Fracture Pattern and Evolution in the Mineral Mountains Batholith, SW Utah
Abstract
Fractures in the 25-17 Ma Mineral Mountains batholith were characterized to predict natural fracturing in the Frontier Observatory for Research in Geothermal Energy (FORGE)site. The batholith lies in the footwall of a Basin-Range normal fault that seismic imaging and inversion of gravity data indicate dips 30° west. Fractures were mapped in eight 0.03-0.08 km2 areas of nearly 100% exposure in the northern half of the batholith to document orientations, spacing, continuity, intersection relations, and slickenlines. Fractures in each map area form three main sets: 1) strike 280±15, dip 70-90°; 2) 200±20, 70-90°; and 3) 180±30, 0-40°W. Sparse fracture data from the southern half of the batholith depart somewhat from this pattern but were not studied in detail. Mean fracture spacing decreases from 8 m in the center of the batholith to <2 m approaching the northern intrusive contact; qualitative observations suggest a similar trend toward the southern contact. Lengths and relative abundances of fracture sets seem to vary randomly on a scale of 100s of m.
Fracture sets 2 and 3 appear to be a conjugate set formed with a vertical maximum compressive stress, followed by 40° eastward tilt. Eastward tilting of the range is confirmed by strata as young as Miocene dipping to the east; wall-rock map pattern with Proterozoic basement to the west and Phanerozoic strata to the east; scattered paleomagnetic data from intrusive rocks that suggest eastward tilt; and an 11 Ma dike swarm that dips 40-50° west. The range tilted during unroofing by the range-bounding fault and thus sets 2 and 3 probably formed prior to late Miocene fault movement. Fractures in set 3 commonly have N-S lengths of only tens of meters, terminate against fractures of set 1, and vary markedly in abundance across throughgoing set 1 fractures, indicating that set 1 fractures predate set 3 and were barriers to its propagation. All three sets thus appear to have formed before or early during Basin-Range faulting. Slickenlines are sparse and mainly found on set 3 fractures. Riedel shears indicate top-west slip. The slipped joints are interpreted to be high-angle normal faults that were later tilted to low dips. The mean orientation of the slipped joints parallels the basin-bounding fault, suggesting that it grew by coalescence of a major joint set in the batholith.- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.T32D..14B
- Keywords:
-
- 3642 Intrusive structures and rocks;
- MINERALOGY AND PETROLOGYDE: 8010 Fractures and faults;
- STRUCTURAL GEOLOGYDE: 8035 Pluton emplacement;
- STRUCTURAL GEOLOGYDE: 8178 Tectonics and magmatism;
- TECTONOPHYSICS