SYSTEMATIC CHARACTERIZATION OF MORPHO-TECTONIC VARIABILITY ALONG THE CASCADIA CONVERGENT MARGIN: IMPLICATIONS FOR OUTER WEDGE DYNAMICS AND SHALLOW MEGATHRUST BEHAVIOR

We systematically characterize and synthesize first-order along-strike morpho-tectonic variability in the marine forearc along the Cascadia subduction zone to better understand geologic controls on outer wedge dynamics and implications for megathrust behavior. Recent studies along subduction margins that have hosted some of the largest and most destructive megathrust earthquakes and tsunamis in recent history (i.e. Japan, Sumatra, Chile) have linked forearc morphology and structure to megathrust behavior. This assertion is based on the hypothesis that spatial variations in the frictional behavior of the megathrust are linked to the tectono-morphological evolution of the upper plate. We characterize along-strike variation in outer-wedge geometry based on 25 km-spaced topo-bathymetric profiles and divide the margin into four broad regions with spatial extents (10’s to 100’s of km) equivalent to rupture lengths of large earthquakes (≥M 6.7) . Each region is characterized by unique seafloor morphology and displays distinct patterns of structural vergence. Margin-wide synthesis of and comparison with various subduction zone parameters suggests this morpho-tectonic variability is controlled primarily by upper-plate backstop dynamics and glacial sediment loading. Critical taper theory provides a framework in which to describe how the development of various backstop configurations within the wedge may influence the frictional properties of the megathrust through modulation of wedge strength. Delineation of both static and dynamic backstop location and geometry along the margin also highlights potential megasplay fault structures. Geologically controlled variability in outer-wedge morphology and structure is broadly associated with along-strike changes in earthquake behavior inferred from distributions of near-plate-interface microseismicity, patterns of episodic tremor and slow-slip, offshore paleoseismic records, and onshore tsunami and subsidence records. Based on our synthesis, we argue that the first-order along-strike morpho-tectonic variability in the outer wedge of Cascadia reflects spatial variations in shallow megathrust behavior occurring over roughly the last few million years.