变分配的发展可能对大陆断裂的局部化起着重要作用。晚中新世（1200万至600万年前）原始加利福尼亚湾伸展和右旋剪切的分割仍然是一个尚未解决的基本问题，阻碍了对海洋盆地形成初始条件的理解。来自索诺拉海岸的初步证据支持一种区域模式，即在大约600万年前加利福尼亚湾板块边界运动定位之前，在一个宽度不确定的区域促进了右旋伸展。尚不清楚晚中新世的右旋剪切是否足够快或集中，以改变流变条件，并将裂陷模式从分布的盆地-山脉式伸展改变为局部的岩石圈颈缩、沉降和大陆破裂。在这个项目中，美国和墨西哥科学家的目标是在板块边界定位过程中限制右向剪切随时间的结构分布。具体来说，他们将确定加利福尼亚湾的晚中新世伸展是由同时期走滑和正断层的宽区域内的弥散三维应变调节的，还是发生在离散的右旋断层和/或嵌入在更宽的东西延伸区域内的旋转块体的狭窄区域内。为了区分这些模型，该团队正在研究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索诺拉大陆架的近海：(1)通过详细的构造测绘和古地磁分析记录了右向剪切的分布；(2)通过构造重建、层间火山岩的40Ar/39Ar定年、同构造沉积盆地和火山盆地的沉降分析，量化拉张应变率；(3)通过对海上Tiburon盆地现有多道地震资料的解释，将陆上地质记录与海上构造和盆地相结合。加利福尼亚湾为研究大陆分裂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人们对北美西部温暖地壳如何以及为什么破裂形成加利福尼亚湾的理解，加利福尼亚湾从大约600万年前开始开放。本项目由EAR构造学项目、NSF国际科学与工程办公室和NSF/OCE边际项目支持。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.