QUANTITATIVE TECTONIC GEOMORPHOLOGY WITH APPLICATIONS TO NEOTECTONICS OF NORTHWESTERN ARIZONA

Abstract

Quantitative tectonic geomorphology is based on a conceptual synthesis between historical and process geomorphology that integrates physical models of geodynamics and erosional kinematics. Geodynamic models of flexure, heat flow and isostasy provide a framework for regional investigation. Topographic analysis and related filtering techniques allow for the description or delineation of regional features. Erosional kinematics modeled by a diffusion equation of heat flow allow the quantitative dating of uplifts across known structures. For large uplifts, the geometry of streams crossing faults is the key element in analyzing age. Theoretically, these faulted stream channels will readjust their gradients with time and these changes can be described as a diffusion process. At this time, however, sufficient data are not available to rigorously constrain estimates of uplift age. For small uplifts, those that result in piedmont fault scarps, statistical analysis of hundreds of scarp profiles suggests that the age of the faulting can be constrained. The apparent morphologic age of fault scarps is strongly affected by variables such as the resistance to erosion of the faulted rock type, but for scarps in alluvium, morphologic dating methods appear to be robust. Quantitative tectonic geomorphology of northwest Arizona and environs suggests that regional topographic patterns can be explained by nonuniform extension of the lithosphere. This process probably began as sub lithospheric erosion about 35 m.y. ago and resulted from a prolonged period of plate convergence and subduction. Superimposed on this predominantly thermal event was younger Basin and Range crustal extension and the opening of the Gulf of California. The Grand Wash fault, the boundary between the Basin and Range and Colorado Plateau physiographic provinces, has been active, though in decreasing amounts, from late Miocene to late Pleistocene time. Morphology of Quaternary piedmont fault scarps that exhibit repeated faulting suggest recurrence intervals on the order of 50,000-100,000 years. The Pitaycachi fault in Sonora, Mexico, which has similar recurrence intervals, broke in 1877, suggesting perhaps that many of the late Pleistocene faults in northwestern Arizona and environs are also due to break.