A number of theoretical studies and large scale mass balance
calculations have attempted to quantify the rates and flow patterns
associated with crustal scale flow systems in sedimentary basins.
The results must be viewed as order of magnitude estimates, at
best, due to the dearth of accurate permeability data at the regional
scale for sedimentary basins [ Oreskes et al., 1994]. Even less
certainty must be assigned to estimates of flow rates within the
deep crust where hydrologic conditions are poorly constrained.
Estimates taken from numerical modeling studies of the magnitude
of groundwater flow rates due to sediment compaction associated
with overthrusting [ Deming et al., 1990], accretionary prisms
[ Wang et al., 1993], and tectonic shortening during continental
collision [ Ge and Garven, 1992] would only produce a
maximum Darcy velocity of 1 cm/yr. and temperature anomalies of
about 5 C. However, if fluid dissipation is focused along fault
zones and released episodically, then higher flow rates and some
thermal perturbations may occur [Roberts and Nunn, in press].
Topography-driven groundwater flow, on the other hand, can
produce groundwater flow rates of up to 10 m/yr. and generate
significant thermal and salinity anomalies at the margins of basins.
The extent of heat and mass transfer by topography-driven flow is
subject to debate, however, due to the limited availability of
observational data. Numerical calculations of density-driven flow
within sedimentary basins, produce groundwater flow rates as high
as 0.1 to 1.0 m/yr. [ Evans et al., 1991; Person and
Garven, 1994]. However, numerical studies of thermal convection
near shallow magma chambers within high permeability crystalline
rocks (> 10
m
) predict convective flows with velocities
as high as 100 m/yr. [ Furlong et al., 1991].
Fluid flow in most deep crustal systems is probably episodic in
nature [ Nur and Walder, 1990; Walther, 1990;
Rumble, 1994]. No direct measure-ment of fluid velocity is
possible in these deep seated systems, and all published estimates
are based on indirect observation and/or calculations. Time
averaged fluid velocities were estimated from observed rock
alterations and mineral changes to be as high as 10
m/year
[ Baumgartner and Ferry, 1991; Ferry and Dipple, 1991].
The vertical fluid velocity in subduction zones was estimated at
10
to 10
m/year by Peacock [1990] from thermal
modeling of the dehydration of the descending slab. Estimates of
fluid velocities in regional terrains based on devolatization of
sediments undergoing metamorph-ism range from 10
to a few
cm/year [ Walther, 1990]. Calculations on fracture propagation
[ Walther, 1990; Nishiyama, 1991; Nakashima,
1993] indicate that individual cracks could propagate up to 1 km/h.
All of the above values are viewed as first approximations and are
thus subject to large uncertainties.