Cross section, Alaska Peninsula-Kodiak Island—Aleutian Trench: Summary

GEORGE W^MOORE^ ] Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 J. CASEY MOORE Earth Sciences Board, University of California, Santa Cruz, California 95064 CHRISTOPHER D. STEPHENS U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025

SCOPE earthquake is recorded, but a numerical precision is difficult to give, and accuracy probably varies greatly. Relative accuracy in the The U.S. Geodynamics Committee has sponsored preparation Benioff zone can be estimated by comparing the scatter of hypocen- and publication of geologic sections across the nation's continental ters evident in two nearby compilations from local networks of margins. The sections are at a scale of 1:250,000 without vertical seismographs in the Shumagin Islands and Cook Inlet areas. The exaggeration and include the basic data from which they were Benioff zone is 10 km thick below the Shumagin Islands network constructed. The section described here (von Huene and others, 600 km southwest of the Kodiak group of islands (Davies and 1979)1 crosses a seismically active continental margin in the Gulf of House, 1979) and 15 km thick below the Cook Inlet network 600 Alaska that includes the Aleutian Trench, the Aleutian volcanic km northeast (Lahr and others, 1974). In our data, a 15-km-thick chain, and the intervening accretionary terrane. Between the vol- zone includes most of the hypocenters that were recorded at more canic arc and the oceanic trench are tectonic features common to than 50 stations along the Benioff zone, but many of the hypocen- many other convergent margins. The surface geology includes evi- ters recorded at fewer stations fall outside this zone. Thus the pre- dence of early Mesozoic to Holocene magmatic arcs, probably re- cision of hypocenters recorded at more than 50 stations may ap- lated to Benioff zones, and belts of deformed Jurassic to Neogene proach that of hypocenters determined with local networks. The rocks composed of deep-ocean, trench, and continental-slope ma- other group of hypocenters clustering beneath the continental shelf terials; these belts record the accreted growth of the continent. The has greater scatter, as is also observed in the compilations from subsurface geology shows a forearc basin, a zone of deformation local networks, and the accuracies are uncertain. In the Kodiak along the trench lower slope, and the hypocenters of a Benioff zone. area, earthquakes seaward of the islands occur mainly below the Modern plate convergence is also deduced from thrust-fault first- edge of the shelf near Albatross Bank. Some hypocenters recorded motion determinations of major earthquakes, and late Cenozoic by more than 50 stations may occur below the crust-mantle bound- plate convergence is deduced from global studies of plate motion. ary, but enough uncertainty exists .in the determination of hypo- Information from studies of surface geology, standard marine center depth and depth to the crust-mantle boundary that this rela- and Deep Sea Drilling Project sampling, reflection and refraction tion might be attributed to imprecise data. An interesting detail of seismic sections, a gravity traverse, and earthquake hypocenters, is the earthquake focal data is a distinct line of hypocenters that ex- presented singly and is also merged in the cross section. The differ- tends about 90 km vertically from the Benioff zone to the roots of ence between steeply dipping strata observed in surface geologic Snowy Mountain in the volcanic arc; these hypocenters are pro- mapping and gently dipping subsurface horizons defined by the jected mainly from northeast of Snowy Mountain in the vicinity of marine seismic-reflection data show well at the 1:1 scale. Some of Cape Douglas. the detail lost in adapting all of the data to a composite section is The seismic-refraction data, interpreted by connecting zones of shown in the basic data insets. A simplified version of the compo- nearly equal velocity, may not correctly reflect the configuration of site section at 2:1 vertical exaggeration is shown in Figure 1. the upper geologic layers at the front of the subduction zone. Under the lower slope, seismic-refraction velocity boundaries and DATA BASE seismic-reflection horizons diverge (von Huene, 1979). Divergence may result from tectonic consolidation that may affect velocity as Earthquake data (Fig. 1) are summarized from the Preliminary much as sediment composition and lithostatic load. Determinations of Epicenters for 1967-1976, which use teleseismic In the forearc basin, refraction data (Holmes and others, 1978) records of the World Wide Seismic Net. There are two groups of indicate a large increase in velocity near the base of the main se- earthquakes, one forming a Benioff zone and the other clustering quence of seismic reflections in common depth point (CDP) seismic below the continental shelf. The precision of location generally in- records. In Shelikof Strait, the depth of young sediment is inferred creases with magnitude and the number of stations at which an from single-channel analog shipboard monitor recording of CDP seismic-reflection records, and only the shallow, near-horizontal ' Geological Society of America Map and Chart Series MC-28A. layering is depicted. At greater depths, dipping strata have been

Geological Society of America Bulletin, Part 1, v. 90, p. 427-430, 1 fig., May 1979, Doc. no. S90504.

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A

2 A • A

50- A

A ^A

A A A AA A A • A

A A A A A

A& A

A A KM 50 A 100- A V.E = 2:1

Figure 1. Simplified section from von Huene and others (1979) at 2:1 vertical exaggeration. Earthquake epicenters from 1967 to 1976. Seismic-reflection interpretation under the shelf and slope after von Huene (1979).

better defined by processed records (Michael A. Fisher, 1978, oral geologic mapping, the tracing of seismic-reflection horizons that commun.). are clearly geologic horizons and not multiples, and the straight- line connection of refraction-determined velocity interfaces. Fault- INTERPRETATION ing along the lower trench slope has been extrapolated, as shown by dashed lines beyond the reflection data, to merge into a master The interpretations shown in the composite section do not go thrust fault in accord with the models generally used for a subduc- much beyond those commonly used in data reduction from field in- tion zone. formation such as the downward projection of attitudes from On land, the cross section shows rocks accreted in Mesozoic and

