Anatomy of an Orogen
Anatomy of an Orogen
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造山带解剖
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发表时间：
1974
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通讯作者：
J. C. Maxwell
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作者：
J. C. Maxwell
我和我的学生同事绘制了横跨加州北方海岸山脉的15′四边形地带的地图，结果是建立了一个模型，用于研究方济各会建筑群的起源和就位，以及它与同时代大峡谷序列的关系，该模型与早期模型不同。方济会和大峡谷的岩石主要是杂砂岩、粉砂质泥岩和局部砾岩。大峡谷的大部分岩石都是分级的，底部标记很常见;这些特征存在，但不是方济各会岩石的特征。方济杂砂岩和粉砂质泥岩倾向于破碎和剪切，或混乱变形。混乱带通常包含外来块体，主要是蛇绿岩，但也有蓝片岩，云母片岩，变杂砂岩，和类似于封闭剪切基质的块体。这些混乱的单位最好的特点是混杂。在方济各复杂，映射单元已被定义，根据岩性的矩阵和内容的外来块。这些单元倾斜，面向东方，最古老的是东部（提托阶或更古老的南福克山片岩）。逐渐年轻的岩石出现在西部，最年轻的是海岸相，主要是始新世。可能的起源涉及方济各会沉积物沉积在一个海沟，然后由逆冲的沉积物对和下的向陆侧的海沟，而海沟的轴逐渐向西迁移。这一过程开始于提托时期，在白垩纪晚期放缓，并在始新世停止。模式提出了在沉积和俯冲过程中形成混杂岩。每个混杂岩单元都包含一组独特的外来岩块。这表明，这些组合部分反映了沉积物沉积的基底岩石的岩性。如果这是真的，基底的成分从主要的蛇绿岩套岩到主要的蓝片岩。一个混杂的蛇绿岩块也形成了，基底下面的一部分大峡谷沉积物。埃米尔·佩萨尼奥（Emile Pessagno）确定了整个方济各会蛇绿岩套中的放射虫硅质岩样品的蒂托期。如果蛇绿岩代表洋底，形成于一个扩张的山脊，那么在提顿时期，山脊一定位于海沟附近;提顿之后，方济各会的堆积物中没有海洋岩石。提出了底辟成因的提通蛇绿岩。西北走向的孤立板片的大峡谷型岩石和浅水大陆含煤碎屑岩的白垩纪至中新世的年龄分散在弗朗西斯复杂。证据表明，这些沉积在单独的，可能是断层控制的盆地，开发的表面上的构造活跃的Franciscan复杂的期间和之后的下冲。他们似乎并不像其他人所建议的那样，代表海岸山脉逆冲断层的飞来峰。
Mapping a strip of 15′ quadrangles across the northern Coast Ranges of California by student colleagues and myself has resulted in the development of a model for the origin and emplacement of the Franciscan complex and its relation to the coeval Great Valley sequence that differs from earlier models. Rocks of both Franciscan and Great Valley are predominantly graywacke, silty mudstone, and local conglomerate. Most of the Great Valley rocks are graded, and bottom markings are common; these features are present but not characteristic of the Franciscan rocks. Franciscan graywacke and silty mudstone tend to be broken and sheared, or chaotically deformed. The chaotic zones typically contain exotic blocks, mostly of ophiolitic rocks, but also blue-schist, mica schist, metagraywacke, and blocks similar to the enclosing sheared matrix. These chaotic units are best characterized as melanges. Within the Franciscan complex, mappable units have been defined, based on lithology of matrix and content of exotic blocks. The units dip and face easterly and are oldest to the east (Tithonian or older for the South Fork Mountain Schist). Progressively younger rocks occur down section to the west; the youngest are the Coastal Facies largely of Eocene age. The probable origin involves deposition of Franciscan sediments in a trench, followed by thrusting of the sediments against and under the landward side of the trench, while the axis of the trench migrated progressively westward. The process began in Tithonian time, slowed during Late Cretaceous, and ceased in the Eocene. Models are suggested for the formation of melanges during sedimentation and underthrusting. Each melange unit contains a unique assemblage of exotic blocks. It is suggested that these assemblages in part reflect the lithology of the basement rocks on which the sediments were deposited. If true, the composition of the basement varied from dominantly ophiolite-suite rocks to dominantly blueschist. A melange of ophiolite blocks also formed, the basement beneath a part of the Great Valley sediments. Samples of radiolarian chert from the ophiolite suite throughout the Franciscan were determined to be of Tithonian age by Emile Pessagno. If the ophiolite represents ocean floor, formed at a spreading ridge, the ridge must have been located adjacent to the trench in Tithonian time; after the Tithonian, no oceanic rocks were added to the Franciscan pile. A diapiric origin for the Tithonian ophiolite is suggested. Northwest-trending isolated slabs of Great Valley—type rocks and of shallow-water to continental coal-bearing clastic rocks of Cretaceous to Miocene age are scattered within the Franciscan complex. Evidence is presented that these were deposited in separate, probably fault-controlled basins, developed on the surface of the tectonically active Franciscan complex during and following underthrusting. They do not appear to represent a klippen of a Coast Range thrust fault, as has been suggested by others.