Magnitudes and spatial patterns of erosional exhumation in the Sevier hinterland, eastern Nevada and western Utah, USA: Insights from a Paleogene paleogeologic map
Magnitudes and spatial patterns of erosional exhumation in the Sevier hinterland, eastern Nevada and western Utah, USA: Insights from a Paleogene paleogeologic map
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美国内华达州东部和犹他州西部塞维尔腹地侵蚀剥露的幅度和空间模式：来自古近纪古地质图的见解
作者：
S. Long
位于内华达州和美国犹他州西部的塞维尔造山带腹地被解释为一个古老的高海拔造山高原，或称内华达高原，在第三纪时期发生了伸展性坍塌。为了说明这一地区伸展前的构造起伏，这里提出了一个新的古地质（或地下露头）图，显示出露在区域性古近系不整合面之下的新元古代至三叠纪岩石的分布。地图区域在最西端的主要塞维尔逆冲断层系统和罗伯茨山脉逆冲断层之间延伸。可以定义三个横向地下露头域：（1）Delamar-Wah Wah-Canyon Range（DWC）逆冲岩席前部的寒武纪至密西西比纪地下露头层，（2）内华达州东部和犹他州最西部的一个广阔地区，没有地表突破的逆冲断层，密西西比-三叠纪的地下露头水平，表明构造起伏较小（2 km）;（3）内华达州中部逆冲带（CNTB）新元古代至二叠纪的地下露头水平变化，表明构造起伏较大（高达8 km）。利用已发表的新元古代至三叠纪沉积岩沉积厚度等厚图，将古地质图转换成一幅描绘三叠纪剖面顶部与古近纪地下露头水平之间侵蚀岩石厚度的等值线图，从而说明同造山侵蚀剥露的幅度和空间模式。三个地下露头区域的特征折返幅度为：（1）DWC逆冲席前部4-8 km;（2）CNTB南部、中部和北方分别为4-10 km、2 km和4-6 km;（3）中间低起伏区域为1-3 km。南部CNTB构造的孤立出露可通过其地下露头和折返模式与两个贯通的逆冲系统相关联，这两个逆冲系统与内华达州南部的塞维尔逆冲带的构造相连接。这支持了先前提出的相关性，并暗示这两个逆冲系统之间存在直接的构造联系。地下露头和剥露模式没有显示出地表突破的逆冲痕迹，这将代表一个南部延续的温德米尔逆冲南部的Pequop或红宝石山脉。因此，如果温德米尔逆冲断层模型是正确的，这意味着要么终止于横向结构，如撕裂断层，或过渡到一个盲目的几何形状。塞维尔腹地大部分地区的2.2公里构造起伏表明，大多数高震级（>1-2公里落差）、区域性分布、破坏地表的正断层作用肢解了造山高地，并产生了今天观察到的高构造起伏，这些正断层作用必须是渐新世后的。尽管识别出了与古近系不整合面重叠的70条正断层的痕迹，但几乎所有这些构造的落差都限制在最大1-2 km。这进一步突出了广泛的晚白垩世和古近纪折返的中地壳岩石，现在暴露在变质核杂岩没有相应的高强度上地壳伸展的矛盾。高折返幅度与主要逆冲断层上盘的密切空间关联表明，侵蚀折返是对造山高原伴随收缩变形的救济代的响应。缺乏显着的沿走向折返的变化在塞维尔腹地的大部分意味着救济一代是相对均匀的沿着走向。之间的DWC片和低折返区的西部的领先部分的陡峭的横向走剥露梯度被解释为屏蔽的西部地区的头向侵蚀的结果，通过被动东平移的DWC逆冲片和反形式的顶点在深度的增长，长期的隆起，侵蚀和救济发展。塞维尔和安第斯山脉中部的造山高原之间的显着差异在这里强调。塞维尔高原的大部分内部是一个侵蚀高地，由造山前岩石组成，具有低的结构起伏，而安第斯高原的内部由一个变形和折返的同造山内陆盆地组成，厚达12公里，具有非常高的结构起伏。因此，前造山期和同造山期岩石之间地层接触的结构水平的比较表明，这两个高原的内部之间的岩石隆起的差异高达14-15公里。
The hinterland of the Sevier orogenic belt in Nevada and western Utah (United States) has been interpreted as an ancient high-elevation orogenic plateau, or Nevadaplano, that collapsed extensionally during Tertiary time. To illustrate the preextensional structural relief of this region, a new paleogeologic (or subcrop) map showing the distribution of Neoproterozoic to Triassic rocks exposed beneath a regional Paleogene unconformity is presented here. The map area extends between the traces of the westernmost major Sevier thrust system and the Roberts Mountains thrust. Three across-strike subcrop domains can be defined: (1) Cambrian to Mississippian subcrop levels in the leading part of the Delamar–Wah Wah–Canyon Range (DWC) thrust sheet, indicating high (as much as 7 km) structural relief; (2) a broad region of eastern Nevada and westernmost Utah devoid of surface-breaching thrust faults, with Mississippian–Triassic subcrop levels, indicating low (2 km) structural relief; and (3) subcrop levels varying between Neoproterozoic and Permian in the central Nevada thrust belt (CNTB), indicating high (as much as 8 km) structural relief. Using published isopach maps of depositional thickness of Neoproterozoic to Triassic sedimentary rocks, the paleogeologic map has been converted into a map that contours the thickness of rock eroded between the top of the Triassic section and the Paleogene subcrop level, and thus