Using the Tertiary Volcanic and Gravelly Sedimentologic Record to Test the Timing and Evolution of the Chocolate Mountains Anticlinorium, SE California

利用第三纪火山和砾石沉积记录来测试加利福尼亚州东南部巧克力山反斜层的形成时间和演化

• 批准号: 0911663
• 负责人: Gary Girty
• 金额: $ 13.28万
• 依托单位: San Diego State University Foundation
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2009
• 资助国家: 美国
• 起止时间: 2009-08-01 至 2012-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0911663&HistoricalAwards=false
• 关键词: Using; Tertiary; Volcanic; Gravelly; Sedimentologic

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).The origin of the Chocolate Mountains anticlinorium (CMA) and its significance to the Tertiary structural and tectonic history of the southwestern United States remains enigmatic. Discontinuously exposed throughout southeastern California, these Mesozoic to Tertiary age rocks may have been derived from a subduction zone or forearc basin setting located initially ~300 - ~400 kilometers to the west within the ancient Pacific Ocean, but they now form dome-like structures commonly referred to as the CMA. The goal is to determine whether or not the CMA formed initially during Paleogene low-angle subduction of the ancient Farallon plate, which lay between the Pacific and North American plates, or somewhat later during middle Tertiary extensional exhumation of the southwestern United States. In addition, the investigators will evaluate the potential role that early strike-slip faults of the Eastern California Shear Zone, a precursor to the modern day San Andreas fault, may have played in the development of the anticlinorium. To address these issues, a series of geochronologically constrained time slices that portray the evolution of the rocks and sediments involved in the formation of the CMA will be developed. The expected outcome of this research is an enhanced model for the development of the Chocolate Mountains and their significance to the formation and evolution of the southwestern U.S.Key to the development of the CMA model time slices is the Neogene Bear Canyon conglomerate, which can be subdivided into three unconformably bound units. Significantly, preliminary work indicates that the lower and middle members of the Bear Canyon lie on both limbs of the anticlinorium and record its incremental growth with the last increment recorded by the tilting of the interstratified ~9-13 million year old basalts of Black Mountain. The lead investigator and a large number of students will commence their work by tracing out the extent of the unconformities bounding the lower, middle, and upper members of the Bear Canyon conglomerate over ~300 km2. Their work will include detailed characterization of bedding types, clast and grain-size distributions, and sedimentary structures and paleocurrent directions. In addition, laser-ablation U-Pb zircon and selected Ar-Ar geochronological ages will be determined for all key Mesozoic and Tertiary volcanic units underlying the Bear Canyon, as well as for clasts included in the Neogene conglomerates. Additional analyses will be made on Eocene(?)/Oligocene(?) sedimentary rocks lying between Mesozoic basement and Tertiary volcanic rocks. By sequentially removing fold growth and fault slip that has accumulated over the last ~60 million years, the PI and students will be able to assess the incremental growth of the anticlinorium and the potential role that subduction, extensional exhumation, and pre-San Andreas strike-slip faulting may have played in its development and the evolving landscape of southeastern California during the Paleogene (~65.5 to ~33.9 million years ago) and Neogene (~23 to ~2.6 million years ago).

该奖项是根据2009年美国复苏和再投资法案（公法111-5）资助的。巧克力山复背斜（CMA）的起源及其对美国西南部第三纪构造和构造历史的意义仍然是个谜。 这些中生代至第三纪岩石在整个加州东南部不连续地出露，可能来自俯冲带或弧前盆地环境，最初位于古太平洋以西约300 - 400公里处，但它们现在形成了通常称为CMA的圆顶状结构。 目的是确定CMA是否最初形成于古Farallon板块（位于太平洋和北美板块之间）的古近纪低角度俯冲过程中，或者稍晚于美国西南部中第三纪伸展折返过程中。 此外，研究人员还将评估东加州剪切带早期走滑断层（现代圣安德烈亚斯断层的前身）可能在背斜发育中发挥的潜在作用。为了解决这些问题，将开发一系列地质年代学约束的时间切片，描绘CMA形成过程中所涉及的岩石和沉积物的演变。 这项研究的预期成果是一个增强模型的发展巧克力山和他们的意义，美国西南部的形成和演化的CMA模型时间切片的发展关键是新近纪熊峡谷砾岩，它可以被细分为三个不整合约束单元。 值得注意的是，初步工作表明，熊峡谷的下部和中部位于复背斜的两翼，并记录了其增量增长，最后一次增量是由黑山约900 - 1300万年的间层玄武岩的倾斜记录的。 首席调查员和大量学生将开始他们的工作，追踪出熊峡谷砾岩约300平方公里范围内的下、中、上部不整合面的范围。他们的工作将包括详细描述层理类型、碎屑和粒度分布、沉积结构和古水流方向。 此外，将确定熊峡谷下所有关键的中生代和第三纪火山单位以及新近纪砾岩中的碎屑的激光烧蚀U-Pb锆石和选定的Ar-Ar年代学年龄。 将对始新世（？）/渐新世（？）中生代基底与第三纪火山岩之间的沉积岩。通过顺序消除过去约6000万年来积累的褶皱生长和断层滑动，PI和学生将能够评估复向斜的增量生长以及俯冲、伸展折返的潜在作用，和前圣安德烈亚斯走滑断层可能在其发展和演变的景观东南部加州在古近纪（约6550万年至约3390万年前）和新近纪（约2300万年至约260万年前）。