FAULT GEOMETRY AND LINKAGE ALONG NORMAL FAULTS: EXAMPLES FROM THE SEVIER FAULT, SW UTAH

Several segmented long normal faults lie along the western Colorado Plateau including the Sevier fault (SF), SW Utah. However, the development and growth of these long faults have not been analyzed in detail. Theories suggest long faults may grow by segment linkage. Fault geometries in linkage zones can be of four types: (1) underlapping tips; (2) overlapping tips; (3) fault capture, a subset of overlapping tips; and (4) breakthrough faults. In overlap zones relay ramps may form prior to or during linkage. Fault-parallel folds within normal-fault bounded relay ramps suggest contraction occurred within the ramp during extension. However, such folds are poorly understood. This study focuses on linkage zones, formation of folds within relay ramps, and age of movement along the SF.

We define two linkage sites along the central SF. In the southern region, we interpret linkage via fault capture with the formation of a relay ramp, named the Orderville relay ramp (ORR). Within the ORR is a plunging syncline (8^o, N8^oW). North of the ORR is an overlap zone of five ~NNE striking faults with no surface connection. Cross section analyses suggest that in the NE part of the overlap zone, the faults do not connect at depth. However, in the SE part of the area, the faults appear to connect at depth. The western part of the overlap zone contains the Glendale relay ramp (GRR) that contains an anticline oriented 4^o, N55^oE. Fold axes within the ORR and GRR are subparallel to the bounding faults. These contractile structures may have formed by fault drag or space accommodation. We suggest that they formed by space accommodation because the folds occur only within the relay ramps.

To constrain the age of movement along the SF, basalt samples were collected from each side of the fault. The whole rock ⁴⁰Ar/³⁹Ar dates are 564 +/- 20 ka and 580 +/-50 ka. These ages and map data indicate that the SF cut the basalt after the flow crossed the fault. However, some motion along the SF may be Holocene because several M<5.0 earthquakes occurred in the area.

Models suggest long normal faults can form by linkage of shorter faults. Faults that overlap and link or link by fault capture typically form relay ramps in the linkage zone. This study documents and models two different types of fault linkage and fault-parallel folds within the linkage zones along normal faults.