A CASE OF MISTAKEN IDENTITY: LOW-ANGLE NORMAL FAULTS VERSUS GRAVITY SLIDES IN THE BASIN AND RANGE PROVINCE OF SOUTHWEST UTAH

The Basin and Range Province (BRP) is a treasure trove of geologic structures, including Mesozoic compressional features (classic thrust faults and folds) overprinted by Cenozoic extensional and transtensional features (low-angle and high-angle normal faults and strike slip faults). Decades of mapping and study were needed to unravel this complex area and place structures into their proper tectonic context. Today, detailed mapping continues and helps us fine tune our geologic interpretations. However, if structures are placed in the wrong tectonic process based on a mistaken identity of fault types, then our geologic understanding of an area may be hampered. A case in point is the misidentification of low-angle normal faults (LANF) from extensional tectonics versus gravity slides (GS) from gravity tectonics. The debate between LANF or GS structures within the BRP is not new (Anders, 2006) but could result in overestimating regional extension.

Our continued work in the Cenozoic Marysvale volcanic field has documented three gigantic gravity slides forming an overlapping contiguous complex >8000 km², with the largest (Markagunt gravity slide) being >3500 km². Previously thought to be part of a LANF (Maldonado, 1995), the allochthonous masses moved along three distinct zones: (1) a slide zone (along with a breakaway area) containing a bedding-plane-parallel segment ~55-65 km long, (2) a ramp zone ~1-2 km long where the slide masses cut up section from the slide zone, and (3) a former land surface zone with >35 km of movement over an erosional landscape. Basal layers of sandstone-conglomerate-like material, clastic dikes (injectites) of the same material, ultracataclastically deformed rocks, jigsaw puzzle fracturing, and pseudotachylytes (frictionites) indicate catastrophic emplacement. Kinematic indicators and strike-slip faults in the upper plate suggest slide movement was southward, perpendicular to BRP extension. New age constraints of sliding events (Sevier GS ~25 Ma, Markagunt GS ~23 Ma, and Black Mountains GS between ~19.5-18 Ma) show a westward progression of sliding corresponding to magmatic growth of the volcanic field. These ~100 km long structures are a reminder that we must be aware of alternative fault interpretations that can profoundly change our understanding of a region’s geologic history.