STRUCTURAL STYLE AND MAGNITUDE OF EXTENSION AT THE SOUTHWEST CORNER OF THE COLORADO PLATEAU IN NORTHWESTERN ARIZONA

The Colorado Plateau ends abruptly in a series of down to the west normal faults in northwestern Arizona. The major faults are, from east to west, the Toroweap, Hurricane, and Grand Wash faults. The Grand Wash Fault forms the topographic edge of the Plateau. These high angle faults strike generally NNE and are hundreds of kilometers long. The offset on major faults is up to a few thousand meters. In north-central Arizona, the boundary between the Colorado Plateau and the Basin and Range Province is a wide transition zone characterized by smaller NW-striking normal faults. The central Arizona Transition Zone-Colorado Plateau boundary is the erosional Mogollon Rim rather than major faults. The Aubrey fault may be a transitional structure between the two different boundary zones. The purpose of this study is to quantify the magnitude of extension and the changes in structural style around the southwest corner of the Colorado Plateau from the southwest Utah-northwest Arizona border to Chino Valley north of Prescott, Arizona.

To show the changes in structural style, existing maps were compiled and a series of seven cross-sections were drawn based on these maps. The cross-sections are drawn perpendicular to the major Plateau-bounding faults at a scale of 1:48,000, covering the transition from the Colorado Plateau to the Basin and Range. To quantify the magnitude of extension, a fault budget was completed for each cross-section line. The fault budgets list the orientation and offset of all mapped faults. The cross-sections and fault budgets are utilized to compare the two main fault systems and shed light on the nature of the transition between the two.

Initial results show that the Colorado Plateau in northwestern Arizona is dominated by down to the west normal faults and half grabens. Movement is transferred eastward by stepovers and smaller NW striking faults. The major faults appear to die out near Peach Springs and Seligman, Arizona. Faults become less numerous, displacement decreases, and faulting jumps to the east as one moves toward the southwest corner of the Colorado Plateau. Additional field work is necessary to measure fault dips and calculate extension in detail, but this study will provide the framework and focus more detailed studies.