Exotic blocks within the early Tertiary Rubio Peak Formation in the north-central Black Range, New Mexico: Occurrence, insights into post-emplacement tectonic activity, economic implications and emplacement hypothesis
Richard W. Harrison
Exotic blocks of Pennsylvanian limestone-shale lithology occur within volcaniclastic deposits of the lower Rubio Peak Formation (early Tertiary) over a region of 180 km2 in the north-central Black Range, New Mexico. Known surface area of these blocks exceeds 10.1 km2, thickness averages 120 m, giving a minimum volume of 1.2 km2. These allochthonous blocks are believed to have been emplaced by gravitysliding during Eocene time. The blocks were accompanied by relatively minor additional slide material and came to rest upon a relatively flat surface. Bedding within the largest exotic blocks is parallel to this slide surface. Both massive, poorly bedded sandstone-siltstonc deposits and heterolithic, limestone-poor, debris-flow deposits underlie and engulf the exotic blocks. Based on a preferred northwest-southeast strike orientation of elastic dikes, lithologic characteristics and flow-direction indicators, the exotic blocks' detachment area is interpreted to have been to the southwest of their present position. Because of their unique geometries and distinctive lithologies, the exotic blocks can be used to document and measure displacement on crosscutting structures that have resulted from post-emplacement tectonic events. In particular, restoring the largest exotic blocks to assumed original geometry indicates that at least 3.14 km of dextral strike-slip displacement has occurred on four structures; this faulting is part of a wide and extensive Eocene wrench-fault system active along the eastern margin of the Colorado Plateau. Furthermore, the exotic blocks made a distinctive marker horizon that aids in documenting normal faulting related to the earliest development of the Rio Grande rift in southwestern New Mexico. Open-space development and brecciation created by deflection of normal-fault attitudes cutting downward through the exotic blocks is an important structural control in localizing economic mineral deposits. It is hypothesized that late Laramide wrench-fault activity along the Santa Rita—Hanover axis, southwest of the exotic-block occurrences, produced local uplifts and possibly steep escarpments from which large segments of Pennsylvanian strata detached and slid under the force of gravity toward the northeast.
- Harrison, Richard W., 1989, Exotic blocks within the early Tertiary Rubio Peak Formation in the north-central Black Range, New Mexico: Occurrence, insights into post-emplacement tectonic activity, economic implications and emplacement hypothesis, in: Southeastern Colorado Plateau, Anderson, Orin J.; Lucas, Spencer G.; Love, David W.; Cather, Steven M., New Mexico Geological Society, Guidebook, 40th Field Conference, pp. 99-106.