PALEOMAGNETIC MAPPING OF LATE MIOCENE-PLIOCENE BASALT FLOWS IN THE NORTHWESTERN BASIN AND RANGE: REVEALING THE RELATIONSHIP BETWEEN FAULTING AND VOLCANISM IN AN EMERGENT EXTENSIONAL ENVIRONMENT
Core samples have been collected from nearly 90 basalt flows in the Larkspur Hills (northeastern CA) and subjected to stepwise demagnetization to ascertain their primary remanence directions. Initial results reveal that flows spanned a minimum of three polarity chrons and record a number of unique transitional magnetic polarities. Magnetostratigraphy reveals continuity and later offset of flows in the southwestern Larkspur Hills, but no lateral continuity further north.
The age of the flows combined with low erosion rates precludes the removal of capping flows, resulting in incomplete magnetostratigraphy. The thickness of many flows combined with their low viscosity suggests that flow volumes limiting spatial coverage would be improbable. A wide distribution of source vents have been identified, and in conjunction with concurrent faulting would explain the apparent isolation of some flow paths. It is interpreted that basalts erupted from these broadly distributed vents and filled topographic lows created by contemporaneous faulting, explaining the isolation of particular flows in the Larkspur Hills. Determining the relationship between concurrent faulting and volcanism in the region may shed greater light on the earliest stages and maturation of other extensional environments.