Using Anisotropy of Magnetic Susceptibility (AMS) to Determine the Flow Characteristics of a Pyroclastic Density Current: The Nine Hill Tuff, Nevada and California
Abstract
Pyroclastic density currents (PDCs) typically travel tens of kilometers from their sources; however, some Great Basin PDCs erupted during the Cenozoic ignimbrite flareup travelled ~200 km from their source (Henry and Faulds, 2010, Geosphere). The long runout distance is attributed to channeling of flow in paleovalleys. The 25.38 Ma Nine Hill tuff of northern Nevada and the Sierra Nevada is one of the most far-travelled tuffs both in the Great Basin and worldwide. To understand the depositional processes of the PDC that emplaced the Nine Hill Tuff, anisotropy of magnetic susceptibility (AMS) was applied. AMS uses the preferred orientation of magnetic minerals as a proxy for flow fabric. Sample sites were chosen at different locations within paleovalleys to compare lineation, foliation, and shear fabrics between different points in the current as it deposited in the paleovalleys. A large geographical region was selected to understand variation in flow due to topographical differences. Anisotropy factors were obtained and illustrated using lower-hemisphere projections and P-T diagrams. Data generally show greater foliation than lineation. The sites farthest west of the caldera show the strongest foliation; this is likely due to laminar flow as the paleovalley widened and caused the flow to decrease velocity. PDC flow direction can be interpreted using one of two techniques: using the K1 tensor to indicate flow azimuth, or using the pole to the foliation plane (K3) to indicate flow. In the latter, flow direction can be determined by subtracting 180° from the declination and subtracting the inclination from 90°. Three distinct facies were identified: 1) K1 tensor and pole to the foliation plane (K3) direction were consistent, 2) K1 tensor and pole to the foliation plane (K3) direction were not consistent, and 3) intermediate (K1 tensor and pole to the foliation plane differ, but not drastically). In distal deposits, the second case is common, possibly due to the variation in velocity as the energy within the PDC decreased. Closer to the caldera, the first case is more likely; this could have been influenced by a more turbulent current.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2021
- Bibcode:
- 2021AGUFMGP25B0409H