Sedimentary Basin Development & Deformation Adjacent to the Mojave-Sonora Megashear, Sonora, Mexico: Evaluation of the Mid-Mesozoic Tectonics of the Southwestern Cordilleran Margin

由于各种原因，古大陆转换边缘的识别一直很困难。转换断层在经历了活跃的横向位移历史之后，通常会在较年轻的构造过程中重新活动。此外，允许对偏移进行评估的穿刺点通常是模棱两可的，并受到多种解释。然而，分析与转换断层，特别是与大陆边缘岩石圈横切的断层相邻的沉积盆地，为古转换边缘的评价提供了强有力的工具。对沉积和变形时间的评估尤为关键，因为在走滑盆地中，这两个过程应该是部分相同的。在北方索诺拉，假设的中生代大陆转换的例子，莫哈韦-索诺拉巨剪，并列大陆地壳碎片，卡博尔卡块，对着现在是一个复杂的构造带的北美大陆。一个狭窄的、变形的晚侏罗世-早白垩世沉积盆地沿着这一构造边界绵延300多公里。盆地的沉降历史与巨剪切带的位移历史基本吻合，因此提供了一个独特的机会来评估巨剪切带假说，并了解晚侏罗世-早白垩世科迪勒支大陆边缘的构造过程。通过研究，可以确定巨型海盆附近沉积盆地的沉积和构造演化，以及Caborca块，形成了盆地的南部边缘。本研究的目的分为三大类：1.盆地起源及其与Caborca地块的中生代接近程度。岩石地层学、生物地层学和地质年代学分析将确定盆地的范围和几何形状，而物源研究将确定Caborca地块是否为盆地的沉积物源。地层中的化石和互层火山碎屑岩使这一目标是可行的.盆地变形运动学与年代学。盆地地层局部紧密褶皱、劈裂和变质。然而，变形强度在较老的岩石中比在较年轻的岩石中大，这种变化可能是时间上的，也可能与变形过程中地层的埋藏深度有关。中尺度和微尺度的结构分析将评估运动学和变形的变化在地层和地理上相对于巨剪切带。变质泥质岩的40 Ar/ 39 Ar年代学研究将确定盆地变形的时代. Caborca地块的隆升时间。根据地层关系和~（40）Ar/~（39）Ar热年代学确定盆地边缘隆升的时代，以确定盆地边缘隆起与盆地变形的相对时间。隆起分析也将允许物源结果的评价。这一综合的沉积学，构造和地质年代学研究将评估转换过程，推断已经创建西南科迪勒拉边缘开始于晚侏罗世。如果巨声假说仍然成立，这项研究应该作为一个模板，为其他分析古代大陆transmingmargin。如果巨剪切假说被推翻，该研究将迫使重新评估西南科迪勒拉边缘中生代构造和墨西哥盘古超大陆分裂的运动学。

The identification of ancient continental transform margins has proven difficultfor a variety of reasons. Transform faults are commonly reactivated subsequent to theiractive transcurrent displacement histories by younger tectonic processes. In addition,piercing points that permit assessment of offset are commonly equivocal and subject tomultiple interpretations. However, analysis of sedimentary basins developed adjacent totransform faults, particularly faults that transect marginal continental lithosphere,provides a powerful tool for evaluation of ancient transform margins. Evaluation of thetiming of deposition and deformation is especially critical, because the two processesshould be partly coeval in strike-slip basins. In northern Sonora, the postulated Mesozoictype example of a continental transform, the Mojave-Sonora megashear, juxtaposes acontinental crustal fragment, the Caborca block, against cratonal North America alongwhat is now a complex structural zone. A narrow, deformed sedimentary basin of LateJurassic-Early Cretaceous age lies along this structural boundary for a distance of morethan 300 km. The subsidence history of the basin largely coincided with the positeddisplacement history of the megashear and so provides a unique opportunity to evaluatethe megashear hypothesis and understand the processes that constructed the LateJurassic-Early Cretaceous Cordilleran continental margin.A two-year field- and laboratory-based study proposed here will determine thedepositional and structural evolution of the sedimentary basin adjacent to the megashearand the uplift history of the Caborca block, which forms the present southern edge of thebasin. The objectives of the research fall into three major categories:1. Basin origin and its Mesozoic proximity to the Caborca block.Lithostratigraphic, biostratigraphic, and geochronologic analysis will determine thelifespan and geometry of the basin, while provenance studies will determine if theCaborca block was a sediment source for the basin. Fossils in the strata and interbeddedvolcaniclastic rocks make this objective practicable.2. Deformation kinematics and chronology of the basin. Strata in the basin arelocally tightly folded, cleaved, and metamorphosed. However, deformation intensity isgreater in older rocks than in younger ones, a variation which may be either temporal orrelated to burial depth of strata during deformation. Mesoscale and microscale structuralanalysis will evaluate kinematics and variability of deformation both stratigraphically andgeographically with respect to the megashear. 40 Ar/ 39 Ar geochronology onmetamorphosed pelites will determine the age of the deformation in the basin.3. Time of uplift of Caborca block. The age of uplift of the basin margin will bedetermined from stratigraphic relations and 40 Ar/ 39 Ar thermochronology in order toestablish time of uplift of the outboard block relative to deformation of the basin. Theuplift analysis will also permit evaluation of provenance results.This integrated sedimentologic, structural and geochronologic study will evaluatethe transform processes that are inferred to have created the southwestern Cordilleranmargin beginning in the Late Jurassic. If the megashear hypothesis remains tenable, thisstudy should serve as a template for other analyses of ancient continental transformmargins. If the megashear hypothesis is refuted, the study will force re-evaluation of theMesozoic tectonics of the southwestern Cordilleran margin and the kinematics ofPangean supercontinent break up in Mexico.