INSIGHTS INTO CRUSTAL STRUCTURE OF THE NORTHERN WALKER LANE (CALIFORNIA-NEVADA) FROM GRAVITY AND AEROMAGNETIC DATA

Gravity and aeromagnetic data provide information on the crustal structure of the northern Walker Lane northwest of Reno, NV. More than 1,400 new gravity measurements and new detailed aeromagnetic data delineate fault and basin structure. The Honey Lake Fault Zone bounds the southwest margin of the most prominent gravity low in the region with an amplitude of more than 60 mGal; the gravity gradient coincident with the fault over a distance of 70 km indicates a very steep to vertical fault dip. The basin fill is more than 4 km thick and is bisected by a narrow northwest-striking buried bedrock ridge that is mapped by its corresponding magnetic high and by two closely spaced gravity profiles. The northeast margin of the bedrock ridge coincides with the Warm Springs Fault Zone southeast of the gravity low and the magnetic anomaly suggests that the fault zone may continue northwest of its mapped extent across the low.

At its southwest end, the Honey Lake Fault Zone makes a right step, forming a 12-km long basin that is ~1.5 km deep. The length of the basin, coupled with a pair of magnetic anomalies that appear to be offset right-laterally across the basin, suggest ~12 km of cumulative fault offset. Additional right-lateral offset may be accommodated farther south in Upper Long Valley beneath a basin that is as much as 3 km deep. Another pair of magnetic anomalies suggest ~2-3 km of apparent right-lateral offset across this basin.

Southwest of the Honey Lake Fault Zone, gravity anomalies in Sierra Valley indicate a complex bedrock surface influenced by the Grizzly and Mohawk Valley Fault Zones. The Grizzly Valley Fault Zone bounds a concealed bedrock ridge that strikes southeast across the valley separating basin fill that is more than 1 km thick. Continuity of magnetic highs across the fault suggests no more than 1 km of apparent right-lateral offset. The Mohawk Valley fault zone appears to have ~5 km of right-lateral offset based on correlation of magnetic anomalies. Farther to the northwest, a gravity low in Mohawk Valley indicates a northwest-trending fault-bounded basin that is at least 1 km deep; its length also supports ~ 5 km apparent right lateral offset. Even farther to the northwest, magnetic data suggest that this fault zone may have taken advantage of a crustal-scale structure that separates Shoo Fly Complex from other accreted arc terranes.