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Subduction of the Kula Ridge at the Aleutian Trench

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Subduction of the Kula Ridge at the Aleutian Trench Subduction of the Kula Ridge at the Aleutian Trench 0 0 SSlTfOX ™ I Department of Geological Sciences, State University of New York at Albany, Albany, New York 12222 FRED W. McDOWELL Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712 ABSTRACT motion of 60 mm/yr throughout Tertiary time between the Pacific and North American plates. Their reconstruction showed that the A simple model of the probable topographic and thermal conse- Kula Ridge reached the Aleutian Trench 30 m.y. ago (with an un- quences of subducting an oceanic spreading center at an island arc certainty of about 10 m.y.), approximately a factor of two later predicts three geologic effects: (1) shoaling and subaerial than the date estimated by Hayes and Pitman (1970). emergence of the crest of the arc, (2) decrease or cessation of Atwater and Molnar's (1973) results indicate that motion be- subduction-related magmatism, and (3) regional low-grade thermal tween the Pacific and North American plates has been continuous metamorphism (AT = 100 to 300 °C) of the arc rocks. All three of during much of Cenozoic time, but with an overall acceleration these phenomena are recorded in the geology of the Aleutian Is- from 20 mm/yr (the average velocity between 38 and 10 m.y. ago) lands, and the following sequence of events is indicated: (1) di- to 55 mm/yr today. Use of these relative motions in a reconstruc- minution of magmatism on approach of the Kula Ridge in middle tion would yield a time for arrival of the Kula Ridge at the Aleutian Eocene time (=45 m.y. ago), marked by a conformable transition Trench somewhat earlier than the nominal 30 m.y. ago obtained from volcanic-rich to volcanic-poor early-series rocks; (2) shoaling from Grow and Atwater's (1970) constant-velocity model, but and emergence of the crest of the Aleutian arc in late Eocene to probably later than the time determined by Hayes and Pitman Oligocene time, marked by a deep- to shallow-marine transition in (1970). sedimentation and then an arc-wide unconformity above the early We present here an approach to the Kula Ridge—Aleutian series and its probable "submarine equivalent," the now-dissected Trench interaction that complements the use of reconstructed plate Aleutian crestal platform; (3) subduction of the Kula Ridge and motions and provides geological constraints for such reconstruc- greenschist metamorphism of the early-series rocks at about 30 to tions. The principal problem reduces to determining the time of ar- 35 m.y. ago, inferred from K-Ar ages; (4) subsidence of the arc rival of the Kula Ridge at the Aleutian Trench. We have therefore down the south flank of the Kula Ridge in middle Oligocene to developed a simple model of geologic events to be expected when Miocene time, as the Pacific plate was subducted; and (5) abrupt an active ridge approaches a subduction zone, determined whether resumption of arc magmatism 15 m.y. ago. This history of events is these events are recorded in the Aleutian arc, and, if so, attempted consistent with the timing of plate motions in the North Pacific and to ascertain their timing. Where they are appropriate, we include suggests that there has been essentially continuous underthrusting calculations that are only as sophisticated as necessary to make or at the Aleutian Trench since at least 45 to 50 m.y. ago, with sub- test predictions with the precision permitted by the geology. For the duction of 900 to 1,000 km of ocean floor since 30 to 35 m.y. ago. available geological data, this precision is generally quite poor, and the tests have a correspondingly large uncertainty; the precision is INTRODUCTION substantially better for some of the relevant marine geophysical data. The Great Magnetic Bight (Elvers and others, 1967) in the North Pacific (Fig. 1) preserves a partial record of sea-floor spreading on MODEL FOR RIDGE SUBDUCTION two arms of a presumed ridge-ridge-ridge triple junction (Pitman and Hayes, 1968). The western limb of the bight is characterized by Previous suggestions for the fate of a ridge upon its arrival at a magnetic anomalies that become progressively younger toward the trench have almost uniformly favored the idea that the ridge is Aleutian Trench. This pattern is attributed (Grow and Atwater, "stifled" or "dies" — that is, that active spreading ceases (Pitman 1970) to successive subduction at the Aleutian Trench of (1) the and Hayes, 1968; Hayes and Pitman, 1970; Marlow and others, now-vanished Kula plate, formerly between the North American 1973; Hayes and Heirtzler, 1968; Atwater, 1970). This idea fol- and Pacific plates, (2) the Kula Ridge, an east-west spreading center lows directly from Atwater's (1970) observation that "In plate between the Kula and Pacific plates, along which the western limb models, a spreading ridge