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Trans-Alaska Crustal Transect and continental evolution involving subduction underplating and synchronous foreland thrusting

¹ U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA
² U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA, and Stanford University, Stanford, California 94305, USA
³ University of Wisconsin, Madison, Wisconsin 53711, USA
⁴ Rice University, Houston, Texas 77251, USA
⁵ U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA, and University of Wisconsin, Madison, Wisconsin 53711, USA
⁶ U.S. Geological Survey, Denver, Colorado 80225, USA
⁷ University of Alaska, Fairbanks, Alaska 99775, USA

View larger version (85K): [in this window] [in a new window]   Figure 1. Tectonic map of southern Alaska. Colors—subducted (lighter) and unsubducted (darker) plates and/or terranes. Stipple pattern—underplated rocks; heavier, sparser stipple where outcrops are seen. Heavy dotted black line—interpreted original (Eocene) outline of Yakutat terrane (YAK) translated to its current position in Gulf of Alaska. Heavy white line—interpreted modern outline of subducted Yakutat terrane; dashed where uncertain. SMA—Slope magnetic anomaly (subducted Transition fault); CB—coupling boundary, drawn from Zweck et al. (2002); SB—seismicity boundary (see Page et al., 1989; Eberhart-Phillips et al., 2006). White arrow (parallel to modern subduction)—difference between original and modern north boundaries of YAK in subducting plate. POL—Pacific oceanic lithosphere. Numbered black arrows—plate motion (mm/yr) with respect to North America. White triangles—receivers for study (BEARR) of Ferris et al. (2003). Blue lines—Trans-Alaska Crustal Transect (TACT) receivers. Red lines—faults; heavier red line—plate boundary; CSEF—Chugach–St. Elias fault; KIZ—Kayak Island zone; PZ—Pamplona zone; TsF—Totschunda fault; 1987, 1988—strike-slip earthquakes (Lahr et al., 1988); DRZ—Dangerous River suture within YAK (Plafker, 1987). Black star—epicenter of M 9.2 1964 earthquake. Doubled subducting crust imaged in offshore seismic profiles (red-brown lines with black outlines) is YAK overlying POL. We infer that the single thickness of subducting oceanic lithosphere imaged onshore beneath TACT line (white line with black outline) is POL alone, and that the missing YAK—the difference between original and modern northern boundaries of YAK—has been transferred from subducting plate to North America (stippled areas); see Plate 1D.

View larger version (55K): [in this window] [in a new window]   Plate 1. Maps of Alaska and geophysical profiles along Trans-Alaska Crustal Transect (TACT). A. Colored, shaded relief map of Alaska showing TACT route. Black lines, faults; teeth on upper plates of thrust faults; heavy black line, plate boundary between Pacific and North American plates. See explanation for other symbols. B. Lithotectonic terrane map of Alaska showing TACT route. See explanations for other symbols. C. Gravity, magnetic, and topographic profiles along and near TACT route. Profiles are constructed along straight lines joining numbered points in A; numbered points shown at top of C. D. Cross section along TACT route (V.E. 2:1) showing seismic velocities, velocity boundaries, near-vertical-incidence reflections, lithotectonic terranes, faults, and hypocenters of major earthquakes. See explanation below cross section E. E. Cross section across Alaska (V.E. 1:1) showing seismic velocities, velocity boundaries, near-vertical-incidence reflections, faults, and earthquake hypocenters. See explanation below cross section. F. Seismicity map and cross sections of seismicity along directions of modern plate motions (A-A', B-B', and C-C') and perpendicular to strike of Wrangell Wadati-Benioff zone (W-W'). See explanations beneath map and cross sections.