Untitled Document

Evolution of Fluid Pathways During Deformation

Structural and stable isotope studies of a one kilometre thick, Lower Devonian carbonate sequence (Murrumbidgee Group) in the Taemas area of the Lachlan Fold Belt (south-eastern Australia) indicate that externally-derived fluids migrated through the sequence during upright folding and associated reverse faulting at depths of several kilometres and temperatures in the range 150ºC to 200ºC. The evolution of fluid pathways during crustal shortening was controlled by growth of fault-related and fold-related vein networks at transiently supralithostatic fluid pressures (see figure below).

Extension vein arrays in competent massive limestone unit (Currajong Limestone). Such units have been stretched and fractured at various stages during progressive fold growth.

Systematic changes in O-isotope compositions of veins up through the carbonate sequence are related to buffering of externally-derived fluid compositions by progressive reaction with the host rocks along the structurally-controlled fluid pathways. At the base of the carbonate sequence, calcite veins are 18 O depleted by up to 23‰ relative to unaltered host-rock limestones. 18O-depleted alteration haloes up to 20 metres wide in wall-rock in areas of intense vein development are related to fluid discharge from faults and associated vein arrays. Higher in the carbonate sequence, fault- and fold-related veins typically exhibit progressively less depletion relative to the distal host rocks (see below). In the upper parts of the sequence, vein d 18 O is usually less than 1-2‰ less than unaltered host rocks. Systematic depletion of vein 18 O by up to 23‰ also occurs immediately adjacent to the high displacement Warroo Fault, which bounds the eastern side of the carbonate sequence.

Decreasing 18 O depletion within faults upwards through the carbonate sequence indicates vein formation was associated with upwards infiltration of fluids having an initial d 18 O of -8‰. These fluids are interpreted to be evolved meteoric fluids or formation waters which migrated through the Black Range Group, a volcanic sequence underlying the Murrumbidgee Group. Reactive transport modelling of this flow system indicates time-integrated fluid fluxes of approximately 10 2 moles H 2 O cm -2 .

 

Variation in vein d 18 O as a function of stratigraphic height in the Murrumbidgee Group.

At the eastern boundary of the area, fluids which percolated through parts of the Warroo Fault also had an initial d 18 O of -8‰. This fault is interpreted to have tapped fluids from the same reservoir which supplied fluids that migrated through faults at the base of the Murrumbidgee Group during contractional deformation.

Within-site variations in d 18 O between veins are interpreted in terms of variations in (1) relative timing of formation of veins during progressive migration of the geochemical front through the sedimentary sequence, (2) changes in connectivity between the backbone part of the fracture-controlled flow network and developing fracture arrays, and (3) local variations in stable isotope compositions of host rocks along fluid pathways.

Systematic variations in stable isotope compositions of veins indicate that most faults were well-connected to an external fluid reservoir. This suggests that growth of fault and fracture networks has been driven largely by invasion of high pore fluid factor fluids.