Notes and Discussions

JOHN A. GROW University of , San Diego, Marine Physical Laboratory of the Scripps TANYA ATWATER Institution of , La Jolla, California 92037

Mid-Tertiary Tectonic Transition

in the Aleutian Arc

ABSTRACT and Alaskan Peninsula suggest that the direc- tion of underthrusting is approximately per- The present lack of volcanic activity in the pendicular to the arc. West of 180°, however, Western Aleutian Arc correlates with the lack the direction of relative motion shifts to under- of underthrusting at the western Aleutian thrusting at an acute angle and then to virtually trench predictable by plate tectonic theory. A tangential strike-slip motion in the far Western simple plate model allows one to predict that Aleutians near the Kurile Trench. Thus, the another plate (Kula plate) and a spreading present configuration of the Western Aleutians ridge (Kula ridge) lay south of the arc in early can be described as a trench-trench transform Tertiary time and that the Aleutian trench fault (McKenzie and Parker, 1967). The consumed the ridge in the middle Tertiary. volcanic inactivity in the Western Aleutians Early Tertiary volcanism in the Western Aleu- can be attributed to this lack of underthrusting tians may be related to underthrusting by the (Isacks and others, 1968). Kula plate. A mid-Tertiary orogeny in the The simplest history of crustal plate move- Central Aleutians and a late Tertiary orogeny ments that can be constructed from magnetic in Alaska may be related to the subduction of anomalies predicts that in early Tertiary time the Kula ridge. an east-west-trending spreading center lay south of the Aleutian trench, contributing an INTRODUCTION additional northerly component of under- The Aleutian Arc forms a volcanic ridge thrusting. In middle Tertiary time, the ridge extending westward from the Alaskan Penin- apparently entered the trench and the direction sula until it intersects the Kamchatka Penin- and rate of underthrusting changed. This model .sula. The arc is considered to have 3 tectonic is compatible with geologic data which indicate subdivisions (Von Huene and Shor, 1969) that the W'estern Aleutians were volcanically which have been referred to as the Western active in the early Tertiary and subsequently (west of 180°), Central (180° to 165° W.), and became inactive. Also, the consumption of the Eastern Aleutians (Alaskan Peninsula, east of ridge by the trench may be related to a middle 165° W.). In this paper, the boundary between Tertiary orogeny for which evidence is found the Central and Western Aleutians will be throughout the Aleutians. taken as 176° E., in order to emphasize the limits of volcanism; there is no indication of TERTIARY PLATE INTERACTIONS late Tertiary volcanism west of this longitude IN THE NORTH PACIFIC (Fig. 1). The Central and Eastern Aleutians South of the Aleutian Trench, magnetic each have about 35 volcanoes which have been anomalies 25 to 32 trend east-west from 175° active in the late Tertiary (Coats, 1950). E. to 160° W., with anomaly 25 on the north The fault mechanism solutions for earth- (Elvers and others, 1967; Hayes and Heirtzler, quakes in the Aleutian Arc (Stauder, 1968) are 1968; Grim and Erickson, 1969; Erickson and remarkably compatible with the northwest- Grim, 1969). The present configuration of these ward underthrusting of the Aleutian Arc by the anomalies is shown in Figure 2D. Ages are North Pacific oceanic lithosphere described by assigned according to the geomagnetic time the hypotheses of sea-floor spreading and plate scale of Heirtzler and others (1968), which has tectonics (Isacksand others, 1968). Occurrences recently been confirmed by deep-sea drilling of earthquakes beneath the Central Aleutians (Maxwell and others, 1970). Pitman and Hayes

Geological Society of America Bulletin, v. 81, p. 3715-3722, 2 figs., December 1970 3715

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Figure 1. Western Aleutian arc. Arrows indicate present relative motion between northern part of and the Aleutian arc predicted by a rotation about 53°N., 53°W. (Morgan, 1968). Volcanic data from Coats (1950).

