The argument about the
kinematic history of detachment
faults in metamorphic core
complexes has become quite
heated. One school believes that
these faults form and slip at
high (65
) dip angles, but
that the footwall bends to near-horizontal dip as it nears the
surface. Buck [1993]
extended this bending-footwall
model for normal fault rotation
and showed why low-angle
detachment faults should only be
found in regions of anomalous
heat flow. The classic example
of the active normal fault along
the front of the Black Mountains
in Death Valley has been shown to
be segmented in dip, from a
maximum of 60
W in the
subsurface to a minimum of
17
W where the footwall is
exposed to the east [
Miller, 1991].
However, striking evidence has
also been presented for the view
that at least some detachments
slip while in a near-horizontal
orientation. Dokka [1993]
used a new technique of
paleodepth determination to show
that the Newberry Mountains
detachment in California had an
initial dip of only 20 to
27
. The Rawhide detachment
fault in Arizona was active at a
low dip [ Scott and Lister,
1992], as shown by a marker tuff
and truncated normal faults (in a
section that cannot be balanced)
in the upper plate. In southeast
Arizona, a Miocene detachment
with a dip of 20
down to 6
km also projects updip to within
100 m of Quaternary scarps [
Johnson and Loy, 1992], which
seems to show that the fault
remains active at this dip.
If well-developed faults of large slip are intrinsically weak, however, these two views may not be incompatible. Detachments faults could form at high dips (while still strong), rotate to low dips by footwall bending, and continue to slip due to an aquired weakness. Some compensating deformation of the hanging wall would be required; in fact, almost all hanging walls seen in the Basin and Range province are extensively fractured and faulted.
An important new idea which has arisen in the last four years is that the lower crust of the Basin and Range province should behave as a viscous fluid, flowing in to fill the voids that extreme extension would otherwise form. Simple two-dimensional calculations [ Bird, 1991] show that in this province the high heat-flow should cause Moho topography to be destroyed in 1-20 million years (Ma) by lateral extrusion of the lower crust. In fact, observed variations of crustal thickness at the edge of the Basin/Range province are much less than they would be in balanced cross-sections, implying either lateral extrusion or massive intrusion; McCarthy and Parsons [1994] use seismic data to limit the amount of intrusion.
The spatial and temporal relationships between the different core complexes (the integration of regional velocity fields) and their ultimate cause remain problems for the future. One place where there is clarity is in a benchmark study of the patterns of faulting around the moving Yellowstone plume, by Pierce and Morgan [1992]; because much of this deformation was clearly distinct from earlier distributed Basin/Range extension, it may serve as a model for the interpretation of structure along other, more ancient plume tracks.