Anatomy of a subduction complex: architecture of the Franciscan Complex, California, at multiple length and time scales
Anatomy of a subduction complex: architecture of the Franciscan Complex, California, at multiple length and time scales
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DOI：
10.1080/00206814.2014.998728
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发表时间：
2015-02
影响因子：
2.6
通讯作者：
J. Wakabayashi
中科院分区：
文献类型：
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作者：
J. Wakabayashi
加利福尼亚州的方济各会杂岩记录了超过1.5亿年的连续E-俯冲俯冲，以转换为右旋转换板块边界而终止。方理石由混杂和相干的单元组成，形成一叠逆冲推覆构造，具有显著的沿走向变化，变质等级和吸积年龄向下递减。方济各会记录了递进的俯冲、吸积、变质和折返，跨越了延长的俯冲时期，而不是叠加在先前存在的地层上的事件。高压变质岩缺乏热叠加，表明从大约165 Ma开始俯冲到大约25 Ma结束是俯冲的连续性。吸积期可能与俯冲侵蚀交替发生，使一些先前被吸积的物质消失，但该杂岩共同反映了对上板块物质的净增加。具有奇异块体的混杂岩(蛇纹岩和碎屑岩基质)具有海底物质输运沉积的沉积成因，而由构造作用形成的混杂岩包括破碎的大洋板块地层学，缺乏外来块体。前者与相干硅质碎屑单元互层，并形成层状硅屑单元。将不同时期沉积的推覆体分隔开的古巨型砂砾岩层，似乎仅限于100米厚的狭窄地带。方济各会单位的折返，无论是连贯的还是混杂的，都是通过悬壁的显著延伸和横截面挤压来实现的。在将方济各会建筑与现代和古代的俯冲综合体进行比较时，需要考虑到总掘出的数量以及自俯冲终止以来的掘出。在(俯冲后)圣安德烈亚斯断裂系统的股线上，褶皱的方济西推覆体和晚新生代(俯冲后)单元的等右旋分离表明，推覆体的褶皱发生在俯冲终止之前。方济各会和同时代大峡谷群弧前盆地中类似碎屑沉积岩套的右旋分离与圣安德烈亚斯断裂系统的右旋分离大致相同，排除了方济各会内部的主要俯冲-走滑位移。
The Franciscan Complex of California records over 150 million years of continuous E-dipping subduction that terminated with conversion to a dextral transform plate boundary. The Franciscan comprises mélange and coherent units forming a stack of thrust nappes, with significant along-strike variability, and downward-decreasing metamorphic grade and accretion ages. The Franciscan records progressive subduction, accretion, metamorphism, and exhumation, spanning the extended period of subduction, rather than events superimposed on pre-existing stratigraphy. High-pressure (HP) metamorphic rocks lack a thermal overprint, indicating continuity of subduction from subduction initiation at ca. 165 Ma to termination at ca. 25 Ma. Accretionary periods may have alternated with episodes of subduction erosion that removed some previously accreted material, but the complex collectively reflects a net addition of material to the upper plate. Mélanges (serpentinite and siliciclastic matrix) with exotic blocks have sedimentary origins as submarine mass transport deposits, whereas mélanges formed by tectonism comprise disrupted ocean plate stratigraphy and lack exotic blocks. The former are interbedded with and grade into coherent siliciclastic units. Palaeomegathrust horizons, separating nappes accreted at different times, appear restricted to narrow zones of <100 m thickness. Exhumation of Franciscan units, both coherent and mélange, was accommodated by significant extension of the hanging wall and cross-sectional extrusion. The amount of total exhumation, as well as exhumation since subduction termination, needs to be considered when comparing Franciscan architecture to modern and ancient subduction complexes. Equal dextral separation of folded Franciscan nappes and late Cenozoic (post-subduction) units across strands of the (post-subduction) San Andreas fault system shows that the folding of nappes took place prior to subduction termination. Dextral separation of similar clastic sedimentary suites in the Franciscan and the coeval Great Valley Group forearc basin is approximately that of the San Andreas fault system, precluding major syn-subduction strike-slip displacement within the Franciscan.