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

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 R_A 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 R_A 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.