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
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.