Evolution of the middle and lower crust during the transition from contraction to extension in Fiordland, New Zealand
Evolution of the middle and lower crust during the transition from contraction to extension in Fiordland, New Zealand
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新西兰峡湾地区中下地壳从收缩到伸展转变过程中的演化
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发表时间：
2009
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通讯作者：
D. King
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作者：
K. Klepeis;D. King
新西兰西南部的一个深度侵蚀造山带保存了从收缩和地壳增厚到伸展和地壳减薄的过渡期间中地壳和下地壳流动模式变化的记录。新西兰的暴露表明，在中低地壳深度的变形模式强烈影响的地壳结构，温度，成分，岩浆活动和流变学的局部变化。运动学参数，包括剪切带边界的方向，非同轴度和运动学分区，应变对称性的程度，以及剪切带是否增厚或变薄在不同的观察平面，是非常可变的空间和反复改变了8-10马期间。然而，尽管存在这种变异性，叠加变形的几个方面仍然保持不变，可以分配到独特的构造环境。在北方峡湾119-111 Ma期间形成的所有剪切带都记录了大量的水平（平行层）缩短、垂直（垂直层）增厚和>50%的纯剪切，而与剪切带的方向、非同轴度、应变对称性和温度条件无关。与此相反，所有剪切带，在114-90 Ma期间形成的中央峡湾记录流涉及垂直减薄，近水平拉伸和40%-50%的纯剪切。这些格局分别与区域收缩和区域扩张相关。这些数据表明，在~100 km的长度尺度和约100 km的时间尺度上，10 Ma，板块边界动力学变化对中下地壳变形模式的影响可以与局部边界条件变化的影响区分开来，包括陡峭的温度梯度和可变的流变学。* 基思. uvm.edu Klepeis，K.A.，和国王，D.S.，2009年，新西兰峡湾中地壳和下地壳从收缩到伸展过渡期间的演化，米勒，R.B.，和斯诺克，A.W.，编辑，来自北美西部科迪勒拉山脉和其他地方的地壳横截面：对构造和岩石学过程的影响：美国地质学会特别论文456，第243-266页，doi：10.1130/2009.2456（09）。如需复制许可，请联系editing@geosociety.org。©2009美国地质学会。All rights reserved. 244 Klepeis and King spe 456-09 page 244 regions than it is in ocean regions（Royden，1996;博蒙等人，2001年）。然而，尽管认识到它的重要性，低地壳流动的特点和后果是地球动力学最不了解的方面。例如，随着构造环境和大陆变形驱动力的变化，下地壳中演化出什么类型的流动模式？当地壳深部的局部流变学和物理条件发生变化时，流动模式是如何变化的？中下地壳不均匀性的长度和时间尺度是什么？深地壳暴露的研究为回答这些问题提供了一个重要而有用的途径。地球物理图像提供了下地壳结构的瞬时视图，但深地壳组构的年龄和运动学意义通常难以解决（Nemes等人，1997; McBride和克纳普，2002;杰克逊，H.R.，2002年）。古代中下地壳的自然暴露可能使我们能够直接确定地壳深部的变形与其他可观察到的特征之间的关系。然而，对不同背景下的中地壳和下地壳的研究也表明，下地壳的流变学和热结构极其不均匀，在造山循环期间会迅速变化（米勒和帕特森，2001年，惠特尼等人，2004年; Rusmore等人，2005; Karlstrom和威廉姆斯，2006）。在大多数自然系统中，变形的这种时间依赖性和非均匀性通常掩盖了控制下地壳变形的力的性质，并使数值模型在自然现象中的应用复杂化。许多数值和分析技术采用需要稳态条件或限制在地质长度和时间尺度上同时操作的变量的数量的算法（Royden，1996; Lin等人，1998，Jiang和威廉姆斯，1998; Jiang等人，2001;博蒙等人，2001年）。我们预测大陆岩石圈变形对驱动力变化（包括板块运动）的响应的能力，依赖于对造山过程中下地壳行为的充分确定。在本章中，我们报告了新西兰峡湾暴露的中下地壳深度侵蚀剖面中的韧性流的半定量、基于野外的调查结果（图1）。这些暴露的年轻时代和良好的构造环境使我们能够检查中下地壳对图1中主要构造变化的物理响应。伍德（1972年）、奥利弗和科贡（1979年）、J.Y. Bradshaw（1989年a）、Daczko等人（2002年a）、Tulloch和Kimbrough（2003年）以及Klepeis等人（2004年）。Turnbull等人（2005年）的Resolution岛附近区域。缩写如下：CS-Caswell Sound; DS-Doubtful Sound; LM-Lake Manapouri; LTA-Lake Te Anau; MS-Milford Sound; RI-Resolution Island。方框显示研究领域。50公里0
A deeply eroded orogen in southwest New Zealand preserves a record of changing flow patterns in the middle and lower crust during a transition from contraction and crustal thickening to extension and crustal thinning. The New Zealand exposures show that deformation patterns at mid-lower crustal depths were strongly influenced by local variations in crustal structure, temperature, composition, magmatic activity and rheology. Kinematic parameters, including the orientation of shear zone boundaries, the degree of non-coaxiality and kinematic partitioning, strain symmetry, and whether shear zones were thickening or thinning in different planes of observation, were extremely variable spatially and changed repeatedly over an 8–10 Ma period. However, despite this variability, several aspects of superposed deformations remained constant and can be assigned to distinctive tectonic settings. All shear zones that formed during the 119–111 Ma period in Northern Fiordland record flow involving bulk horizontal (layer-parallel) shortening, vertical (layer-perpendicular) thickening and >50% pure shear regardless of shear zone orientation, degree of noncoaxiality, strain symmetry, and temperature conditions. In contrast, all shear zones that formed during the 114–90 Ma period in Central Fiordland record flow involving vertical thinning, subhorizontal stretching and 40%–50% pure shear. These patterns are correlative with regional contraction and regional extension, respectively. The data