Composition of Aleutian forearc sediments

The ultimate composition of any sandstone is affected by a host of primary and secondary factors, including the lithologies present in source terranes, climate, depositional environment and diagenesis. In the case of a subduction complex, however, unequivocal identification of detrital provenance may be impossible because of the cumulative effects of tectonic and sedimentary transport. Long-distance sedimentary transport (> 1000 km) is common within trenches, and abyssal-plain turbidites can be tectonically transported for long distances as the underlying oceanic basement drifts towards a subduction front. Post-accretionary displacement can occur as a consequence of strike-slip faulting, and the total distance of tectonic dislocation may reach several thousand kilometers. The present-day Aleutian forearc region (North Pacific Ocean) illustrates many of the “problems” which typify subduction zones. Several petrologic suites can be identified, and there are significant variations in detrital modes in both time and space. The Aleutian region serves as a sobering modern analog for accreted rock units such as the Franciscan Complex of California, where intercalations of discrete sandstone suites have been noted. In the absence of paleomagnetic control, interpretations of sediment provenance within ancient subduction complexes probably should be restricted to the generic level.

任何砂岩（sandstone）的最终组成均受诸多原生与次生因素影响，涵盖物源地体（source terranes）的岩性、气候、沉积环境以及成岩作用（diagenesis）。然而针对俯冲杂岩（subduction complex）而言，受构造与沉积搬运的累积效应制约，往往无法明确识别其碎屑物源（detrital provenance）。海沟内长距离沉积搬运（>1000km）现象十分普遍，且随着下伏洋壳基底向俯冲前锋漂移，深海平原浊积岩（abyssal-plain turbidites）可被长距离构造搬运。走滑断层作用（strike-slip faulting）可引发增生后位移，构造错位的总距离可达数千公里。现今的阿留申前弧区（Aleutian forearc region，北太平洋）便是典型俯冲带诸多典型“难题”的直观例证。该区域可识别出多种岩石组合（petrologic suites），且碎屑组分模式（detrital modes）在时间与空间维度上均存在显著差异。阿留申地区可作为加利福尼亚弗朗西斯科杂岩（Franciscan Complex）这类增生岩石单元的极具警示意义的现代类比实例，此前研究已在该杂岩中观测到离散砂岩组合的互层现象。若缺乏古地磁约束（paleomagnetic control），对古俯冲杂岩（ancient subduction complexes）内沉积物源的解释或许仅能局限于一般性归类层级。