Deep crustal reflection profiling offshore southern central California
Deep crustal reflection profiling offshore southern central California
复制标题
加利福尼亚州中南部近海深部地壳反射剖面
DOI：
--
复制
发表时间：
1991
期刊：
影响因子：
--
通讯作者：
A. Levander
中科院分区：
文献类型：
--
作者：
A. Meltzer;A. Levander
加利福尼亚中南部近海深部地壳反射剖面显示，近海圣玛丽亚盆地及其毗邻的陆架和斜坡为洋壳所覆盖，洋壳逆冲边缘。在共中点（CMP）剖面上，洋壳顶部表现为发生在6 s双向时间（twt）的强相干反射。从大陆架下的深海盆地向东，可以连续追踪到50公里以上的反射。深度转换将海洋地壳顶部置于6-16公里深度，向海岸倾斜8°。在圣卢西亚高地下，地壳反射分裂成几个向东倾斜的部分。在海洋地壳之上，一个增生楔的厚度从斜坡底部的3公里增加到圣卢西亚高地下面的12公里。增生棱柱上方的新近纪沉积剖面变形相对较小，表明大部分增生发生在前新近纪北美板块与西部大洋板块的会聚过程中。圣玛利亚近海盆地至少经历了两期新近纪变形：中新世下至中新世中期的早期伸展期伴走滑断裂，中新世上至上新世下部的挤压和盆地反转。盆地构造发育始于西北，向东南逐渐年轻化。在近海沉积柱中发现的断层偏移了声波基底，但在2-3 s twt以下的方济各基底物质中没有成像，也没有延伸到或偏移深反射。我们认为洋壳是一个剥离面，在其上发生了浅层变形。近海盆地新近纪沉积物的缩短幅度仅为1 ~ 3 km。这表明，由于新近纪太平洋和北美板块之间的斜向板块运动，缩短延伸到圣玛丽亚盆地以东，并分布在整个转换边缘。
Deep crustal reflection profiling offshore southern central California shows that the offshore Santa Maria Basin and adjacent shelf and slope are underlain by oceanic crust which underthrusts the margin. On common-midpoint (CMP) sections the top of the oceanic crust appears as a strong coherent reflection occurring at 6 s two-way time (twt). The reflection can be traced continuously more than 50 km from the deep ocean basin eastward beneath the continental shelf. Depth conversion places the top of oceanic crust at 6–16 km depth with a dip of 8° toward the coast. Beneath the Santa Lucia High the crustal reflection breaks up into several east dipping segments. Above the oceanic crust an accretionary wedge increases in thickness from 3 km at the base of the slope to over 12 km beneath the Santa Lucia High. The Neogene sedimentary section above the accretionary prism exhibits relatively little deformation, implying that most of the accretion occurred during pre-Neogene convergence between the North American plate and oceanic plates located to the west. The offshore Santa Maria Basin exhibits at least two periods of Neogene deformation: an early extensional phase, accompanied by strike-slip faulting, occurred in the lower to middle Miocene followed by compression and basin inversion in the upper Miocene-lower Pliocene. The development of structures in the basin began in the northwest and is progressively younger to the southeast. Faults identified in the offshore sedimentary column offset acoustic basement but are not imaged in the Franciscan basement material below 2–3 s twt and do not extend to, or offset, the deep reflections. We suggest that the oceanic crust served as a detachment surface above which the shallow deformation occurred. Neogene sediments in the offshore basins show only 1–3 km of shortening. This implies that shortening, due to Neogene oblique plate motion between the Pacific and North American plates, extends east of the offshore Santa Maria Basin and is distributed across the entire transform margin.