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首页> 外文期刊>International Geology Review >Whither the megathrust? Localization of large-scale subduction slip along the contact of a melange
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Whither the megathrust? Localization of large-scale subduction slip along the contact of a melange

机译：还需要更大的推力吗？大型俯冲滑移沿混杂体接触的局部化

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Long-lived subduction complexes, such as the Franciscan Complex of California, include tectonic contacts that represent exhumed megathrust horizons that collectively accommodated thousands of kilometres of slip. The chaotic nature of melanges in subduction complexes has spawned proposals that these melanges form as a result of megathrust displacement. Detailed field and petrographic relationships, however, show that most Franciscan melanges with exotic blocks formed by submarine landsliding. Field relationships at El Cerrito Quarry in the eastern San Francisco Bay area suggest that subduction slip may have been accommodated between the blueschist facies metagreywacke of the Angel Island nappe above and the prehnite-pumpellyite facies metagreywacke of the Alcatraz nappe below. Although a 100-200m-thick melange zone separates the nappes, this melange is a variably deformed, prehnite-pumpellyite facies sedimentary breccia and conglomerate deposited on the underlying coherent sandstone, so the melange is part of the lower nappe. A 20-30m-thick fault zone between the top of the melange, and the base of the Angel Island nappe displays an inverted metamorphic gradient with jadeite-glaucophane-lawsonite above lawsonite-albite assemblages. This zone has a strong seaward (SW)-vergent shear fabric and hosts ultracataclasite and pseudotachylite. These relationships suggest that significant subduction megathrust displacement at depths of 15-30km was accommodated within the 20-30m-thick fault zone. Field studies elsewhere in the Franciscan Complex suggest similar localization of megathrust slip, with some examples lacking melanges. The narrow megathrust zone at El Cerrito Quarry, its uniform sense-of-shear, and the localization of slip along the contact of, rather than within a melange, contrast sharply with the predictions of numerical models for subduction channels.
机译：长期的俯冲复合体，例如加利福尼亚的方济各会复合体，包括构造接触，这些构造接触代表了发掘出的巨推力层位，共同容纳了数千公里的滑移。俯冲复合体中混杂物的混沌性质催生了这样的建议，即由于巨大推力位移，这些混杂物形成了。但是，详细的田野和岩石学关系表明，大多数方济各会的混血儿具有由海底滑坡形成的奇异块体。旧金山湾东部地区El Cerrito Quarry的田间关系表明，俯冲滑移可能已被容纳在上方的安吉尔岛推覆岩的蓝片岩相metagreywacke和下方的恶魔岩浆岩的海蛇纹岩-pumpellyite岩相metagreywacke之间。尽管厚度为100-200m的混杂岩带分隔了喷口，但该混杂岩是沉积在下伏相干砂岩上的形变的，早老岩-粉晶岩相的沉积角砾岩和砾岩，因此，混杂岩是下推覆岩体的一部分。混杂岩顶部与天使岛推覆底部之间的20-30m厚断层带显示出倒斜变质梯度，其上的钙钠石-葡甲烷-钙钠石在钙钠石-轨道组合上方。该区域具有强大的向海（SW）边缘剪力布，并具有超催化硅酸盐和假速滑石。这些关系表明，在20-30m厚的断层带中，在15-30km深度处有大量的俯冲巨推力位移。方济各会综合体其他地方的野外研究表明，巨大的推力滑移也有类似的局限性，有些例子中没有混杂现象。 El Cerrito采石场的狭窄超大推力带，均匀的剪切感以及沿接触而不是混杂区域的滑动局部，与俯冲通道数值模型的预测形成了鲜明的对比。

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