Fault controlled sequential vein dilation: competition between slip and precipitation rates in the Austin Chalk, Texas

Multi-layered calcite veins in a dilatant jog of a left-stepping, left-slipping shallowly buried fault segment are composed of alternating millimeter- to submillimeter-thick calcite veinlets and host lithons forming a coarse 'crack-seal' texture. The grain fabrics in calcite veinlets are mostly equant or irregular, suggesting face-controlled grain growth in a fluid-filled cavity. The relatively thick veinlets can be developed by progressive fault slip and veinlet opening simultaneously with calcite precipitation under low effective stress. Continuous changes in the oxygen isotopic compositions of the calcite veinlets along the length of veins suggest that the individual calcite veinlets were sequentially developed from the footwall to the hanging wall. There is no particular evidence that these veins represent excursions in fluid pressure or instantaneous fracture opening related to episodic fault slip; the fracture formation and filling cycle could have taken place along a continuously slipping fault contained within a porous rock with normal fluid pressure. (C) 2000 Elsevier Science Ltd. All rights reserved.