Three-Dimensional Models of Faulting During Oblique Continental Extension

该奖项是根据2009年美国复苏和再投资法案（公法111-5）资助的。将采用新开发的可处理弹粘塑性变形的三维数值模拟程序来研究大陆斜向伸展对断裂的控制作用。本项目分为两个阶段，重点研究大陆岩石圈构造伸展模式的控制作用。该项目的第一阶段处理有关断层发育物理的一般问题，并直接建立在二维伸展断层的先前工作基础上：(1)确定断层弱化参数，这些参数需要与自然和模拟研究中的正断层种群统计相匹配：将考虑模型和观测断层的大小频率分布和长度偏移比；(2)弹性对断层模式的影响，这关系到何种数值方法能最有效地模拟岩石圈伸展过程中的断层。在项目的第二阶段，将模拟3个自然类型的斜裂例子，每个例子都扩展了不同的初始热条件和地壳厚度：(1)一个区域，地壳在裂陷时可能是冷的，以评估裂谷带的预热程度，这是解释苏伊士湾断层沿轴分割和跨轴结构所需要的；(2)一个中等厚度、温暖地壳的区域，在那里，高度倾斜的伸展显然被两种不同的拉伸几何形状所容纳，这决定了产生加利福尼亚湾所见的从应变分割到拉伸的变化所需要的条件；(3)在科罗拉多河伸展走廊等地，厚热地壳的伸展可能导致变质核复合体的形成，以评估是否需要浮力岩体侵位来形成变质核复合体的穹状核。所有的模型都将考虑薄岩石圈地区的极端倾斜延伸，其中一套模型将包括浮力的中地壳层。大陆的分裂控制着地球表面的形状，并直接影响区域和全球气候。除了对地理和气候的好奇之外，研究断裂或裂谷是如何发生的还有经济上的原因。世界上大部分的油气储量都是在大陆裂谷边缘的地质构造中形成和保存下来的。研究小组最近提出了一种假设，即大陆分裂只发生在两种特殊情况中的一种：岩浆岩脉的注入加热并削弱岩石圈，或者在造山过程中地壳变厚也使岩石圈变暖并削弱。该项目的目标是研究这些非常不同的过程如何产生在大陆裂谷边缘观察到的各种各样的结构。为了研究裂缝结构的发展，将采用先进的数值模型，类似于工程师设计在合理荷载下不会破坏的结构。这些模型与大多数工程模型的不同之处在于，它们考虑了断层上的多个失效或滑动事件如何在非常大的空间和时间尺度上组合产生结构。以前，这些结构中的一些已经在二维中建模。现在，更好的数值方法和更大的计算机集群相结合，可以检查这些结构是如何在三维空间中发展的。这一点尤其重要，因为有些结构本质上是三维的。通过尝试再现观察到的断层、地壳厚度和地形的模式，断层的强度可能会受到限制，并有助于理解地震行为。

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).A newly developed three dimensional numerical modeling code that can deal with elasto-visco-plastic deformation will be used to study the controls on faulting during oblique continental extension. This project is a two-phase study that focuses on the controls on patterns of tectonic extension of continental lithosphere. The first phase of the project deals with general questions about the physics of fault development and builds directly on previous work on two dimensional extensional faulting: (1) determination of fault weakening parameters that are needed to match normal fault population statistics for natural and analog studies: size frequency distribution and length offset ratios of model and observed faults would be considered; (2) the effect of elasticity on the pattern of faulting, which has bearing on which numerical methods most efficiently simulate faulting during lithospheric extension. In the second phase of the project, three natural type examples of oblique rifting will be modeled, each of which extended with different initial conditions of thermal and crustal thickness: (1) a region where the crust was likely cold at the time of rifting in order to assess the degree of pre-heating of the rift zone that is needed to explain the along-axis segmentation and across-axis structure of faulting in the Gulf of Suez; (2) a region of moderately thick, warm crust, where highly oblique extension was apparently accommodated by two distinct geometries of transtension to determine what is needed to produce the change from strain partitioning to transtension seen in the Gulf of California; (3) extension of thick, hot crust that may have led to the formation of metamorphic core complexes in places like the Colorado River Extensional Corridor to evaluate if buoyant pluton emplacement is needed for the formation of domal cores of metamorphic core complexes. All the models would consider extreme amounts of oblique extension of areas with thin lithosphere and one suite of models would include a buoyant mid-crustal layer.The breakup of continents controls the shape of the Earth's surface and directly affects regional and global climate. Besides curiosity about geography and climate there are economic reasons to study the how breakup, or rifting, occurs. Much of the world's hydrocarbon reserves were formed and preserved in the geologic structures of rifted continental margins. The research team has recently advanced the hypothesis that continental breakup occurs only under one of two special circumstances: when injection of magmatic dikes heats and weakens the lithosphere or when thickening the crust during mountain building also warms and weakens the lithosphere. The goal of this project is to investigate how these very different processes may produce the great variety of structures observed at rifted continental margins. To study the development of rift structures advanced numerical models, similar to those used by engineers to design structures that do not fail under reasonable loads will be employed. These models differ from most engineering models in that how multiple failure or slip events on faults combine to produce structures on a very large spatial and temporal scale are considered. Previously, some of these structures have been modeled in two dimensions. Now, the combination of better numerical methods and bigger computer clusters permits examination of how these structures develop in three dimensions. This is particularly important since some of the structures are intrinsically three dimensional. By trying to reproduce the observed patterns of faults, crustal thickness, and topography the strength of faults may be constrained and aid in understanding earthquake behavior.