Plots of dip measurements taken from syntectonic deposits in extended terranes provide a simple and useful technique that can help to unravel complex deformation histories. In the Castaneda Hills-Signal area of west-central Arizona, dip analysis has enabled us to identify an unconformity that is at best equivocally displayed in the field. This unconformity marks the inception of extension that formed the highly extended terranes (HET). The technique also highlights a later unconformity that marks the onset of basin-range (BR) extension, and one that formed because extension ceased and through-flowing drainage developed.
Northeastward decrease of dips related to HET-type extension is evidence for a decrease in deformation toward the Colorado Plateau. Conversely, a mild increase of dips of younger basin-fill deposits in that direction suggests that the locus of strongest deformation shifted toward the Colorado Plateau in BR time. The variability in amount and timing of tilting displayed by the dip data supports listric and "rolling-hinge" models of extensional tectonism more than the "domino" model of upper-plate, HET-type, normal faults.
Dip data and stratigraphic data enable us to construct a sequence of events for middle Tertiary extension in the Castaneda Hills-Signal area. The first phase started ca. 27 Ma and consisted of upwarp of the area now occupied by the Buckskin-Rawhide metamorphic core complex. No rotation of blocks or volcanism accompanied this phase. The second phase (19 to 13 Ma) is characteristic of HET-type extension and was marked by movement on a basal detachment fault and upper-plate normal faults, rotation of blocks along near-horizontal axes, and widespread basalt-rhyolite volcanism. The third phase (about 13 to 7.5 Ma) is characteristic of BR extension and was marked by movement on high-angle normal faults, minor rotation of blocks, and basaltic volcanism. The fourth and final phase (ca. 7.5 to 0 Ma) saw the waning of tectonism, the inception of through-flowing drainage related to the Colorado River, and the effusion of thin megacryst-bearing basalt flows.
The early uplift (probably dome-like and thermally driven), the areal variability of rotation, and the migration of deformation toward the northeast do not support the concept that the extended terranes result from simple shear along a low-angle fault of crustal dimensions or from a rolling-hinge mechanism. A more attractive mechanism is a thermal welt in the upper mantle and lower crust that moves northeast with time in a manner analogous to that proposed for the Tyrrhenian Sea-Apennine system.