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

Helium and carbon isotopic data from 12 springs along a similar to 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 He-3/He-4 as high as 2.4 R-C/R-A (air-corrected He-3/He-4 relative to air ratio) indicating similar to 30% mantle He. Corresponding delta C-13 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 He-3/He-4 of 0.65-0.99 R-C/R-A (similar to 8%-12% mantle He), with delta C-13 values (similar to 0%) from carbonates. Results confirm the Denali fault system is a lithosphericscale feature tapping mantle volatiles. Springs along the 2002 rupture yield air-like He-3/He-4 of 1 R/R-A and delta C-13 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 He-3/He-4, whereas an along-strike high-angle normal splay yields He-3/He-4 of 1.3 R-C/R-A (similar to 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.