is considered to be just the weakest place of the bight was generated, and (3) some part of the Pacific plate. between two plates . ." Hence, when the plate preceding a ridge Reconstructions of plate motions involving interaction of the into a trench has been entirely subducted, the ridge ceases to exist Kula Ridge and Aleutian Trench have started from different prem- as a spreading center that produces new sea floor. We adopt this ises and achieved quite different results. Hayes and Pitman (1970) viewpoint here, but point out elsewhere (DeLong and Fox, 1977) assumed no relative motion between the Pacific and North Ameri- the possibility that the two plates defining the former ridge may can plates throughout most of Tertiary time and determined that continue to separate as they subduct (Uyeda and Miyashiro, 1974), the Kula Ridge reached the Aleutian Trench in late Paleocene to permitting upward flow of asthenosphere to fill the resultant hole early Eocene time. Marlow and others (1973) and Scholl and others or gap. We refer to this gap below in relation to thermal effects of (1975a) have suggested that geological data from the Aleutian arc ridge subduction. and from Deep Sea Drilling Project (DSDP) drilling are compatible Grow and Atwater (1970, p. 3720) commented that "Although with such a history and, in particular, that underthrusting at the the nature of the thermal anomalies introduced by putting a re- Aleutian arc stopped (or proceeded at a much slower rate) from the cently active ridge crest into the Benioff zone [is] problematic, it time that the Kula Ridge arrived until some time in the middle seems probable that increased magmatic activity and uplift would Miocene or Pliocene. Grow and Atwater (1970) assumed constant result." Bird and Dewey (1970) and Uyeda and Miyashiro (1974) Geological Society of America Bulletin, v. 89, p. 83 -95, 4 figs., 3 tables, January 1978, Doc. no. 80106. 83 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/89/1/83/3444349/i0016-7606-89-1-83.pdf by guest on 29 September 2021 160° 170° 180° 170° 160° Figure 1. Location map of Aleutian arc area. Bathymetry (in fathoms) generalized from Chase and others (1970, charts 2, 3), trenches defined by 2,600-fin contour. Magnetic anomaly numbering and location from Pitman and others (1974). Islands: 1, Komandorsky Islands; 2, Attu; 3, Agattu; 4, Shemya; 5, Kiska; 6, Rat; 7, Amchitka; 8, Amatignak; 9, Ulak; 10, Adak; 11, Unalaska. Bering shelf submarine canyons: A, Zemdiug; B, Pribilof; C, Bering. Open squares = DSDP sites from Leg 19. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/89/1/83/3444349/i0016-7606-89-1-83.pdf by guest on 29 September 2021 SUBDUCTION OF THE KULA RIDGE 85 have also suggested that enhanced magmatism would occur, in part appropriate to magma generation (Fox and DeLong, 1976, and in on the basis of evidence of widespread volcanism in the Appala- prep.). chians and Japan, respectively, at times when a ridge could have All of these mechanisms, operating individually or collectively to been subducted. These times of ridge subduction are not well decrease arc magmatism, can be reversed to reactivate magmatism. known, however, so that any assumed correlation of increased For example, subduction of the progressively older plate following magmatism with ridge subduction is very uncertain. the ridge will eventually restore the viscosity and density contrasts We summarize below a model of ridge subduction (DeLong and across the slab-mantle interface, thereby restoring frictional heating Fox, 1977) that examines in somewhat more detail the two man- across the slip zone and steeper dip of the slab. Thus, we can pre- ifestations suggested by Grow and Atwater (1970) — uplift and dict for the situation in which the plates preceding and following magmatic activity. This model is restricted to the geometric situa- the ridge are both subducted that the likely effect of these processes tion where the ridge was approximately parallel to the trench, as is a hiatus in arc magmatism, bounded by periods of more extensive indicated by orientation of magmatic anomalies south of the cen- magmatic activity. tral Aleutian Trench. Oblique approach of a ridge as in the east- An additional thermal manifestation — low-grade thermal ernmost Aleutian Trench (Grow and Atwater 1970), Japan (Uyeda metamorphism — may occur in the overlying arc as the center of and Miyashiro, 1974), and the present-day northernmost Middle the former ridge crest passes beneath it. We suggest elsewhere (De- America Trench (Atwater, 1970), or subduction of a spreading cen- Long and Fox, 1977), on the basis of a simple, nonrigorous calcu- ter at high angles to the trench, as seen in the Woodlark Basin area lation, that heat from the hot material injected at the former ridge (Luyendyk and others, 1973), may not necessarily produce the ef- crest may be adequate to produce a near-surface temperature in- fects predicted below.
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