29 Cryptic and Slow-Moving Quaternary-Active Faults in the Western United States

Wednesday, 16 March 2022
1:30 PM - 5:30 PM
CBC-A 108

MANTLE FLUID CONTRIBUTION TO SPRINGS ALONG THE DENALI FAULT SYSTEM: 3HE/4HE CONSTRAINTS ON THE CRUSTAL SCALE NATURE OF THE MAIN STRAND, HINES CREEK AND SPLAYS

Paper number: 29-13
Program: Theme Sessions
Time: 5:10 PM – 5:30 PM

Authors

Abstract

The He and C isotopic signatures of springs are ideal for identifying mantle-to-crust connections and testing geophysical and conceptual models for strike-slip fault systems. We present new 3He/4He and δ13C results from 14 springs along a ~400 km segment of the Denali Fault in Alaska to address if it is a lithospheric structure with characteristics consistent with a transform plate boundary. The Denali Fault is an ancient and active >2000-km-long right lateral strike-slip system evidenced by the M7.9 2002 earthquake with a 336 km surface rupture. In places, the Denali Fault separates the peri-Laurentian Yukon-Tanana composite terrane to the north and late Paleozoic and Mesozoic intra-oceanic arc rocks of the accreted Wrangellia composite terrane to the south, and debate persists among geophysicists on if this boundary is a lithospheric scale structure. We sampled mineral springs along the Denali, Hines Creek, and Totschunda faults, and linked thrust and normal fault splays to fingerprint the provenance of entrained volatiles to better resolve the 3-d geometry of the fault system.

Springs along the 2002 earthquake rupture yielded air-like 3He/4He of ~1 RA and δ13C values from -20 to 0.1 ‰ (VPDB), which we interpret to reflect shallow groundwater circulation through organic and carbonate-bearing strata in a permafrost environ. Although travertine is present at most locations, these springs were not bubbling, but some had observed gas releases before the 2002 earthquake. A spring along the Mesozoic Hines Creek Fault system had Ra values of 1.5, or ~20% mantle He, confirming the lithospheric scale of this structure based on geophysical imaging. This is an example of an ancient “dead” strike-slip fault maintaining a Moho offset.

Two bubbling warm springs (8-12 °C) along the Denali Fault, near Cantwell AK, had 3He/4He up to 2.4 RA indicating ~30% mantle He. CO2/3He and δ13C values in these springs were ~108 and -9.1 to -8.1 ‰, respectively, which we interpret as mantle derived. These results suggest that the Denali Fault is a lithospheric feature tapping mantle volatiles. In addition, we suggest that because the Denali Fault separates stable North America from the independently moving southern Alaska plate that the fault meets criteria for being a transform boundary, similar to other structures like the North Anatolian Fault.

Geological Society of America Abstracts with Programs. Vol. 54, No. 2

doi: 10.1130/abs/2022CD-373880

© Copyright 2022 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.