Roadblocks and speed limits: Mantle-to-surface volatile flux through the lithospheric-scale Denali fault, Alaska

Abstract

Abstract Helium and carbon isotopic data from 12 springs along a ~400 km segment of the Denali fault system (Alaska, USA), inform mantle-to-surface connections of this enigmatic structure. Warm springs on the main strand, west of the 2002 M7.9 Denali fault earthquake rupture, have 3He/4He as high as 2.4 RC/RA (air-corrected 3He/4He relative to air ratio) indicating ~30% mantle He. Corresponding δ13C values are −9.1‰ to −7.8‰ (relative to Vienna Peedee belemnite), suggesting that the CO2 at these western springs is partially mantle derived. At the eastern end of the 2002 rupture, Totschunda fault springs have 3He/4He of 0.65–0.99 RC/RA (~8%–12% mantle He), with δ13C values (~0‰) from carbonates. Results confirm the Denali fault system is a lithosphericscale feature tapping mantle volatiles. Springs along the 2002 rupture yield air-like 3He/4He of 1 R/RA and δ13C values from −9.2‰ to −3.4‰, interpreted as representing shallow groundwater circulation through shales and carbonates without mantle contributions. A thrust splay parallel to the rupture zone has air-like 3He/4He, whereas an along-strike high-angle normal splay yields 3He/4He of 1.3 RC/RA (~16% mantle He), implying that flow paths along the ruptured strand are disrupted in the upper 10 km of the fault zone. Because the Denali fault is a lithospheric-scale, transcurrent structure separating North America from independently moving southern Alaska, we suggest that it has characteristics of a transform boundary. The similarity of our results to helium isotope values at analogous tectonic settings suggest that without magmatism influence, there is a maximum mantle fluid flux through continental strike-slip faults.