MID-CRUSTAL VELOCITY ANOMALIES IN THE CENTRAL WASHINGTON CASCADES: EVIDENCE FOR STRUCTURES AFFECTING REGIONAL TECTONICS AND VOLCANISM?

We performed a local earthquake tomographic imaging experiment in southwestern Washington using over 75,000 P-wave arrival times from 3760 local earthquakes recorded by the Pacific Northwest Seismograph Network (PNSN). Amongst the more significant features of the resultant 3-D velocity model are two large (~30 x 70 km) north-south trending low-velocity regions (5.9-6.4 km/sec) separated by a northwest-trending ~15 km-wide ridge-like zone of higher velocities (6.4-6.8 km/sec) at depths of 10-23 km beneath the axis of the Cascade Range. Although resolution in this part of our model is 10-15 km, impulse response tests indicate that the anomalies are not a result of smearing that can result from poor data coverage. The southern low-velocity anomaly broadly correlates with an isostatic residual gravity low and an area of elevated heat flow values. The ridge separating the two low-velocity anomalies is parallel to the Olympic-Wallowa Lineament (OWL), a structure inferred to represent a Tertiary tectonic boundary. The ridge also coincides with an axis along which primary stress directions derived from fault plane solutions rotate from north-northeast south of the ridge to north-northwest north of the ridge, as well as a region of low heat flow values.

In aggregate, these observations suggest that the anomaly pattern may correspond to structures that have played, and may continue to play, a role in regional tectonics. Seismicity is largely absent within the anomalies and along the ridge, suggesting that either they are not tectonically active or temperatures are too high to allow for brittle failure. However, rotation of stress axes along the OWL suggests that it could still be influencing seismotectonics in the upper crust. Based on seismicity, seismic velocities, gravity and heat-flow data, we infer that the southern low-velocity anomaly is a region of hot, low-density, highly fractured and altered volcanic or sedimentary rocks. Virtually all Quaternary vents north of Mount St. Helens and south of Glacier Peak occur within or along the margins of this anomaly. We infer that the anomaly represents a body that plays a structural role in controlling the locus of central Washington volcanism, possibly as a broad zone of mid-crustal magma accumulation.