suggest that at length scales of ~100 km and time scales of ca. 10 Ma, the effects of changing plate boundary dynamics on deformation patterns in the middle and lower crust can be distinguished from the effects of changing local boundary conditions, including steep temperature gradients and variable rheology. *Keith.Klepeis@uvm.edu Klepeis, K.A., and King, D.S., 2009, Evolution of the middle and lower crust during the transition from contraction to extension in Fiordland, New Zealand, in Miller, R.B., and Snoke, A.W., eds., Crustal cross sections from the western North American Cordillera and elsewhere: Implications for tectonic and petrologic processes: Geological Society of America Special Paper 456, p. 243–266, doi: 10.1130/2009.2456(09). For permission to copy, contact editing@geosociety.org. ©2009 The Geological Society of America. All rights reserved. 244 Klepeis and King spe 456-09 page 244 regions than it is in oceanic regions (Royden, 1996; Beaumont et al., 2001). However, despite recognition of its importance, the characteristics and consequences of lower crustal flow are among the least understood aspects of geodynamics. For example, what types of flow patterns evolve in the lower crust as tectonic settings and the driving forces of continental deformation change? How do flow patterns change as local rheologies and physical conditions in the deep crust change? What are the length and time scales of middle and lower crustal heterogeneity? The study of deep-crustal exposures provides an important and useful approach to answering these questions. Geophysical images provide instantaneous views of lower-crustal structure but the age and kinematic significance of deep-crustal fabrics commonly is difficult to resolve (Nemes et al., 1997; McBride and Knapp, 2002; Jackson, H.R., 2002). Natural exposures of ancient middle and lower crust potentially allow us to determine directly how deformation in the deep crust relates to other observable features. However, studies of the middle and lower crust in different settings also have shown that the rheology and thermal structure of the lower crust are extremely heterogeneous and can change rapidly during orogenic cycles (Miller and Paterson, 2001, Whitney et al., 2004; Rusmore et al., 2005; Karlstrom and Williams, 2006). This time-dependent, heterogeneous nature of deformation in most natural systems commonly obscures the nature of the forces that control lower-crustal deformation and complicates the application of numerical models to natural phenomena. Many numerical and analytical techniques employ algorithms that require steady-state conditions or limit the number of variables that operate simultaneously over geologic length and time scales (Royden, 1996; Lin et al., 1998, Jiang and Williams, 1998; Jiang et al., 2001; Beaumont et al., 2001). Our ability to predict the response of deforming continental lithosphere to changes in driving forces, including plate motions, relies on an adequate determination of lower-crustal behavior during orogenesis. In this chapter, we report the results of a semiquantitative, field-based investigation of ductile flow in a deeply eroded section of exposed middle and lower crust in Fiordland, New Zealand (Fig. 1). The young age and well-constrained tectonic setting of these exposures allowed us to examine the physical response of the middle and lower crust to a major tectonic change from Figure 1. General geologic map of western New Zealand after Wood (1972), Oliver and Coggon (1979), J.Y. Bradshaw (1989a), Daczko et al. (2002a), Tulloch and Kimbrough (2003), and Klepeis et al. (2004). Area near Resolution Island from Turnbull et al. (2005). Abbreviations are as follows: CS—Caswell Sound; DS—Doubtful Sound; LM—Lake Manapouri; LTA—Lake Te Anau; MS—Milford Sound; RI—Resolution Island. Boxes show areas of study. 50 km 0