TECTONICS, VOL. 4, NO. 1, PAGES 107-125, JANUARY 1985 GEOMETRY OF CENOZOIC EXTENSIONAL FAULTING: DIXIE VALLEY, NEVADA David A. Okaya and George A. Thompson Department of Geophysics, Stanford University, Stanford, California Abstract. Precise definition of geometric relationships between individual basins and ranges may help to reveal the mechanical processes of Basin and Range Cenozoic extensional faulting at depth. Previous studies have attempted to identi- fy simple horsts and grabens, tilted cru- stal blocks with planar faulting, or tilt- ed crustal blocks with listric faulting in the shallow crust. Normal faults defining these crustal blocks may root (1) indivi- dually in the ductile lower crust, (2) in regional or local low-angle detachment faults, or (3) in igneous intrusions or decoupling surfaces produced by the intru- sions. The present study, in Dixie Val- ley, west-central Nevada, makes use of a seismic reflection survey, gravity models, seismograms from earthquakes occurring on December 16, 1954, and geometrical block models. These data show a strutrurally asymmetric basin bounded by a single zone of faulting on the northwest and by a downbowed and step-faulted floor to the southeast. The northwest bounding fault is moderately dipping (50 0 ) and planar to a depth of 3 kin. The southeast boundary is step-faulted, and altogether the faults indicate an extension of 20% across the valley at the rate of 0.38 mm/y for the Copyright 1985 by the American Geophysical Union. Paper number 4T0980. 0278-7407/85/004T-0980510. O0 last 8 my. Synthetic earthquake seismo- grams confirm a focal depth of 15 km and fault dip of 62 o for the Fairview Peak earthquake and suggest that the focal depth of the Dixie Valley earthquake was also 15 km instead of the previously re- ported 40 kin. Local microearthquakes cluster around 10-15 km. The geometrical block models indicate that crustal horst- graben faulting and planar, high-angle normal faults rooted in a low-angle de- tachment surface do not readily account for development of the subsidiary (step) faults found in Dixie Valley. Extension of the crust by intrusion may develop high-angle faults and, with further intru- sion, may develop the subsidiary faults and produce a complex, sagged, asymmetric graben like Dixie Valley. INTRODUCTION Formation of the basins and ranges in the northern Basin and Range province represents a late stage of Cenozoic cru- stal extension. While the shallow Basin and Range structure is generally known from geologic studies, geophysical methods have only recently begun to reveal the structure at greater depth. In particu- lar, extensional normal faults which separate the crustal blocks defining the basins and ranges are steeply dipping (>500) near the surface; geophysical meas- urements are necessary to define the deeper geometry of the crustal blocks and