The grabens of Canyonlands National Park, Utah: Geometry, mechanics, and kinematics
The grabens of Canyonlands National Park, Utah: Geometry, mechanics, and kinematics
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DOI：
10.1029/jb084ib09p04547
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发表时间：
1979-08
影响因子：
--
通讯作者：
G. Mcgill;Albert W. Stromquist
中科院分区：
文献类型：
--
作者：
G. Mcgill;Albert W. Stromquist
犹他州峡谷地国家公园针子区的地块位于一块460米厚的板块中，板块上覆盖着韧性蒸发岩，上面覆盖着未折叠的脆性岩石。这些岩石向西北方向缓缓倾斜，指向科罗拉多河的白内障峡谷。河流完全侵蚀了脆性板块，从而允许蒸发岩向下倾斜，随之而来的是延伸和脆性板块内的断层，产生了系统的间隔地堑，在平面上是弯曲的，倾角方向是凹陷的。在延伸流动中，脆性板块的应力类似于山谷冰川脆性表面冰的应力，裂隙几何的应力分析适用于峡谷地堑；地堑曲率是由于垂直于蒸发岩流动带侧边界的垂直平面上的剪应力所致。蒸发岩中的流动松弛了脆性板块上的基面剪应力，因此地堑间距只是脆性板块厚度和所含岩石强度的函数。因为两者在整个区域内基本上是一致的，所以地堑往往是均匀分布的。来自现场和实验的证据表明，地堑断裂起源于脆性岩石和韧性岩石之间的接触处或接近接触处，并在脆性板块内向上和横向传播。因此，在地表，由此产生的初始地块的宽度只是脆性板块厚度和断层倾角的函数。由于与层理平行的岩石的板厚和力学性质在该地区内变化不大，因此所有主要地堑形成的初始宽度都是相似的。坡道终止于坡道，坡道保持该初始宽度。基于这个地块几何学和运动学模型的方程将断裂板的厚度与可在照片上测量的地块参数联系起来。在地表附近，地块的几何形状和走向表明，弯曲的中间主应力轨迹与两套预先存在的、近乎直线的垂直节理之间存在折衷关系，导致地块具有以梯形偏移和锯齿状墙为特征的特征，其边界断层在地表附近垂直，但在深部向内倾斜。
The grabens of the Needles District, Canyonlands National Park, Utah occur in a 460 m-thick plate of unfolded, brittle rocks overlying ductile evaporites. These rocks dip gently northwest towards Cataract Canyon of the Colorado River. The river has eroded entirely through the brittle plate, thus permitting downdip flow in the evaporites with consequent extension and faulting within the brittle plate producing systematically spaced grabens which in plan are curved, concave in the direction of dip. Stresses in the brittle plate are similar to those in the brittle surface ice of a valley glacier in extending flow, and stress analyses of crevasse geometry are applicable to the Canyonlands grabens; graben curvature is due to shear stresses acting on vertical planes parallel to the lateral boundaries of the zone of evaporite flow. Flow in the evaporites relaxes basal shear stress on the brittle plate, hence the spacing of grabens is a function only of the thickness of the brittle plate and the strength of the contained rocks. Because both are essentially uniform throughout the area, the grabens tend to be uniformly spaced. Evidence from the field and from experiments suggests that the graben faults initiated at or close to the contact between brittle and ductile rocks and propagated both upward and laterally within the brittle plate. At the surface, the resulting incipient grabens thus had initial widths that were functions only of thickness of the brittle plate and dips of the faults. Because plate thickness and mechanical properties of the rocks parallel to bedding do not vary significantly within the area, these initial widths were similar for all major grabens formed. Grabens terminate in ramps which retain this initial width. An equation based on this model of graben geometry and kinematics relates thickness of faulted plate to graben parameters measurable on photographs. Near the surface, graben geometry and trend indicate a compromise between a curved intermediate principal stress trajectory and two sets of pre-existing, nearly rectilinear, vertical joints, resulting in grabens characterized by en-echelon offsets and saw-toothed walls and which have bounding faults that are vertical near the surface but dip inward at depth.