Footwall dip of a core complex detachment fault: thermobarometric constraints from the northern Snake Range (Basin and Range, USA)
Footwall dip of a core complex detachment fault: thermobarometric constraints from the northern Snake Range (Basin and Range, USA)
复制标题
DOI：
10.1111/j.1525-1314.2010.00907.x
复制
发表时间：
2010-12
影响因子：
3.4
通讯作者：
Frances J. Cooper;John P. Platt;R. Anczkiewicz;M. Whitehouse
中科院分区：
文献类型：
--
作者：
Frances J. Cooper;John P. Platt;R. Anczkiewicz;M. Whitehouse
低角度滑脱断层是大规模大陆伸展区域的常见特征，通常与变质核杂岩相关，它们将上部板块的脆性伸展与下部板块的韧性拉伸和变质作用分开。在许多核心杂岩体中，下盘岩石从中地壳深度到下地壳深度被折返，导致关于上盘和下盘岩石之间的关系以及滑脱断层在下盘折返中所起的作用的大量争论。这里介绍了蛇岭滑脱北部 (NSRD) 下盘变泥质岩内七个地点的石榴石 - 黑云母测温和石榴石 - 白云母 - 黑云母 - 斜长石气压测量结果，以及石榴石和锆石年代学数据。这些位置既平行又垂直于下盘输送方向，以限制下盘折返前的几何形状。为了精确确定下盘内的 P-T 梯度，采用了 Worley & Powell (2000) 的 ΔPT 方法，该方法最大限度地减少了系统不确定性对热压计算的影响。结果表明，下盘岩石的压力达到 6-8 kbar，温度达到 500-650 °C，相当于埋藏深度 23-30 km。下盘运移的 WNW-ESE 方向埋深保持不变，但从南向北逐渐增加。下盘运移方向上缺乏埋藏梯度意味着，如今定义断层运移方向上的亚水平基准的下盘岩石，在白垩纪晚期也定义了深度上的亚水平基准。这表明在折返过程中下盘并未相对于断层输导方向的法线倾斜，因此NSRD并未形成为切穿下地壳的低角度正断层。相反，提出了蛇岭北部下盘的以下演变。 (i) 中生代收缩通过斜地层序的重复和折叠导致地壳显着增厚，并伴有上部绿片岩到角闪岩相变质作用。 (ii) 大约一半的折返总量是通过晚白垩世至早第三纪的大致同轴拉伸和变薄完成的，同时伴有退缩和糜棱岩变形。 (iii) 然后，下盘岩石沿着适度倾斜的 NSRD 从中地壳“捕获”，该 NSRD 深入中地壳，在上端和下端均具有滚动铰链几何形状。
Low‐angle detachment faults are common features in areas of large‐scale continental extension and are typically associated with metamorphic core complexes, where they separate upper plate brittle extension from lower plate ductile stretching and metamorphism. In many core complexes, the footwall rocks have been exhumed from middle to lower crustal depths, leading to considerable debate about the relationship between hangingwall and footwall rocks, and the role that detachment faults play in footwall exhumation. Here, garnet–biotite thermometry and garnet–muscovite–biotite–plagioclase barometry results are presented, together with garnet and zircon geochronology data, from seven locations within metapelitic rocks in the footwall of the northern Snake Range décollement (NSRD). These locations lie both parallel and normal to the direction of footwall transport to constrain the pre‐exhumation geometry of the footwall. To determine P–T gradients precisely within the footwall, the ΔPT method of Worley & Powell (2000) has been employed, which minimizes the contribution of systematic uncertainties to thermobarometric calculations. The results show that footwall rocks reached pressures of 6–8 kbar and temperatures of 500–650 °C, equivalent to burial depths of 23–30 km. Burial depth remains constant in the WNW–ESE direction of footwall transport, but increases from south to north. The lack of a burial gradient in the direction of footwall transport implies that the footwall rocks, which today define a sub‐horizontal datum in the direction of fault transport, also defined a sub‐horizontal datum at depth in Late Cretaceous time. This suggests that the footwall was not tilted about the normal to the fault transport direction during exhumation, and hence that the NSRD did not form as a low‐angle normal fault cutting down through the lower crust. Instead, the following evolution for the northern Snake Range footwall is proposed. (i) Mesozoic contraction caused substantial crustal thickening by duplication and folding of the miogeoclinal sequence, accompanied by upper greenschist to amphibolite facies metamorphism. (ii) About half of the total exhumation was accomplished by roughly coaxial stretching and thinning in Late Cretaceous to Early Tertiary time, accompanied by retrogression and mylonitic deformation. (iii) The footwall rocks were then ‘captured’ from the middle crust along a moderately dipping NSRD that soled into the middle crust with a rolling‐hinge geometry at both upper and lower terminations.