AN ENIGMATIC MOUNTAIN-BLOCK EXTENSIONAL TRANSFER ZONE WITHIN THE CENTRAL COLORADO RIO GRANDE RIFT

Understanding of extensional strain transfer and accommodation in continental rifts has grown considerably, but few studied transfer zones exhibit high internal topographic and structural relief. In the Rio Grande rift of Colorado the WNW-trending northern tip of the Sangre de Cristo Range separates the opposite-tilted Upper Arkansas River (UAR) and San Luis half grabens. We investigated the development of faults flanking this “Poncha” intrarift mountain block and their role in transferring extension between rift basins and contributing to mountain block surface uplift and landscape evolution. The elevated high-relief core of the Poncha block consists of Proterozoic metamorphic and plutonic rocks overlain on its west and southwest flanks by 34.6-33-Ma volcanic rocks and Mio-Pliocene basin-fill deposits of the Dry Union Formation. Similar Dry Union sediments underlie a moderately elevated, strongly dissected older piedmont along the northern mountain flank. All the flanking units are tilted 10-35º to the W and SW. A broad WNW-trending, right-stepping fault system ~33 km in length separates the piedmont Dry Union sediments and UAR basin from the steep northern Poncha mountain front underlain by Proterozoic rocks. The fault system continues west to the N-striking Sawatch range-front fault. Slip measurements along the Poncha frontal fault system within Dry Union and overlying deposits as young as ~200 ka indicate mainly dextral-normal oblique movement on WNW-striking fault segments and sinistral-normal movement on faults striking ~NNW. The southern continuation of the Sawatch range-front fault forms the western terminus of the Poncha block. Gently tilted proximal diamicton and alluvial deposits on the downthrown blocks of both range-front faults record Plio-Pleistocene mountain-block uplift. Preliminary paleomagnetic data from the volcanic rocks and Dry Union sediments suggest modest (25±11^o) clockwise vertical-axis rotations accompanied oblique-slip faulting on the Poncha flanks. Tentatively, we view the Poncha frontal fault system as a broad normal-oblique relay zone that developed in the late Neogene to transfer extensional strain across the left step between the Sawatch and Sangre de Cristo rift-bounding faults, coevally accommodating surface uplift of the Poncha mountain block.