Summary. The crustal structure beneath the exposed terranes of southern. Alaska has been explored using coincident seismic refraction and reflection profiling.
Geophys. J . R . astr. Soc. (1987), 89,73-78
Crustal structure beneath exposed accreted terranes of Southern Alaska G.S. Fuis, E.L. Ambos, W.D. Mooney, R.A. Page, M.A. Fisher and T.M. Brocher, u.s. Geological Swyey, Menlo Park, CA, USA
J.J. Taber, Lumont-Doherty Geological Observatory, Palisades, NY, USA Summary. The crustal structure beneath the exposed terranes of southern Alaska has been explored using coincident seismic refraction and reflection profiling. A wide-angle reflector at 8-9 km depth, at the base of an inferred low-velocity zone, underlies the Peninsular and Chugach terranes, appears to truncate their boundary, and may represent a horizontal decollement beneath the terranes. The crust beneath the Chugach terrane is characterized by a series of north-dipping paired layers having low and high velocities that may represent subducted slices of oceanic crust and mantle. This layered series may continue northward under the Peninsular terrane. Earthquake locations in the Wrangell Benioff zone indicate that at least the upper two low-high velocity layer pairs are tectonically inactive and that they appear to have been accreted to the base of the continental crust. The refraction data suggest that the Contact fault between two similar terranes, the Chugach and Prince William terranes, is a deeply penetrating feature that separates lower crust (deeper than 10 km) with paired dipping reflectors, from crust without such reflectors.
1. Introduction
In 1984, the U.S. Geological Survey began the Trans-Alaska Crustal Transect (TACT) program, a coordinated geological and geophysical study of the structure, cornphition and evolution of the Alaskan crust along the Trans-Alaska oil pipeline corridor from the Pacific to Arctic continental margins (Page et al. 1986). In the first two years, 850 km of seismic-refraction data were collected in southern Alaska, comprising a north-south profile from the Alaska Range to the Pacific coast (330 km) and four shorter cross profiles (Fig. 1). The north-south profile crosses structure in a roughly perpendcular fashion, and the shorter profiles are strike lines. In 1986, the TACT program collected 130 km of seismic-reflection data along parts of the north-south profile. 2. Geologic and geophysical setting
The TACT route in southern Alaska traverses a mosaic of tectonostratigraphic terranes that were accreted to North America during Mesozoic and Cenozoic time (Jones et al. 1984). The transect passes through the volcanic gap between andesitic volcanism of the Alaska Peninsula and the Wrangell Mountains and lies over the Wrangell Benioff zone (Fig. 1)
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Figure 1. Map of south-central Alaska showing tectonostratigraphic terranes, suture zones, volcanoes (stars) and isobaths (heavy lines) on top of Aleutian and Wrangell Benioff zones (from Jones ef al. 1984; Stephens el al. 1984) TACT seismic-refraction lines (double lines) and seismic-reflection lines (heavy dashed lines). Refraction lines are labelled: WG--West Glenn Highway SR--South Richardson Highway, CG--Chugach, CP--Cordova Peak, an MG--Montague. Small circles are shotpoints; filled circles with numbers are shotpoints for which data are shown. Offset shotpoints are linked to profiles by doubIe-dashed lines. Dashed line separates Peninsular terrane (on SW) from Wrangellia terrane.
(Stephens et al. 1984). The transect crosses, from north to south, the Wrangellia, Peninsular, Chugach, and Prince William terranes. The Wrangellia and Peninsular terranes are Palaeozoic and Mesozoic island-arc sequences. A complex of mafic and ultrarnafic rocks, uplifted at the southern boundary of the Peninsular terrane may represent the root of one or more magmatic arcs in the Peninsular terrane (Bums 1985). These rocks produce marked magnetic and gravity highs. The Chugach terrane consists of fault-bounded, highly deformed sequences of flysch, volcanic rocks, and melange of varying metamorphic facies and ages (Late Cretaceous to Early Jurassic or older) (Winkler et al. 1981). Like the Peninsular terrane, the Chugach terrane is characterized by gravity and magnetic highs near its south margin (Campbell & Barnes 1985). The Prince William terrane consists of Paleocene and Eocene(?) flysch and volcanic rocks that are similar to those of the Chugach terrane but are largely unmetamorphosed (Winkler 8z Plafker 1981). Eocene plutons intrude the Prince William terrane, and Eocene dikes in the Chugach terrane may indicate larger plutons at depth there (Page et al. 1986). The Border Ranges fault separates the Peninsular and Wrangellia terranes from the Chugach terrane, and the Contact fault separates the Chugach terrane from the Prince William terrane.
