Hidden Intra-Basin Extension: Evidence for Dike-Fault Interaction from Magnetic, Gravity, and Seismic Reflection Data in Surprise Valley, NE California

Abstract:

In continental rift zones, the proportion of strain accommodated by tectonic and magmatic processes, as well as their spatial and temporal distribution, is highly variable. Magnetic, gravity, and seismic reflection data in Surprise Valley, CA, in the northwest Basin and Range, reveal an intra-basin, fault-controlled, ~10 m thick dike at a depth of ~150 m, providing a unique example of the interplay between faulting and magmatism in the process of continental rifting.

The dike, likely a composite structure representing multiple successive intrusions, is inferred from modeling a positive magnetic anomaly that extends ~35 km and parallels the basin-bounding Surprise Valley normal fault on the west side of the valley. A 2D high-resolution seismic reflection profile acquired across the magnetic high images a normal fault dipping 56°E with ~275 m of throw. Densely spaced gravity measurements reveal a <1 mGal gravity low consistent with the offset along the fault inferred from the seismic data.

Co-location of the magnetic high and gravity low for ~6 km implies normal fault control of the dike along that length. The unusually shallow angle of the dike suggests that motion along the fault (perhaps aided by reduced friction along the dike) and associated block rotation resulted in post-intrusion tilting of the dike. The source of the dike is likely related to a shallow brittle-ductile transition zone that was elevated following rapid slip on the Surprise Valley fault post-3 Ma.

Prior to this work, the Surprise Valley fault was assumed to accommodate the vast majority of extension across the region. Our results indicate that subsurface features, though no longer active, are significant contributors to the processes, timing, and total amount of extension observed in continental rift environments.