摘要:造山带中原始大地构造单元具有诸多相似的地质特征，要区分它们是很困难的。因此，本研究综述了保留完好的加利福尼亚海岸带的古弧前单元特征，旨在为其他造山带研究提供对比实例。加利福尼亚海岸带古弧前单元长约1 000 km、宽100 km，包含上部板块物质和俯冲—增生过程中从下部板块铲刮下来而拼贴的俯冲杂岩。上部板块的构造单元包含弧前盆地、沉积有最大厚度可达10 km的大峡谷群碎屑物，其基底为海岸山脉蛇绿岩；该蛇绿岩不均匀性地出露有蛇纹石化橄榄岩、和（或）辉长岩—辉绿岩—玄武岩及硅质岩，最大视厚度约为4 km。上部板块的岩石单元构造叠覆在增生杂岩之上，后者包括弗朗西斯科杂岩，其构造厚度为20 km或更薄一些。这3个构造单元的边界和内部构造在区域上呈低角度展布，但局部受紧闭褶皱影响、倾角变为中等到陡立状。除了大峡谷群底部沉积混杂岩中出露一些可与弗朗西斯科杂岩类比的变质岩岩块，海岸山脉蛇绿岩和大峡谷群总体缺少埋藏变质岩石。弗朗西斯科杂岩产出一系列埋深变质的岩石，从沸石相到角闪岩相和榴辉岩相，包括连续单元（无“基质夹岩块”结构）和混杂岩单元。大部分海岸山脉蛇绿岩和大峡谷群都缺少混杂岩的特征；弗朗西斯科杂岩中常见混杂岩，但主体仍为连续单元。大峡谷群为正常的地层沉积序列，而弗朗西斯科杂岩中俯冲—增生岩片呈断层接触，向下方（译者注：向大洋方向）逐渐变年轻。尽管大峡谷群、海岸山脉蛇绿岩和弗朗西斯科杂岩在岩性、构造和变质特征方面存在较大差别，但它们也拥有大量的相同之处，因此，在区分它们时需要慎重。世界上大多数造山带中的古弧前单元常遭受了更为严重的俯冲后的构造热事件的干扰和改造，因此要辨别它们的特征就更为困难了。
关键词: 加利福尼亚/
混杂岩/
蛇绿岩/
弧前结构
Abstract:Primary tectonic components of orogenic belts are difficult to distinguish because they share many geologic characteristics. Accordingly, I present a summary of well-preserved paleo-forearc components of coastal California with the goal of providing a guide to investigators in other orogenic belts. The paleo-forearc components of coastal California crop out over an area of about 1 000 km along strike and 100 km across strike, and comprise upper plate components that were above the subduction zone throughout their history, and a subduction complex comprising materials, that were tectonically transferred from the subducting （lower） plate to the upper plate, by the process of subduction-accretion when the subduction interface sliced into the top of the subducting plate. The upper plate units comprise the forearc basin clastic strata of the Great Valley Group （GVG） that reaches a maximum thickness of about 10 km and depositionally rests on the Coast Range ophiolite （CRO） that comprises an assemblage of variably serpentinized peridotite and/or gabbro and/or diabase and/or basalt and chert that reaches a maximum pseudostratigraphic thickness of about 4 km.? The upper plate units tectonically overlie the subduction complex represented by the Franciscan complex, that has a structural thickness of 20 km or less.? The three units and their bounding and internal contacts have regionally low-angle contacts that are tightly folded so that local dips are moderate to steep. The CRO and GVG lack burial metamorphism with the exception of some blocks in sedimentary mélanges of the basal GVG that record the same range of metamorphism as rocks in the Franciscan complex. The Franciscan spans a wide range of burial metamorphism from zeolite to amphibolite and eclogite grade rocks and it includes coherent （not block-in-matrix） and mélange units. Most of the CRO and GVG lack a mélange character, whereas mélanges are more common in the Franciscan but do not predominate over coherent units. The GVG exhibits a normal depositional stratigraphic sequence, whereas the Franciscan consists of fault-bounded units that young structurally downward in subduction?accretion age. Although there are some large differences between lithologic, structural, and metamorphic characteristics of the GVG, CRO, and Franciscan, they share many common traits so that careful inspection is necessary to distinguish between them.? The distinguishing characteristics of distinct paleo-forearc components are more difficult to discern in most of the world’s orogenic belts where post-subduction tectonothermal disruption and modification is more severe.
Key words:California/
Mélange/
Ophiolite/
Forearc architecture
PDF全文下载地址:
http://www.dzkx.org/data/article/export-pdf?id=geology_11708
