NEW INSIGHTS INTO THE POST-MIOCENE THERMAL EVOLUTION OF THE NORTH CASCADES BASED ON APATITE FISSION TRACK ANALYSIS OF CUTTINGS FROM GEOTHERMAL EXPLORATION WELL GAR-1

Viable commercial geothermal resources in the Washington Cascades remain elusive, despite several decades of reconnaissance. Recent efforts at development include the drilling of geothermal exploration well GAR-1 at Garland Mineral Springs in 2011. These springs consist of a large (~8,000 m²) surface manifestation of warm water (15-29 °C) and high CO₂ and magmatic He discharge. Snohomish Public Utility District funded the drilling of two wells at these springs: a shallow (~200 m) temperature gradient hole, and GAR-1 (1524 m). Promising temperature gradients observed in the shallow well, in excess of 80 °C/km, reverted to lower gradients (~32 °C/km) at depths >600 m. Both wells encountered CO₂-charged artesian flow and pervasively bleached granite. However, sub-economic temperatures and low flow rates forced the District to abandon the project. Material from this well is uniquely suited to address key questions about the onset, duration, and source of hydrothermal fluid flow in moderate-temperature, high gas flux thermal springs found throughout the Washington Cascades, and to better characterize the geothermal resource in this region.

In this study, 4 depths were selected for apatite fission track analysis: 201 m, 494 m, 920 m, and 1513 m. Calculated apatite fission track ages for these depths are 16.86 (-2.72/+3.24) Ma, 14.06 (-2.61/+3.21) Ma, 10.59 (-5.69/+12.27) Ma, and 14.75 (-2.25/+2.65) Ma, respectively. These apatite fission track ages are generally consistent with mid- to late-Miocene cooling previously documented throughout the North Cascades, which is presumably related to rapid erosional unroofing at that time. However, preliminary analysis of material from GAR-1 indicates that apatite fission track length distributions at all depths are well fit by model thermal histories that include gradual warming to measured downhole temperatures beginning in the early Pliocene (~5 Ma). This warming may represent a combination of post-Miocene change in heat source (magmatism) or change in fluid-flow conduits (tectonism).