The upper crust on its side—steeply tilted slabs in the Basin and Range

Tilted slabs that expose as much as 8–15 km of the upper crust in the Basin and
Range province allowanalysis of pre-tilt depth variations in igneous systems and thermal
structure. Before tilting the slabs were panels between moderately to steeply dipping
Tertiary normal faults. The slabs and their bounding normal faults were tilted to
piggyback positions on deeper footwalls that warped up isostatically beneath them
during crustal stretching and tectonic unloading. Stratal dips within some slabs are
now vertical or even slightly overturned, especially in the southern Basin and Range,
where thick sections of basement granite and gneiss are tilted homoclinally as determined
from both stratified cover and intrusive-sheet proxies of original horizontality.
Some tilted slabs of basement rock display faults that splay upwards into overlying
cover sequences, folding them to shallower dips, so stratified rocks are not always the
best indicators of slab tilt. The 12–15-km maximum exposed paleodepth for the slabs
matches the depth of the modern base of the seismogenic zone of normal-fault earthquakes
in the Basin and Range, and so this depth is inferred to represent the pre-tilt
base of the brittle part of the extending crust. Many highly tilted slabs are upper-
plates to metamorphic core complexes, but not all core complexes expose thick recumbent
upper-plate slabs. Paleozoic stratal sequences in thin upper-plate fault slices
commonly are dragged or sheared subparallel to the faults, thereby obscuring the
original fault-bedding cutoff angles and maximum block tilt. The Ruby Mountains
core complex, for example, preserves only scraps of upper-plate rocks as domed-up
extensional klippen uneven in stratal dips, and most of the thick crustal section that
originally overlay the uplifted metamorphic core nowmust reside at depth to the west.
In contrast, large intact tilted fault-bounded slabs of plutonic and metamorphic basement
rock near core complexes in the Colorado River extensional corridor and the
San Pedro Trough expose 8–15 km paleodepths that originally roofed the
metamorphic cores; the exposed paleodepths imply that deeper core-complex
footwalls rolled up beneath such slabs.