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

Documenting the temporal and spatial relationships between successive basalt flows and contemporaneous normal faults can elucidate the interactions between volcanism and faulting in extensional environments, but requires good exposure, easily identifiable units that can be mapped across faults and precise age constraints. We have attempted to apply this to the northwestern Basin & Range, where 2-8 Ma basalts erupted contemporaneously with the development of normal faults. Although these flows have been precisely dated using ⁴⁰Ar/³⁹Ar geochronology, individual flows cannot be easily distinguished in outcrop or geochemically. Paleomagnetics, however offers a means of mapping common flows across broad regions in order to determine the relative timing of faulting and volcanism, and to establish whether faults act as barriers or pathways for flows. Identifying flows by their unique primary magnetic directions and assembling magnetostratigraphy facilitates correlations between flows that have been faulted and offset.

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.