New Mexico Geological Society Annual Spring Meeting — Abstracts

Detailed geologic mapping and structural analysis define the Joyita Hills (Los Canoncitos) as a north-northeast trending, west-tilted fault block at the southern end of the Albuquerque Basin. Fault and joint orientations preserved within the Phanerozoic stratigraphy of the Joyita Hills record at least two different episodes of normal faulting. Rotation of bedding with contained fault and joint patterns back to horizontal defines two nearly orthogonal stress fields, and allows a logical sequencing of events.

The younger of these normal fault systems offsets Precambrian through Tertiary volcanic units and is associated with the development of the Rio Grande rift. Combined fault and joint orientations define a stress field oriented such that sigma one (axis of maximum compressive stress) was vertical; sigma two was oriented north-northeast, horizontal; and sigma three was oriented west-northwest, horizontal. The dominance of normal faults that dip to the eastsoutheast, abundant slickenside data and the westward rotation of the Joyita block indicate that the hanging wall displacement direction was to the east-southeast.

The older of these normal fault systems is best developed in the southern Joyita Hills. conjugate normal faults and joints define a stress field oriented such that sigma one was vertical; sigma two was oriented northwest, horizontal; and sigma three was oriented northeast, horizontal. To date, fault patterns associated with this stress field have only been observed within Permian and older strata, and at present can only be dated as post-mid Permian.

Strucutral and stratigraphic evidence within the southern Joyita Hills may indicate that the older stress field represents reactivation of a pre-existing normal fault system associated with Ancestral Rocky Mountain deformation. Structural analysis indicates that while northwest-trending normal faults superimposed upon the Permian units display opposing dip directions, those offsetting Pennsylvanian and Precambrian units dip consistently to the southwest and exhibit subtle differences with respect to orientation, style of fault drag and degree of brecciation. Stratigraphic evidence includes erosional thinning and rotation of Pennyslvanian strata prior to burial by compositionally and texturally immature syntectonic deposits of the Wolfcampian Bursum Formation. The combination of both structural and stratigraphic evidence suggests that the Joyita Hills was bordered on the south and southwest by a northwest-trending prong of the Lucero basin.

In addition to normal faulting, numerous exposures of strikeslip faults, thrust faults, and associated folds have been observed within the Paleozoic strata of the Joyita Hills. Future investigations will focus on detailed geologic mapping and structural analyses of Mesozoic and early Tertiary units (to the east in EI Valle de La Joya) in an attempt to correlate these structural styles and associated stress field orientations with specific tectonic events.