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首页> 外文期刊>American Journal of Science >MESOSCOPIC STRUCTURE, STRAIN, AND VOLUME LOSS IN FOLDED COVER STRATA, VALLEY AND RIDGE PROVINCE, MARYLAND
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MESOSCOPIC STRUCTURE, STRAIN, AND VOLUME LOSS IN FOLDED COVER STRATA, VALLEY AND RIDGE PROVINCE, MARYLAND

机译：马里兰州山谷和里奇省折叠盖地层的中观构造，应变和体积损失

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In ～200 m wavelength folds in Silurian strata exposed in the western part of the Valley and Ridge Province in Maryland, two morphologically distinct cleavages contribute to total shortening. Large solution seams that may cross several beds at nearly right angles define an early, pre-folding, cleavage; this early cleavage accommodated at least 10 percent layer-parallel shortening along a northwest azimuth with nearly 10 percent volume loss. Finite-amplitude folding accommodated later layer shortening along a nearly west-northwest azimuth; a second cleavage formed during fold growth in concert with calcite-filled veins, bedding-parallel slip surfaces, and bedding-parallel shear zones which we call cleavage-slip zones. Rotation of beds during fold amplification yields 18 percent shortening in the strata we examine in detail, and the later cleavage and cleavage-slip zones together account for 10 to 15 percent of sub-horizontal shortening and 10 to 15 percent volume loss. Total shortening in these strata must be at least 25 percent (early cleavage shortening plus profile shortening due to folding), and it may be as much as 40 percent when shortening due to the later cleavage is considered. This magnitude of layer-parallel shortening exceeds previous estimates for shortening in Silurian to Devonian strata in this region. Furthermore, different azimuths for shortening have not been suggested before for this portion of the belt. Measurements of strain and quantitative analysis of mesoscopic structures across a symmetrical anticline among these folds suggest fold growth by a simultaneous combination of flexural flow and flexural slip. Flexural slip and flexural flow made different net contributions to fold growth on the two limbs of the anticline. The number of active bedding-parallel slip zones apparently increased on both limbs as the fold grew, causing changes in the bulk behavior of the strata involved in folding. Our study of strain and mesoscopic structures in this fold suggests that buckling was the dominant mechanism of folding and that differences in evolving mesoscopic structures had consequences for the subsequent development of folds.
机译：在马里兰州Valley和Ridge Province西部暴露的志留系地层的〜200 m波长褶皱中，两次形态上截然不同的卵裂导致总缩短。可能以几乎直角穿过几张床的大型溶液接缝定义了早期的，预折叠的裂缝；早期的裂隙沿西北方位至少有10％的平行层缩短，体积损失近10％。有限振幅折叠在近西-西北方向上容纳了较晚的层缩短；在褶皱生长过程中，与方解石填充的脉，层理平行的滑动面和层理平行的剪切带共同形成了第二条卵裂，我们称其为卵裂滑动带。在倍数放大过程中，床层的旋转在我们详细研究的地层中缩短了18％，后来的劈裂和劈裂滑移带共同构成了亚水平缩短的10％至15％，体积损失了10％至15％。这些地层的总缩短量必须至少为25％（早期卵裂缩短加上由于折叠而引起的剖面缩短），而考虑到由于后期卵裂而缩短的话，则可能高达40％。平行平行缩短的幅度超过了该地区志留纪至泥盆纪地层缩短的先前估计。此外，对于皮带的这一部分，以前没有建议用于缩短的不同方位角。测量这些褶皱之间的对称背斜间的介观结构的应变和定量分析表明，弯曲流和弯曲滑移的同时存在会导致褶皱的生长。弯曲滑移和弯曲流动对背斜的两个肢体的褶皱生长产生不同的净贡献。随着褶皱的增加，活跃的平行于两层的滑动带的数量显然增加了，从而导致了涉及褶皱的地层的整体行为发生了变化。我们对这种褶皱的应变和介观结构的研究表明，屈曲是褶皱的主要机制，而不断发展的介观结构的差异会导致褶皱的后续发展。

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