The Olympic Mountains of Washington state are composed of the Olympic Subduction Complex (OSC) in the center of the massif, which is the only onshore exposure of the modern Cascadia accretionary prism, the overlying rocks of the Coast Range terrane, which are mainly Eocene basalts and Eocene to Recent clastic rocks of the Peripheral sequence.  Structurally, the Olympic Mountains form a domal uplift which has tilted the overlying Coast Range and Peripheral sequence rocks and exposed the OSC in its core.

   Reset apatite fission-track ages form a broadly elliptical pattern within which the youngest ages, 5-11 Ma, are located in the core of the Olympic Mountains which corresponds to the area of maximum modern relief.  Totally reset apatite fission-track ages of ~12 to 13 Ma are consistent with erosion of the Olympic Mountain uplift at ~12 Ma as indicated by sedimentological evidence in the Montesano Formation.   Age versus elevation relations for these samples indicate denudation rates of ~1 km/myr.

   The areal extent of reset apatite fission-track ages appears to be generally continuous across structural boundaries such as the Peripheral fault which separates the OSC from the overlying Coast Range Terrane.  There is one area in the northeast corner of the OSC where apatite fission-track ages are discontinuous across the fault boundary.  Apatite fission-track ages on the lower plate (OSC) side of the fault are in the 12-14 Ma range; whereas, on the upper plate (Coast Range) side of the fault, the apatite fission-track ages are significantly older, 35 to 40 Ma.  This difference in ages across the fault suggests normal fault movement along this boundary.

   West of the area of maximum resetting, toward the coast of the Olympic peninsula, apatite fission-track ages become mixed ages and increase to 14 to 28 Ma.  Confined track length measurements for samples of the coastal Oligocene to early Miocene Hoh Formation are consistent with partial resetting.  Mean track lengths vary from 12.5 ± 2 to 14.4 ± 1.3 mm with all track length distributions showing a significant number of tracks < 10 mm in length.

   Using apatite fission-track data from the 9 km thick strata of the Peripheral sequence, the base of the apatite partial annealing zone (PAZ) was located between 3.8 to 5.6 km.  Assuming an 8°C mean surface temperature and 70°C for the top of the apatite partial annealing zone, a paleogeothermal gradient of ~19°C/km is estimated.  This is consistent with modern estimates from adjacent Vancouver Island based on surface heat flow measurements and thermal conductivity.