ALBERT

Description: The relative contributions of tectonic and magmatic processes to continental rifting are highly variable. Magnetic, gravity, and seismic reflection data from Surprise Valley, California, in the northwest Basin and Range, reveal an intrabasin, fault-controlled, ~10-m-thick dike at a depth of ~150 m, providing an excellent example of the interplay between faulting and dike intrusion. 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 two-dimensional high-resolution seismic reflection profile acquired across the magnetic high images a normal fault dipping 56°E with ~275 m of throw buried ~60 m below the surface. Densely spaced gravity measurements reveal a 〈1 mGal gravity low consistent with the fault offset inferred from the seismic data. Collinearity 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 after 3 Ma. Prior to our work, the Surprise Valley fault was assumed to accommodate the vast majority of extension across the region. Our results indicate that subsurface features, although no longer active, are significant contributors to the processes, timing, and total amount of extension observed in continental rift environments.