illustrates magnitudes and spatial patterns of synorogenic erosional exhumation. Characteristic exhumation magnitudes for the three subcrop domains are: (1) 4–8 km in the leading part of the DWC thrust sheet; (2) 4–10 km, 2 km, and 4–6 km in the southern, central, and northern parts of the CNTB, respectively; and (3) 1–3 km in the intervening low-relief region. Isolated exposures of southern CNTB structures can be correlated by their subcrop and exhumation patterns into two through-going thrust systems that connect with structures of the Sevier thrust belt in southern Nevada. This supports previously suggested correlations, and implies a direct structural link between these two thrust systems. Subcrop and exhumation patterns do not reveal a surface-breaching thrust trace that would represent a southern continuation of the Windermere thrust south of the Pequop or Ruby Mountains. Thus, if the Windermere thrust model is correct, this implies either termination at a lateral structure such as a tear fault, or a transition to a blind geometry. The ∼2 km structural relief that characterizes much of the Sevier hinterland indicates that the majority of high-magnitude (>1–2 km throw), regionally distributed, surface-breaking normal faulting that dismembered the orogenic highland and produced the high structural relief observed today had to be post-Oligocene. Although the traces of 70 normal faults that are overlapped by the Paleogene unconformity are identified, the throw on nearly all of these structures is limited to a maximum of 1–2 km. This further highlights the paradox of extensive Late Cretaceous and Paleogene exhumation of mid-crustal rocks now exposed in metamorphic core complexes without corresponding high-magnitude upper crustal extension. The close spatial association of high exhumation magnitudes with the hanging walls of major thrust faults suggests that erosional exhumation is a response to relief generation accompanying contractional deformation in orogenic plateaus. The lack of significant along-strike exhumation variability within much of the Sevier hinterland implies that relief generation was relatively uniform along strike. The steep across-strike exhumation gradient between the leading part of the DWC sheet and the low-exhumation region to the west is interpreted as the result of shielding of the western area from headward erosion by long-lived uplift, erosion, and relief development through passive eastward translation of the DWC thrust sheet and growth of antiformal culminations at depth. A significant difference between the Sevier and central Andean orogenic plateaus is emphasized here. Much of the interior part of the Sevier plateau was an eroding highland, composed of preorogenic rocks, with low structural relief, while the interior part of the Andean plateau consists of a variably deformed and exhumed, synorogenic hinterland basin as much as 12 km thick, with very high structural relief. Thus, comparison of the structural level of the stratigraphic contact between preorogenic and synorogenic rocks indicates a difference in rock uplift between the interior parts of these two plateaus of as much as 14–15 km.