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3. Data collection and analysis We discuss three strike lines, the West Glenn Highway, the Chugach, and the Montague lines, and two overlapping cross-structure lines, the South Richardson Highway and Cordova Peak lines. One-hundred-twenty seismographs were spaced about 1 km apart on all lines. Shotpoints were spaced 45-50 km apart on the strike lines and 25-30 km apart on the cross-structure lines. Offset shots extended all lines to ranges of 225-275 km. The data were forward-modelled using a 2-dimensional ray-tracing code based on the method of Cerveni et al. (1977). Most travel-times are matched to within 0.05 s. Models of aeromagnetic and gravity data (Campbell & Barnes 1985) were used to control steep interfaces such as those bounding the mafic and ultramafic bodies near the south boundaries of the Chugach and Peninsular terranes, and oil-well logs provided constraints on the upper 1-2 km of the model for the West Glenn line. 4. Velocity structure
Sample record sections and velocity-depth functions for strike lines (Fig. 2) indicate considerable differences in velocity structure among the terranes. Basement rocks of W
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(KM) Figure 3. Velocity models for South Richardson (A) and Cordova Peak (B) lines, which overlap about 55 km. Numbers on models are velocities (kmls). Low velocity zones (LVZs) are stippled. Zones of prominent reflectors from reflection profiling are indicated by wavy lines. Inferred top of Wrangell Benioff zone is indicated by heavy dashed line. Contact fault zone (CFZ; diagonally lined) is postulated, poorly defined zone of lateral velocity discontinuity separating Prince William terrane from Chugach terrane.
the Peninsular terrane (5.7 km s-' and greater) correspond to Jurassic volcaniclastic rocks intruded by granitic rocks (mostly diorites), as inferred from scattered well-log data. Later wide-angle reflections (corresponding to layers having velocities of 6.5,6.9, and 7.1 km s-I) are seen; the uppermost reflection is from the base of a low-velocity zone (LVZ). The crust beneath the Chugach terrane has a quite different velocity structure. Basement velocity (5.6 km s-') corresponds to exposed phyllite, metagraywacke, and minor metabasalt. Deeper crustal structure is remarkable, however, in having seismic velocities typical of mafic (6.9-7.1 km s-') and ultramafic (7.7-7.9 km s-')rocks. LVZs between these layers produce shadow zones and delayed wide-angle reflections (Fig. 2). All shotpoints on the Chugach line produce roughly similar record sections, although there is significant lateral variation in velocity on this line (particularly in the upper 10 km). The structure beneath the Prince William terrane is quite different from that beneath the Peninsular and Chugach terranes, having generally lower velocity at all depths. Basement (5.5 km s-') corresponds to exposed graywacke, argillite, and pillowed basalt. Below these these rocks is a distinct layer with a velocity of 6.15 km s-'. Later wide-angle reflections (corresponding to layers having velocities of 6.5 and 7.1 km s-') are seen with LVZ's above
Crustal structure beneath exposed accreted terranes of Southern Alaska
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them. There is considerable lateral velocity change along the line. The overlapping South Richardson Highway and Cordova Peak profiles, which cross the structural grain of southern Alaska, reveal contrasting styles of velocity change across major terrane boundaries marked by the Border Ranges and Contact faults (Fig. 3). The Border Ranges fault is truncated at 8-9 km depth by a wide-angle reflector (representing the base of a LVZ) that extends from the Peninsular terrane southward into the Chugach terrane. Southward, the velocity within the LVZ increases towards Mt. Billy Mitchell; still farther south the reflector itself is truncated near the trace of the Contact fault. The series of low- and high-velocity layers that begin at 9-km depth on the Chugach line dip northward, first at 4 degrees, and then at 13 degrees north of the trace of the Border Ranges fault (Fig. 3). This series does not extend as far north as the West Glenn line. The upper 3 pairs of low- and