THERMAL CONSTRAINTS ON BASIN-SCALE FLOW SYSTEMS

An important constraint on basin-wide flow systems is the thermal regime. Studies of high temperatures above fault zones which penetrate geopressured sediments and heat flow in orogenic zones have been used to document both pathways and velocities of pore fluid movement. Similarly, the high temperature (100-150-degrees-C) at shallow depths (< 1.5 km) inferred from studies of fluid inclusions associated with Late Paleozoic Mississippi Valley-type (MVT) lead-zinc deposits places a very restrictive constraint on their formation by regional fluid migration. We have used numerical models of fluid and heat transport to evaluate the viability of sediment compaction by overthrusting and topographically driven recharge as driving mechanisms for regional fluid migration. Sediment compaction by overthrusting leads to Darcy velocities on the order of cm/yr, too low to have significant effects on the temperature field unless flow is channelized. Topographically driven recharge can lead to fluid velocities on the order of m/yr. However, it is difficult to achieve the extremely high temperatures (100-150-degrees-C) at shallow depths (< 1.5 km) associated with MVT deposits unless heat flow values greater than 100 mW/m2 are used. Thus, there are fundamental limitations on the ability of either mechanism to continuously transport large amounts of heat for millions of years on a basin-wide scale. Temporal or spatial focusing of flow is likely to have played a critical role in the genesis of phenomena such as MVT deposits.