ABSTRACT

New geologic mapping of the Franciscan Complex in the western Mt. Tamalpais area northwest of San Francisco, California, reveals a tectonostratigraphic stack of accretionary units that young westward and structurally downward. Serpentinite-matrix mélange at the top of the section, beneath a possible basal fragment of Coast Range Ophiolite, and a structurally low black-rock-bearing, mudrock-matrix Ocean Plate Stratigraphy (OPS) mélange located near the base of the section, bracket lithic and feldspathic metasandstone-dominated accretionary units. The ages of the metasedimentary accretionary units are constrained by U-Pb age dating of detrital zircons. Maximum depositional ages fall between 124 Ma and 88 Ma. Sediment provenance is dominantly in the Sierra Nevada arc to the east, but minor numbers of detrital zircons were derived from Paleozoic and Precambrian sources. The provenance age profiles (cumulative distribution plots) reveal changing age components of zircon populations over time, reflecting the history of unroofing in the Sierra Nevada arc. An unusual post-accretion thrust fault and atypical greenschist facies metamorphism uncharacteristic of the Franciscan Complex, plus probable Late Cenozoic faults, complicate the character of the geology of the Mt. Tamalpais area.

摘要

加利福尼亚州旧金山西北部的塔马尔佩斯山西部地区的方济各会复合体的新地质图显示了一个构造地层堆叠，这些增生单元向西年轻并在结构上向下。剖面顶部的蛇纹岩-基质混杂岩，在海岸山脉蛇绿岩的可能基底碎片下方，以及位于该剖面底部附近的结构低的含黑岩、泥岩-基质海洋板块地层 (OPS) 混杂岩，支架岩屑和长石变砂岩为主的增生单元。变质沉积增生单元的年龄受到碎屑锆石 U-Pb 年龄定年的限制。最大沉积年龄介于 124 Ma 和 88 Ma 之间。沉积物来源主要位于东部的内华达山脉弧，但少量碎屑锆石来自古生代和前寒武纪源。出处年龄剖面（累积分布图）揭示了锆石种群的年龄成分随时间变化，反映了内华达山脉弧形拆顶的历史。一个不寻常的增生后逆冲断层和非典型的绿片岩相变质作用不符合方济会杂岩的特征，加上可能的晚新生代断层，使塔马尔佩斯山地区的地质特征复杂化。