high-velocity layers terminate just north of the Contact fault: south of that fault is a much simpler structure; its upper part is similar to that on the Montague line. A preliminary stack of 128-fold seismic-reflection data along the South Richardson profile indicates the existence of at least two laminated packages of very strong, continuous north-dipping reflections. The upper package is centered on the interface at the bottom of the second LVZ determined from modeling refraction data. The lower one appears to be coincident with the third LVZ, but the depths to the top and bottom of this LVZ are poorly constrained by the existing refraction data. 5. Discussion an d conclusions 1. The lower crustal structure differs significantly among the Peninsular, Chugach, and Prince William terranes in southern Alaska. These differences are unexpected in the latter two cases given the geologic similarity of the two exposed terranes. 2. The Chugach and Peninsular terranes appear to be truncated at shallow (9 km) depth by a horizontal reflector (decollement?) Thus these terranes appeared rootless. Perhaps the exposed terranes are thin thrust plates analogous to those reported for the Appalachian convergent margin (Cook et al. 1979). 3 . A series of north-dipping low- and high-velocity layers may represent slices of oceanic crust and mantle, respectively, that have been partly subducted northward beneath the exposed terranes. Earthquake hypocentral locations in the Wrangell Benioff zone are below the upper two paired layers (Fig. 3), indicating that at least these two pairs are now accreted to North America (perhaps) in a manner similar to layers inferred beneath Vancouver Island, Canada (Spence et al. 1985). 4. The lower crustal structure beneath the Prince William and the Chugach terranes is quite different. The Contact fault, which separates the terranes at the surface, is a deeply penetrating crustal discontinuity. At this stage of modelling, it is not clear whether this discontinuity extends extends below the inferred top of the Wrangell Benioff zone. 5 . Two packages of strong reflections are observed on a preliminary stack of reflection data. They appear to be coincident with LVZs or the bottoms of LVZ's inferred to be subducted oceanic crust. References Bums, L. E., 1985. The Border Ranges ultramafic and mafic complex, south-central Alaska: cummulate fractionates of island arc volcanics: Can. J . Earth Sci.,22,1020-1038. Campbell, D. L. & Barnes, D. F., 1985. Gravity and magnetic model of part of the 1984 TACT line, Chugach Mountains and southern Copper River basin: US.Geof.SUN. Circ. 967,52-55. cerveny, V., Molotkov, I. A., & PSenEik, I., 1977, Ray method in seismology, Karlova Univ., Prague.
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Cook F., Albaugh D. Brown, L., Kaufman, S., Oliver, J., and Hatcher, R., Jr., 1979. Thin-skinned tectonics in the crystalline southern Appalachians: COCORP seismic reflection profiling of the Blue Ridge and Piedmont, Geology, 7,563-567. Jones, D. L., Silberling, N. J., Coney, P. J., Plafker, G., 1984. Lithotectonic terrane map of Alaska, in Lithofecfonicterrane maps of the North American Cordillera, eds Silberling, N.J., and Jones, D.L. U S . Geol. Surv., Open-File Rep. 84-523, A1-A12, 4 sheets, 1:2,500,000. Page, R. A., Plafker, G., Fuis, G. S., Nokleberg, W. J., Ambos, E. L., Mooney, W. D., & Campbell, D. L., 1986, Accretion and subduction tectonics in the Chugach Mountains and Copper River Basin, Alaska: Initial results of the Trans-Alaska Crustal Transect: Geology, 14,501-505. Spence, G. D., Clowes, R. M., and Ellis, R. M., 1985, Seismic structure across the active subduction zone of western Canada: J . geophys. Res., 90,6754-6712. Stephens, C. D., Fogleman, K. A., Lahr, J. C., and Page, R. A,, 1984, Wrangell Benioff zone, southern Alaska: Geology, 12, 313-376. Winker, G. R., Silbeman, M. L., Grantz, A,, Miller, R. J., and MacKevett, E. M., Jr., 1981, Geologic map and summary geochronology of the Valdez quadrangle, southern Alaska: U.S. Geol. Sum. Open-File Rep. 80-892A, 1:250,000. Winkler, G. R., and Plafker, G., 1981, Geologic map and cross sections of the Cordova and Middleton Island quadrangles, southern Alaska: U S . Geol. Sum. Open-File Rep. 81-1 164, 1:250,000.
