Deformation bands from the toe of the Nankai Accretionary Prism
Deformation bands from the toe of the Nankai Accretionary Prism
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南开吸积棱镜趾部的变形带
DOI：
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发表时间：
1990
期刊：
影响因子：
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通讯作者：
N. Lundberg
中科院分区：
文献类型：
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作者：
D. Karig;N. Lundberg
日本西南部南开增生棱柱的趾部变形不仅发生在地震定义的逆冲断裂上的位移，而且还表现为弥漫性增厚、缩短和孔隙度损失。从这个背景中恢复的沉积物只有很小的倾斜，唯一注意到的结构是微妙的变形带，这是与亚水平缩短有关的脆-韧性剪切的半平面带。大多数条带的倾角在25°~60°之间，与顺层倾角方向成正比，与顺层倾角成反比，随深度和距离海沟的变化趋势不明显。变形带内矿物学变化不大，但颗粒尺寸略有减小，但孔隙度降低了5%。倾角在25°~40°之间的条带通常以裂缝为标志，其表面表现为具有滑移和下倾台阶的逆滑。这些带子横切，比陡峭的带子年轻。所有变形带都由比主带倾角小10°-15°的子带组成，其中粘土组构旋转到接近垂直的位置。这些子带很可能是Riedel切变，在与脆性库仑响应相适应的频带中与01成角度。少数形变带在与主带成85°的∼夹角处表现出许多短带的下沉。这些可能是R‘剪刀。较陡峭的变形带，特别是倾角为50°-60°的共轭组，被解释为被韧性流动被动地旋转向最大有限伸展方向(E1)。旋转这些带所需的应变将需要∼2.5的椭圆度R=(1+E1)/(1+E3)。根据地震剖面和孔隙度数据，R只有1.5，这表明如果韧性流动是带状旋转的原因，这种流动不能代表脚趾的大部分。横跨原冲断带的地震剖面显示出45°倾角的“地震间断”，这似乎是由略陡峭的顺层倾角(10°-15°)和可能更大的变形组成的面板。南开脚趾压实变形的沉积物中存在韧性流动，但脆性剪切与脆韧性剪切共存的现象很难用实验土力学解释。
Deformation in the toe of the Nankai accretionary prism of SW Japan occurs not only by displacement on seismically defined thrust faults but also by diffuse thickening, shortening, and porosity loss. Sediments recovered from this setting are only very slightly tilted, and the only structures noted were subtle deformation bands, which are semiplanar zones of brittle-ductile shear associated with subhorizontal shortening. Most bands dip between 25° and 60°, both in and opposite to the direction of bedding dip, and show no obvious trends with depth or distance from the trench. There is no change of mineralogy within deformation bands and only a very slight reduction of grain size, but porosity is reduced by 5%. Bands with dips between 25° and 40° are often marked by fractures, the surfaces of which show reverse slip with slickenlines and downdip steps. These bands crosscut and are younger than the steeper bands. All deformation bands consist of subbands that dip 10°–15° less steeply than the main band and within which the clay fabric is rotated to near vertical. The subbands are probably Riedel shears, oriented at angles to 01 in the bands appropriate to a brittle Coulomb response. A few deformation bands show subsidary sets of many short bands, at angles of ∼85° to the main bands. These may be R′ shears. The steeper deformation bands, especially the conjugate sets that dip 50°–60°, are interpreted to have been passively rotated toward the maximum finite extension direction (e1) by ductile flow. The strain necessary to rotate these bands would require an ellipticity, R = (1+e1)/(1+e3), of ∼2.5. Based on seismic profiles and porosity data, R is only 1.5, demonstrating that if ductile flow were responsible for band rotation, such flow is not representative of the bulk of the toe. Seismic profiles across the proto thrust zone show 45° dipping “seismic discontinuities,” which appear to be panels of slightly steeper bedding dips (10°–15°) and perhaps greater deformation. Ductile flow is to be expected in the compactively deforming sediments of the Nankai toe, but the coexistence with brittle to brittle-ductile shears is not easily explained with experimental soil mechanics.