In northwestern Arizona, the high-standing, relatively unextended Colorado Plateau abruptly gives way across a system of major west-dipping normal faults to a highly extended part of the Basin and Range province known as the northern Colorado River extensional corridor. The transition from unextended to highly extended upper crust is unusually sharp within this region, contrasting with a broad transition zone elsewhere. The southern White Hills lie near the eastern margin of the extensional corridor in northwestern Arizona and contain a large east-tilted half graben that chronicles Miocene extension and constrains the timing of structural demarcation between the Colorado Plateau and Basin and Range province during Neogene time. This growth-fault basin is bounded on the east by the west-dipping Cyclopic and Cerbat Mountains fault zones. Greater tilts in the hanging walls suggest that these faults have listric geometries. The stratigraphy in the half graben consists of Miocene vol canic rocks intercalated with an eastward-thickening wedge of synextensional fanglomerates. Tilts in the Miocene units decrease up section from ~75° to 5°. Recent 40 Ar/ 39 Ar dating (11 new dates) of variably tilted volcanic rocks in the growth-fault basin and regional relations constrain the timing of east-west extension between ca. 16.6 and <9 Ma, with peak extension from ca. 16.6 to 15.2 Ma. Capping 8.7 Ma basalts are tilted 5°–10° and record the waning stages of extension. Thus, the sharp boundary between the Colorado Plateau and Basin and Range began developing by ca. 16.5 Ma and has changed little since ca. 9 Ma. Major extension and basin development significantly lowered base level within the extensional corridor and induced headward erosion into the western margin of the Colorado Plateau, which ultimately facilitated development of the western Grand Canyon. Abundant clasts of 1.7 Ga megacrystic granite in the eastward-thickening fanglomerates within the growth-fault basin suggest a partial provenance from the Garnet Mountain area along or near the western margin of the Colorado Plateau beginning as early as ca. 16 Ma and continuing to ca. 9 Ma.

Abstract The Neogene stratigraphic section in the Split Mountain area exposes megabreccia deposits up to 12 km long with volumes up to 3 × 10 8 m 3 . Shattered-rock domains still portray the bedrock distribution of lithologies. Jigsaw-puzzle fabric occurs on a variety of scales from microscopic to outcrop. Broken and stretched pegmatites tend to rise upward as step-ups in the inferred down-flow directions. Upper Miocene subaerial megabreccias about 65 m thick disturbed the underlying strata to depths less than a meter during their emplacement. This includes producing grooved and decapitated stones both in the substrate and below shear surfaces especially within the basal few meters of the megabreccia deposits. The lower portions of a megabreccia are rich in step-ups, ramps, and crushed-rock streamers that rise upward in the down-flow direction. After flooding of the basin by the ancestral Gulf of California, a lower Pliocene megabreccia moved across the sea floor deforming underlying sedimentary layers by injections and sunken megabreccia lobes that locally caused tightly folded bottom-sediment packages >35 m thick to rise as diapirs. Near the leading edge, on the southwest corner of the deposit, there is a small volume of more traditional sandy conglomerate deposited as the mass rapidly slowed and stopped. Both subaerial and subaqueous megabreccias contain lithologic domains that preserve the distribution of bedrock lithologies, jigsaw-puzzle fabric, step-ups and crushed-rock streamers; these features all require non-turbulent flow. These huge volumes of shattered bedrock moved 10–12 km distance in late Miocene as dry subaerial masses, and again across the floor of an early Pliocene inland sea. All the observed features strongly indicate flow as sturzstroms.

A systematic relationship exists between seismically defined, master normal faults of the Albuquerque Basin and the structural style of rift shoulders on the basin margins. Rift shoulders occupying the footwalls of these master faults have at least three characteristics distinguishing them from the other shoulders of the basin: 1. They are the source areas for the basin’s major fanglomerates in the syn-rift Santa Fe Group. 2. They display distinctly Neogene apatite fission-track ages. 3. They have undergone the greatest amount of rift-related uplift in the Neogene. These relationships imply that uplift of these rift shoulders may be in part, the result of isostatic rebound of the lithosphere, accompanying tectonic unloading by the seismically defined master faults of the basin.

A 1.3-km-thick section of basin-fill sediments within the Skardu intermontane basin of the Karakoram Himalaya, termed the Bunthang sequence, is predominantly reversely magnetized. Glacial deposits and landforms are closely associated with the Bunthang sequence. This implies that basin filling took place between glacial advances and prior to the present Brunhes normal chron, i.e., between 3.2 and 0.73 Ma. Four major facies interfinger within the Bunthang sequence: glacial facies; lacustrine facies, indicative of periods during which the Indus River was ponded within the basin; aggradational fluvial facies that represent periods during which the gradient of the Indus River was controlled by rising base level downstream; and alluvial fanglomerates that prograde transversely from the basin margin into the basin at different stratigraphic levels. The last facies represents periods of decreased sedimentation by the Indus River relative to alluvial sedimentation from the basin margin. Downstream from the Skardu Basin, the Indus River crosses the Nanga Parbat–Haramosh syntaxis; this is an area of rapid late Cenozoic uplift (as much as 1 cm/yr). Differential uplift of the Nanga Parbat–Haramosh syntaxis relative to its surrounding terrain led to local variations in Indus River gradient. Temporary ponding of the Indus River is inferred to have occurred when the uplift rate of the Nanga Parbat–Haramosh syntaxis exceeded the rate of downcutting by the Indus River. Temporary blockage of the Indus River Gorge through the Nanga Parbat–Haramosh syntaxis by glaciers or by major landslides may also have led to variations in base level. Temporal variations in the gradient and base level of the Indus River downstream from the Skardu Basin, are reflected in the different facies that interfinger within the Bunthang sequence at Skardu.