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(1968) demonstrated that these anomalies could apaskan Indian dialect. The short solid arrows be explained by an east-west spreading center indicate the 7 cm/yr northwesterly motion of south of the trench during the late Cretaceous the Pacific plate with respect to North America and early Tertiary. By assuming that the Pacific (6 to 8 cm/yr, depending on distance from plate had remained fixed with respect to the rotation pole). The white arrows indicate the , they concluded that 7.0 cm/yr movement of the Kula plate with the east-west spreading center was carried respect to the Pacific plate (twice the 3.5 cm/yr down the Benioff zone in the late Cretaceous or spreading rate). The long solid arrows show the Paleocene. However, paleomagnetic data show motion of the Kula plate with respect to North that the Pacific and North American plates America (the vectorial sum of the short solid were not fixed, but rather moved a great deal arrow and the white arrow). The small line relative to one another since the Cretaceous arrows indicate spreading of the east-west ridge, (Francheteau and others, 1970). A likely model for which we propose the name "Kula" ridge. is that the relative motion throughout the Modest differences in the rate or direction of Cenozoic was approximately constant and was North America-Pacific motion or in the rate equal to the present motion (McKenzie and of Kula-Pacific spreading will cause some varia- Morgan, 1969; Atwater, 1970). If the latter tion in this reconstruction. For example, if the model is correct, then the age for disappearance North America-Pacific rate of motion were of the spreading center into the trench must be 25 percent faster or if the Kula-Pacific rate were much younger than Paleocene. 25 percent slower, the first contact between the A reconstruction of the North Pacific mag- ridge and trench would occur about 5 m.y. netic anomalies assuming uniform motion of later; if the North America-Pacific motion the plates (Fig. 2) suggests the east-west center were more northerly, the interaction would be did not enter the Aleutian trench until about later while a more westerly-motion would make 30 m.y. ago. For this reconstruction: (1) the it earlier, and so on. A combination of these North American plate and Aleutian arc are effects could shift the interaction by as much as arbitrarily held fixed; (2) the motion of the 10 m.y. Pacific plate with respect to North America In spite of these uncertainties, it is quite is described as a rotation about a pole at 53° N., clear that the Kula plate and ridge existed in 53° W. (Morgan, 1968); (3) the rate of motion early Tertiary time, and that they do not exist indicated by 6 cm/yr at the San Andreas trans- at present. If the basic assumptions of uniform form is held constant throughout the Tertiary motion of the Pacific plate and constant spread- (McKenzie and Morgan, 1969; this does not im- ing rate for the Kula ridge are approximately ply that the San Andreas itself existed through- correct, then a mid-Tertiary interaction be- out the Tertiary); and (4) the half-spread- tween the ridge and the Aleutian-Alaskan ing rate of the east-west ridge, which was 3.5 system must have occurred. cm/yr between 63 and 72 m.y. ago, is held constant. In addition to the above, the con- EARLY TERTIARY VOLCANISM IN tinuation of the east-west spreading center well THE CENTRAL AND WESTERN west of 180° (on Fig. 2D) is postulated in order ALEUTIANS to discuss the early Tertiary volcanism in the The oldest fossiliferous materials discovered Western Aleutians. Although this region is not in the Aleutians are Eocene volcanic sediments well surveyed and is complicated by the on Northern Adak (Coats, 1956a; Scholl and Emperor seamounts and fracture zone, the others, 1969). These sediments have been magnetic anomalies still trend east-west at 165° correlated with altered basaltic and andesitic E. (Erickson and others, 1970). The real ridge flow breccias, pillow lavas, and volcaniclastic probably had fracture zones and ridge offsets sediments which make up much of Southern in this westward extension, but only a simplified Adak and are the oldest formation on the island linear ridge can be considered here. (Fraser and Snyder, 1959). Paleogene sedi- Given these assumed conditions, the recon- ments and old, deeply eroded volcanic rem- struction for 40 m.y. ago (Fig. 2A) shows that nants are reported on the Komandorskiye the spreading center was well south of the Islands with no evidence of late Tertiary cones Aleutian trench, and an intermediate plate or craters (Dawson, 1894; Nalivkin, 1960). existed between the North American and Basalts, pillow lavas, and sediments containing Pacific plates. For this plate, we propose the early Tertiary fossils are found on Attu (Gates name "Kula," meaning "all gone" in an Ath- and Gibson, 1956). Volcanic flows, breccias,

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A. 40 M.Y AGO 180 160

. NORTH AMERCA

B. 30 M.Y. AGO Figure2. Schematic evolution of northern Pacific plates and magnetic pattern, assuming constant relative motions of the Pacific, Kula, and North American plates. Solid magnetic lines indicate presently observed anomalies. The

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40

30

C. 20 M.Y. AGO 160 ISO 160 140

30

D. PRESENT Figure 2. Continued, ages 32, 49, 63, and 72 m.y. correspond to magnetic anomalies number 10, 20, 25, and 31. Solid arrows indicate direction of motion of plates with respect to North America (see text for other arrows).

