Crustal-scale fluid flow can be regarded as a bimodal transport mechanism. At low hydraulic head gradients, fluid flow through rock porosity is slow and can be described as diffusional. Structures such as hydraulic breccias and hydrothermal veins both form when fluid velocities and pressures are high, which can be achieved by localized fluid transport in space and time, via hydrofractures. Hydrofracture propagation and simultaneous fluid flow can be regarded as a “ballistic” transport mechanism, which is activated when transport by diffusion alone is insufficient to release the local fluid overpressure. The activation of a ballistic system locally reduces the driving force, through allowing the escape of fluid. We use a numerical model to investigate the properties of the two transport modes in general and the transition between them in particular. We developed a numerical model in order to study patterns that result from bimodal transport. When hydrofractures are activated due to low permeability relative to fluid flux, many hydrofractures form that do not extend through the whole system. These abundant hydrofractures follow a power-law size distribution. A Hurst factor of ~0.9 indicates that the system self-organizes. The abundant small-scale hydrofractures organize the formation of large-scale hydrofractures that ascend through the whole system and drain fluids in large bursts. As the relative contribution of porous flow increases, escaping fluid bursts become less frequent, but more regular in time and larger in volume. We propose that metamorphic rocks with abundant veins, such as in the Kodiak accretionary prism (Alaska) and Otago schists (New Zealand), represent regions with abundant hydrofractures near the fluid source, while hydrothermal breccias are formed by the large fluid bursts that can ascend the crust to shallower levels.

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地壳尺度达西与水力压裂流体输送之间的相互作用：数值研究

地壳尺度的流体流动可以被视为一种双峰运输机制。在低水头梯度下，流体流过岩石孔隙的速度很慢，可以描述为扩散的。当流体速度和压力很高时，水力角砾岩和热液脉等结构都会形成，这可以通过水力压裂在空间和时间上进行局部流体传输来实现。水力压裂传播和同时流体流动可以被视为一种“弹道”传输机制，当单独通过扩散传输不足以释放局部流体超压时，该机制被激活。通过允许流体逸出，弹道系统的激活局部地降低了驱动力。我们使用数值模型来研究两种运输方式的一般特性，特别是它们之间的过渡。我们开发了一个数值模型来研究由双模式运输产生的模式。当水力压裂由于相对于流体通量的低渗透性而被激活时，会形成许多不延伸到整个系统的水力压裂。这些丰富的水力压裂遵循幂律尺寸分布。~0.9 的 Hurst 因子表明系统是自组织的。大量的小型水力压裂组织形成大型水力压裂，这些水力压裂在整个系统中上升并以大爆发的方式排出流体。随着多孔流动的相对贡献增加，流出的流体爆发变得不那么频繁，但在时间上更规律且体积更大。我们建议具有丰富矿脉的变质岩，例如科迪亚克增生棱柱（阿拉斯加）和奥塔哥片岩（新西兰），