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SE

Figure 1. (Continued).

early Cenozoic time. At sea, the section shows the initial stages of shelf and slope environments to the northwest from the more- Pliocene to present-day accretion along the Aleutian Trench. The deformed accreted rock deposited in ocean basin, trench, and landward boundary of the accretionary complex is a fault zone trench-slope environments. The tectonic style in section A-A' along the northwestern shores of the Kodiak group of islands changes from broad folds at the northwest to the steeply dipping (Shelikof Strait side) that may be the structural equivalent of the and isoclinally folded rocks at the southeast that are exposed on Border Ranges fault (MacKevett and Plafker, 1974; Fisher and Kodiak Island. Chert, blueschist, tectonic mélange, and ultramafic Magoon, 1978), a fundamental geologic feature around the Gulf of rocks crop out near the boundary (Moore, 1969; Connelly and Alaska. The fault zone separates less-deformed rock deposited in Moore, 1977; Moore and Connelly, 1979).

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A second fault zone occurs along the southeast shore of Kodiak REFERENCES CITED Island (Pacific side), but its nature is obscured by the shoreline where data change from terrestrial to marine, and merging these Connelly, W., and Moore, J. C., 1977, Geologic map of the northwest side dissimilar data is difficult. Near Kodiak Island, seismic signals are of the Kodiak Islands, Alaska: U.S. Geological Survey Open-File Re- port, scale 1:250,000. scattered or absorbed below the Holocene sediment, perhaps by a Davies, J. N., and House, L., 1979, Aleutian subduction zone seismicity, fault zone because active faults are indicated from bathymetric volcano-trench separation and their relation to great thrust-type studies. Therefore a zone of faults like those mapped on land is in- earthquakes: Journal of Geophysical Research (in press). ferred to continue seaward. Corresponding to this fault zone is the Fisher, M. A., and Magoon, L.., 1978, Geologic framework of Lower Cook Inlet, Alaska: American Association of Petroleum Geologists Bulletin, separation between the well-defined Benioff zone at depth and a v. 62, no. 3, p. 373-402. forearc zone of diffuse seismicity. The steeply dipping isoclinally Holmes, M. L., Meeder, C. A., and Creager, K. C., 1978, Sonobuoy refrac- deformed rocks of Kodiak Island are on one side of the fault zone, tion data near Kodiak, Alaska: U.S. Geological Survey Open-File Re- and the gently downwarped strata of the forearc basin are on the port 78-368. other. The fault zone also corresponds to a major change in crustal Lahr, J. C., Page, R. A., and Thomas, J. A., 1974, Catalog of earthquakes in south-central Alaska, April-June 1972: U.S. Geological Survey structure (Shor and von Huene, 1972), and thus a fundamental Open-File Report. tectonic break between the insular and the forearc basin areas. MacKevett, E. M., Jr., and Plafker, G., 1974, The Border Ranges fault in However, its relation to convergent margin processes, especially south-central Alaska: U.S. Geological Survey Journal of Research, subduction, is not understood. v. 2, no. 3, p. 323-329. Moore, G. W., 1969, New formations on Kodiak and adjacent islands, A third major tectonic feature in the cross section is along the Alaska: U.S. Geological Survey Bulletin 1274-A, p. A27-A35. Aleutian Trench and the trench lower slope. Although seismic- Moore, J. C., and Connelly, W., 1979, Tectonic history of the continental reflection data can generally show only strata dipping less than margin of southwestern Alaska: Late Triassic to earliest Tertiary, in about 30° at these water depths, the present-day tectonism along Proceedings of the 1977 Alaska Geological Society Symposium: An- the leading edge of the subduction zone is shown well in our data. chorage, Alaska Geological Society (in press). Seely, D. R., 1977, The significance of landward vergence and oblique Whether this style of tectonism and other styles shown by Seely structural trends on trench inner slopes, in Talwani, M., and Pitman, (1977) are analogous to those of the ancient margins whose prod- III, W. C., eds., Island arcs, deep-sea trenches, and back-arc basins: ucts are exposed on Kodiak Island is conjectural. However, the American Geophysical Union (M. Ewing Series, Vol. 1), p. 187-198. cross section does show contrasting structure between the initial Shor, G. G., Jr., and von Huene, R., 1972, Marine seismic refraction studies near Kodiak, Alaska: Geophysics, v. 37, p. 697—700. stages of deformation along the Aleutian Trench and the deformed von Huene, R., 1979, Structure of the outer convergent margin off Kodiak rocks on Kodiak Island. The intense compression at the trench is Island, Alaska, from multichannel seismic records, in Watkins, J., and reflected in the upper kilometre of sediment, only a short distance Montadert, L., eds., Geological investigations of continental margins: landward from the shelf edge. The modern environment of isoclinal American Association of Petroleum Geologists Memoir 29, p. 261 — folding and high-pressure metamorphism analogous to that which 272. von Huene, R., Moore, G. W., and Moore, J.C., 1979, Cross section, has affected the rocks on Kodiak Island must be deeper than that Alaska Peninsula—Kodiak Island-Aleutian Trench: Geological Socie- depicted by our data. Thus, this cross section shows both the initial ty of America Map and Chart Series MC-28A, scale 1:250,000. stages of subduction and some of the rocks affected by early Ter-

tiary convergence, and it shows a Mesozoic and Cenozoic sequence MANUSCRIPT RECEIVED BY THE SOCIETY JANUARY 5, 1979 that has been tectonically added to a continental landmass. MANUSCRIPT ACCEPTED JANUARY 10, 1979

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