STRAIN PARTITIONING ACROSS THE CENTRAL WALKER LANE: GEODETIC AND GEOLOGIC CONSTRAINTS

The Walker Lane, a zone of dextral motion on the western margin of the Basin and Range province, accommodates 15 - 25% of the differential motion between the Pacific and North American plates. At the latitude of Lake Tahoe, the Walker Lane has been considered a zone of transtension, with strain accommodated by dip-slip, strike-slip, and oblique-slip faults. Published GPS data, when combined with existing geologic and geophysical data, suggest that strain is strongly partitioned across the region. Dip-slip motion resulting in E-W extension is focused along the present-day margin of the Basin and Range and dextral strike-slip motion takes place across a discrete zone further east. GPS velocity vectors at this latitude (relative to stable North America) vary across four domains determined by consistent vector orientation. In Domain 1, east of the Walker Lane, components of GPS velocities average 3.0 mm/y west and 0.9 mm/y north. Domain 2 (easternmost Walker Lane) vectors display average velocity components of 4.6 mm/y west and 2.6 mm/y north; these indicate a relative motion between Domains 1 and 2 of 1.6 mm west and 1.7 mm/y north, consistent with previous transtensional interpretations. From the western edge of this domain to the western flank of the Wassuk Range, NV (Domain 3), GPS velocity components average 5.6 mm/y west and 6.4 mm/y north, demonstrating the dominance of dextral motion between Domains 2 and 3 (1.1 mm/y west and 3.7 mm/y north). Geologists have previously mapped dextral strike-slip faults between these domains, and the locus of this deformation is sub-parallel to the Central Nevada seismic belt. The westernmost domain (4), which includes the Sierra Nevada, displays average GPS velocity components of 10.5 mm/y west and 6.4 mm/y north, demonstrating the dominance of extension and virtual absence of dextral motion between domains (4.5 mm/y west; 0.0 mm/y north, relative to Domain 3). In addition, no significant dextral or oblique slip faults related to the Walker Lane have been mapped across Domain 4. In fact, geologic relations argue for dip-slip motion on all extensional faults back to 15 m.y.a. GPS and geologic data therefore constrain significant present-day dextral motion to a 75-km wide region to the east of the Wassuk Range, thus reducing the width of the Walker Lane at this latitude.