Evolution of porosity, permeability and fluid pressure in dilatant faults post-failure: implications for fluid flow and mineralization

Mineralization of brittle fault zones is associated with sudden dilation, and the corresponding changes in porosity, permeability and fluid pressure, that occur during fault slip events. The resulting fluid pressure gradients cause fluid to flow into and along the fault until it is sealed. The volume of fluid that can pass through the deforming region depends on the degree of dilation, the porosity and permeability of the fault and wall rocks, and the rate of fault sealing. A numerical model representing a steep fault cutting through a horizontal seal is used to investigate patterns of fluid flow following a dilatant fault slip event. The model is initialized with porosity, permeability and fluid pressure representing the static mechanical state of the system immediately after such an event. Fault sealing is represented by a specified evolution of porosity, coupled to changes in permeability and fluid pressure, with the rate of porosity reduction being constrained by independent estimates of the rate of fault sealing by pressure solution. The general pattern of fluid flow predicted by the model is of initial flow into the fault from all directions, followed by upward flow driven by overpressure beneath the seal. The integrated fluid flux through the fault after a single failure event is insufficient to account for observed mineralization in faults; mineralization would require multiple fault slip events. Downward flow is predicted if the wall rocks below the seal are less permeable than those above. This phenomenon could at least partially explain the occurrence of uranium deposits in reactivated basement faults that cross an unconformity between relatively impermeable basement and overlying sedimentary rocks.     

1. PRETEXTURES OF TIME

Textures of Time is a rich and challenging book that raises a host of important and hard questions about historical narrative, form, and style; the sociology of texts; and the core problem of ascertaining historical truth. Two that pertain to the book's main claims are of special interest to nonspecialist readers: Is register or style—“texture”—necessarily and everywhere diagnostic of “history”? Does a new kind of “historical consciousness” emerge in south India beginning in the sixteenth century, indeed as a sign of an Indian early modernity?Textures is not the first book to argue that historical discourse is constitutively marked by a peculiar style, but the claim is beset by difficulties that scholars since Barthes have detailed. Rather than textures of time—accounts of what really happened in history—what these works offer us may be only pretextures of time, textualized forms of a human experience that make claims about its degrees and types of truth through representations of various states of temporality. Instead of assessing, then, whether these works are history or something else like “myth,” we might ask whether they invite us to transcend this very dichotomy, to try, that is, to make sense of historical forms of consciousness rather than to identify forms of historical consciousness. As for modernity, nothing in south Indian historiography from 1500–1800 remotely compares to the conceptual revolution of Europe. But why should we expect the newness of the early modern world to have been experienced the same way everywhere? Modernity across Asia may have shown simultaneity without symmetry. Should this asymmetry turn out to reveal continuity and not rupture, however, no need to lament the fact. There is no shame in premodernity.