Eastern boundary of the Siletz terrane in the Puget Lowland from gravity and magnetic modeling with implications for seismic hazard analysis
Abstract
The forearc of the Cascadia subduction zone in coastal Oregon and Washington is largely composed of a 15-30 km-thick stack of basalt flows comprising the Crescent Formation (WA) and Siletz River Volcanics (OR), and collectively termed the Siletz terrane. We are developing 3-D structural maps of the Puget Lowland to distinguish older and currently active structures for seismic hazard analysis. The boundaries of the Siletz terrane in particular may strongly influence crustal rheology and neotectonic structures of the region. Careful analysis of the areal extent of this terrane will also facilitate more accurate interpretation of seismic data and gravity anomalies, which will help define the extent and shape of overlying basins. Absence of extensive outcrop in the Lowland and a widespread veneer of Quaternary deposits require extensive subsurface geophysical studies to establish Lowland-wide crustal structure. Previous studies have used active seismic surveys and interpretation of existing industry seismic data, with several studies using gravity and magnetic data or passive-source tomography support. However, steeply dipping boundaries in the mid-crust are difficult targets for seismic study. We need to independently discriminate between potential models established by seismic data using gravity and magnetic datasets. In the Puget Lowland the Siletz is a region of high seismic wave speed, density, and magnetic susceptibility, and therefore its mid-crustal boundaries are good targets for definition by gravity and magnetic data. We present interpretations of gravity and magnetic anomalies for the Puget Lowland region that together establish the most likely position and structure of the Crescent Formation boundary in the mid-upper crust. Well-constrained physical properties of Crescent basalts inform our aeromagnetic map interpretation and give us baseline values for constructing three two-dimensional models by simultaneous forward modeling of aeromagnetic and isostatic gravity anomalies for the Lowland. Based on this work, the likely position of the eastern boundary of the Siletz terrane is east of the Puget Sound and west of the foothills of the Cascade arc, extending in a north-trending line through Lake Washington and merging to the north with the Southern Whidbey Island fault zone. Our preferred location agrees with suggested locations from past study of seismic data targeted at the Seattle basin, but we extend that location through the entire Puget Lowland by analysis of magnetic potential calculated from aeromagnetic data. We also find that the boundary is sharp and most likely dips west, suggesting a reverse-fault juxtaposition of Crescent rocks against Western Melange belt lithologies. The Crescent itself contains steeply dipping packages of basalt of contrasting magnetic character, indicating significant deformation within the Crescent formation under the Seattle uplift. Finally, the boundary location implies that the eastern third of the Seattle basin is shallower than previously estimated from gravity data.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMGP33B..06A
- Keywords:
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- 1219 GEODESY AND GRAVITY / Gravity anomalies and Earth structure;
- 1517 GEOMAGNETISM AND PALEOMAGNETISM / Magnetic anomalies: modeling and interpretation;
- 9350 GEOGRAPHIC LOCATION / North America;
- 4319 NATURAL HAZARDS / Spatial modeling