Seismic reflection images of the crust of the northern part of the Chugach Terrane, Alaska: Results of a survey for the Trans‐Alaska Crustal Transect (TACT)
Seismic reflection images of the crust of the northern part of the Chugach Terrane, Alaska: Results of a survey for the Trans‐Alaska Crustal Transect (TACT)
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阿拉斯加楚加奇地体北部地壳的地震反射图像：跨阿拉斯加地壳横断面 (TACT) 调查结果
DOI：
10.1029/jb094ib04p04424
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发表时间：
1989
影响因子：
--
通讯作者：
Gregory L. Smith
中科院分区：
文献类型：
--
作者：
M. Fisher;T. Brocher;W. Nokleberg;G. Plafker;Gregory L. Smith
深地壳地震反射数据显示，阿留申海沟东端附近汇聚大陆边缘的中地壳和下地壳有较强的反射。这些数据是跨越边界山脉断层系统收集的，边界山脉断层系统是分隔半岛和楚加奇构造地层带的主要缝合带。楚加奇火山由变质的、强烈变形的复理石、俯冲混杂岩以及蓝片岩相和绿片岩相岩石组成。即便如此，高反射率的岩石位于这个洞穴之内或之下的很深的地方。地震剖面的浅部（0-5秒）仅显示出少量起伏交织的反射，表明其中一些反射源自地震剖面平面之外。其他起伏的事件可能揭示了早期新生代花岗质岩体或沿沿着逆冲断层叠瓦状的反形式岩石堆。与这种浅层反射模式相反，三个反射带包含了地震剖面中部（5-12 s或12-34 km）的大部分同相轴。我们解释的发散反射内的最高带（5-5.5秒）为轻微的北倾逆冲断层，和近平行反射截断的边界上，中带可能会成像变质叶理或逆冲断层。如果截断的反射波代表逆冲断层，那么反射波的几何形状表明，近平行的顶底板逆冲断层约束着叠瓦状岩石，形成双重结构。上部和中部频带被地震剖面上的一个区域隔开，该区域几乎没有反射。这一地区在深度上与折射模型中明显的高速（7.5公里/秒）层大致相关。反射性差的高速单元可能是火山岩，就像楚加奇河构造最低部分暴露的火山岩一样。如果是这样的话，那么反射性差的岩石的底部就表明了楚加奇河的底部。引起上部和中部反射带的岩石以及介于其间的反射差的火山岩可能形成约10 km厚的中地壳剪切带。最深的近平行反射带（30-35公里）与太平洋板块俯冲的Wadati-Benioff带的顶部深度密切相关。这些反射可能源于俯冲带内分离岩石圈板块的滑脱作用。这条反射带的厚度突然增加，可能表明俯冲沉积物的局部底侵。大洋莫霍面的事件尚未得到确认。边界山脉断层系统、楚加奇山脉和半岛山脉之间的缝合线以及半岛山脉的地壳深层在这些地震反射数据中都不明显。
Deep crustal seismic reflection data show strong reflections from the middle and lower crust of the convergent continental margin near the eastern end of the Aleutian trench. These data were collected across the Border Ranges fault system, a major suture zone that separates the Peninsular and Chugach tectonostratigraphic terranes. The Chugach terrane consists of metamorphosed, strongly deformed flysch, subduction melange as well as blueschist- and greenschist-facies rocks. Even so, highly reflective rocks lie at great depth within or beneath this terrane. The shallow part of the seismic section (0–5 s) reveals only a few reflections, which undulate and interweave, indicating that some of them originate outside of the plane of the seismic section. Other undulating events could reveal early Cenozoic granitic plutons or antiformal stacks of rock imbricated along thrust faults. In contrast to this shallow reflection pattern, three reflection bands contain most of the events on the middle part of the seismic section (5–12 s or 12–34 km). We interpret divergent reflections within the uppermost band (5–5.5 s) as gently north dipping thrust faults, and subparallel reflections truncated at the boundaries of the upper and middle bands may image a metamorphic foliation or thrust faults. If the truncated reflections represent thrust faults, then the reflection geometry suggests that subparallel roof and floor thrust faults bound imbricated rocks, forming duplex structures. The upper and middle bands are separated by an area on the seismic section that reveals few reflections. This area correlates approximately in depth with a high velocity (7.5 km/s) layer evident in refraction models. The poorly reflective, high velocity unit might be volcanic rocks like those exposed in the structurally lowest part of the Chugach terrane. If so, then the bottom of the poorly reflective rocks indicates the base of the Chugach terrane. Rocks that cause the upper and middle reflection bands and the intervening, poorly reflective volcanic rocks may form a midcrustal shear zone that is about 10 km thick. The deepest band of subparallel reflections (30–35 km) correlates closely in depth with the top of the Wadati-Benioff zone associated with the underthrusting Pacific plate. The reflections probably stem from the decollement that separates lithospheric plates within the subduction zone. This reflection band increases abruptly in thickness, possibly indicating local underplating of subducted sediment. No events from the oceanic Moho have been recognized. Neither the Border Ranges fault system, the suture between the Chugach and Peninsular terranes, nor deep crustal layers of the Peninsular terrane are evident in these seismic reflection data.