Influence of Strain Symmetry and Vorticity of Flow on Thrust Sheet Evolution at Mid-Crustal Levels in the Caledonides of NW Scotland: Implications for Transport-Parallel Extrusion

对造山带逆冲系统的几何分析通常假设应变基本上是二维的，没有沿走向的伸展或收缩。然而，大多数造山带，显示出广泛的颗粒形状组构范围从纯S通过L-S到L构造岩，这表明平面应变变形可能是例外而不是规则。S和L构造岩的形成提出了主要的视空间问题。例如，S构造岩是否表示与沿走向伸展相关的压扁应变，而L构造岩是否表示与沿着走向缩短相关的收缩应变？ 如果这些3D应变是真实的，那么为了保持应变相容性，如何适应沿走向的伸展和收缩？ 或者，这些构造岩是否仅仅反映了由应变叠加或体积变化引起的“表观”应变，而没有真实的沿走向的伸展或收缩？ 即使应变是二维的，如果显微结构或岩组数据表明纯剪切（同轴）变形的重要组成部分，可能仍然存在潜在的空间问题。这是因为纯剪切变形反过来又意味着一个重要的组成部分，近水平运输平行延伸。一些作者认为这个空间问题是假设简单剪切变形的一个令人信服的理由，而另一些作者则认为，纯剪切的成分可能是至关重要的中地壳岩石向地形表面挤压。 与逆冲岩片侵位相关的应变的数值模拟几乎完全采用了二维方法，假设观察到的三维应变是由平行于运输方向的平面应变（纯剪切和简单剪切）变形的各种组合产生的“表观”应变。可以说，现在已经达到了一个阶段，应变建模比现场数据需要约束它。未来有意义的进展只能通过确定实际的应变路径（包括应变大小，对称性和流动涡）与结构良好的约束自然系统的演变，从而“地面真实”的假设，在这些复杂的模型。该项目涉及生成这样的自然应变路径数据的综合实地测绘，应变和涡度分析，在代表性的运输平行和造山走向平行横穿Moine推覆体和下伏糜棱岩的Moine逆冲带。加里东造山带的这一部分是研究中地壳水平三维应变和流动涡度域变化的杰出天然实验室，因为：a）我们的勘察工作表明，各种应变、岩石组构和涡度分析方法适用于莫因推覆体和下伏莫因逆冲断层的塑性变形岩石，B）英国地质调查局和学术界几代地质学家在过去120年中进行的先前测绘为我们的应变/晶体组构和涡度数据提供了一个非常好的约束结构框架。在与同事的合作中，这项研究产生的自然数据被用于开发与造山运动相关的更逼真的三维数值模型。这项研究为博士学位提供了支持。该项目的研究成果将成为2007年5月在苏格兰举行的“大陆构造与造山运动”会议相关的专题讨论会和实地考察的核心。

Geometric analyses of thrust systems in mountain belts commonly make the general assumption that strain is essentially two dimensional, with no along-strike extension or contraction. Most orogenic belts, however, display a wide spectrum of grain shape fabrics ranging from pure S through L-S to L tectonites, suggesting that plane strain deformation may be more the exception than the rule. Formation of S and L tectonites presents major apparent space problems. For example, do S tectonites indicate flattening strains associated with along-strike extension, while L tectonites indicate constriction associated with along strike shortening? If these 3D strains are real then, in order to maintain strain compatibility, how is along-strike extension and contraction accommodated? Alternatively, do these tectonites merely reflect 'apparent' strains caused by strain superposition or volume change, with no real along-strike extension or contraction? Even if strain is two dimensional potential space problems may remain if microstructural or petrofabric data indicate a significant component of pure shear (coaxial) deformation. This is because pure shear deformation in turn implies a significant component of sub-horizontal transport-parallel extension. Some authors regard this space problem as a compelling reason for assuming simple shear deformation, while others have argued that components of pure shear may be of critical importance in extruding mid-crustal rocks towards the topographic surface. Numerical modeling of strains associated with thrust sheet emplacement has almost exclusively taken a 2D approach by assuming that observed 3D strains are 'apparent' strains produced by various combinations of plane strain (pure and simple shear) deformation oriented parallel to the transport direction. Arguably a stage has now been reached where strain modeling is more advanced than the field data needed to constrain it. Future meaningful progress can only be made by determining the actual strain paths (including strain magnitudes, symmetries and flow vorticities) associated with evolution of structurally well-constrained natural systems, thereby "ground-truthing" the assumptions made in these sophisticated models. This project involves generating such natural strain path data by integrated field mapping, strain and vorticity analysis in representative transport-parallel and orogenic strike-parallel traverses across the Moine Nappe and underlying mylonites of the Moine thrust zone. This part of the Caledonian Orogen is an outstanding natural laboratory for investigating domainal variation in 3D strain and vorticity of flow at mid-crustal levels because: a) our reconnaissance work indicates a wide range of strain, petrofabric and vorticity analysis methods are applicable to the plastically deformed rocks of the Moine Nappe and underlying Moine thrust, b) previous mapping over the last 120 years by generations of geologists from the British Geological Survey and academia has resulted in an exceptionally well-constrained structural framework for our strain/crystal fabric and vorticity data. In collaboration with co-workers, the natural data generated by this study are being used to develop more realistic 3D numerical models for flow associated with mountain building. The research is providing support for a Ph.D. student, will facilitate collaboration between U.S. universities and the British Geological Survey, and results from the project will form the nucleus for symposia and fieldtrips associated with a conference on 'Continental Tectonics and Mountain Building' that will be based in Scotland in May 2007.