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and their sedimentary derivatives which occur Therefore, the orogeny in the Central Aleu- on Agattu and Shemya are unfossiliferous tians probably occurred sometime during and/ (Coats, 1956B; Sharp, 1946), but probably or after the early Miocene. Farther east, the correlate with the early Tertiary formation on Alaskan Peninsula experienced an orogeny Attu. On Rat and Amchitka Islands, basaltic during the Pliocene (Burk, 1965). formations of Oligocene-Miocene age overlie It is tempting to correlate the mid-Tertiary andesitic formations (Powers and others, 1960; (Miocene?) orogeny in the Central Aleutians Lewis and others, 1960) which are probably of with the subduction of the Kula ridge into the Eocene-Oligocene age. Although the number Aleutian BeniofF zone. Although the of of fossil dates is small, there is ample evidence the thermal anomalies introduced by putting a to infer that both the Western and Central recently active ridge crest into the Benioff zone Aleutians were volcanically active in the early are problematic, it seems probable that in- Tertiary. Assuming that the volcanic activity creased magmatic activity and uplift would is related to the direction and rate of under- result. The Miocene plutonsim and uplift in thrusting, then the relative motion between the Central Aleutians could be explained by the Western Aleutians and the adjacent ocean such a relationship. However, if this orogenesis plate to the south during the early Tertiary is related to the subduction of the Kula ridge, was not tangential, but had a larger component the first ridge-trench interaction apparently of motion perpendicular to the arc. Thus, the occurred 20 to 25 m.y. ago, 5 to 10 m.y. later motions predicted for the Kula plate are com- than suggested by Figure 2B (this is within the patible with these data. limits of variation discussed above). Since the geologic literature on the Koman- Between 170° E. and 170° W., the con- dorskiyes, Attu, Agattu, and Shemya is not sumption of the Kula ridge would have occurred adequate to estimate at what point in the over a relatively short time span (less than Tertiary the volcanism in the Western Aleu- five m.y.) and, within the resolution of avail- tians ceased, nothing can be learned in the able data, may appear to be a simultaneous Western Aleutians concerning the timing of event. In the Alaskan Peninsula at about 150° the interaction of the Kula ridge with the W., the ridge-trench interaction would have trench. However, certain relationships in the occurred about 10 m.y. later than it did at 180° Central and Eastern Aleutians may provide (see B and C of Fig. 2). This interaction could some clues. have been even later if the Kula plate frag- mented as it became smaller and the spreading MIOCENE-PLIOCENE OROGENIES IN rate decreased. Such a fragmentation and slow- THE ALEUTIAN-ALASKAN ARC ing has been observed in the Farallon and Juan Gates and Gibson (1956) have reported that de Fuca plates (Atwater, 1970). Therefore, the a major "unconformity separating lower to Pliocene orogeny in the Alaskan Peninsula middle Tertiary rocks from upper Tertiary may also be related to the passing of the Kula rocks has been mapped on Kiska, Adak, Umnak ridge. [168° W.], and Unalaska [166° W.] islands, almost the length of the Aleutian arc, and ACKNOWLEDGMENTS marks a turning point in the geologic history," We wish to thank James Hawkins and John (p. 128-130) and that "the cutting of the Mudie for numerous helpful discussions. David middle Tertiary erosion surface followed a Scholl and Robert Coats critically read the major orogeny accompanied by extensive fold- manuscript and made several helpful sugges- ing and faulting as well as intrusion of igneous tions. Donna Hawkins suggested the Athapas- rocks" (p. 130). The dating of the orogeny, kan name "Kula." This work was supported by intrusions, and erosion surface has been ham- the Office of Naval Research. pered on most of the islands by the scarcity of fossils in the early Tertiary volcanic materials REFERENCES CITED and by the extensive chemical alteration which Atwater, Tanya, 1970, Implications of plate tec- accompanied the intrusion of the small plutons tonics for the Cenozoic tectonic evolution of (Eraser and Snyder, 1959). However, the western North America: Geol. Soc. America plutons cut through Oligocene-Miocene (un- Bull., v. 81, p. 3513-3536. differentiated) formations on Amchitka (Powers Burk, C. A., 1965, of the Alaskan penin- and others, 1960) and an early Miocene forma- sula island arc and continental margin: Geol. tion on Unalaska (Drewes and others, 1961). Soc. America Mem. 99, 250 p.

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