Episodic accretion and plutonism in southwestern Alaska

Earth and Planetary, Science Letters, 49 (1980) 29--33

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© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands [31

EPISODIC ACCRETION AND PLUTONISM IN SOUTHWESTERN ALASKA LOREN A. RAYMOND Department of Geology, Appalachian State University, Boone, NC 28608 (U.S.A.)

Received July 30, 1979 Revised version received April 7, 1980

Paleontologic and radiometric dating of the accretionary prism and magmatic arc of southwestern Alaska reveal an history of episodic accretion and plutonism. Possible accretion events in the Triassic (220-195 m.y.) and Early Jurassic (184-176 m.y.) were followed by Middle Cretaceous (108-83 m.y.), earliest Paleogene (65-60 m.y,), Middle Paleogene (50-40 m.y.), and Neogene (25-0 m.y.) accretion episodes. Plutonic events, which alternate with the accretion events, occurred in the Early Jurassic (193-184 m.y.), Middle/Late Jurassic (176-145 m.y.), Late Cretaceous/Early Paleogene (83-50 m.y.), and Late Paleogene (38-26 m.y.). Episodicity of accretion events is an apparent cause of incomplete stratigraphic records in the accretionary prism and forearc basin.

1. Introduction Southwestern Alaska contains a series of belts of deformed sedimentary and volcanic rock typical of oceanic crust, which is widely interpreted as a Mesozoic/Cenozoic accretionary terrane [ 1 - 5 ] . These belts are relatively unaffected by extensive post-accretion events, such as transform or strike-slip faulting, plutonism, and volcanism, which cloud the history of more publicized accretionary terranes like the Franciscan Complex of the California-Oregon Coast Ranges. Consequently, this area provides an ideal orogenic zone for the study of the timing of events along Andean-type convergent plate margins. Raymond and Swanson [6] have developed an empirical model, the Episodic Accretion-Plutonism Model, that predicts an intricate series of alternating and episodic accretive and plutonic events for convergent margins. This model calls into question three generally accepted views: (1) that accretion and plutonism are coeval [ 4 , 7 - 1 0 ] ; ( 2 ) t h a t the plutonism and volcanism in magmatic arcs are necessarily coeval [7,11,1 2] ; and (3) that the plutonic and volcanic rocks of magmatic arcs are essentially subsurface and surficial products of the same parental magmas [7,11, 13]. The purpose of this paper is to show that data

from southwestern Alaska support the view that accretion and plutonism are episodic rather than coeval, thereby challenging view (1). Questions about view (2) are raised by the correlation chart of geologic events in magmatic arc, forearc, and accretionary terrane presented by Moore and Connelly [2]. Views (2) and (3) will be dealt with in subsequent papers.

2. Episodic plutonism Gilluly [14] reviewed the magmatic history of western North America and concluded that on the scale of the entire Cordillera, igneous activity has been more or less continuous since the Triassic. Nevertheless, local to regional episodicity of magmatism has been documented along several convergent plate margins, including the western margin of North America [7,9,15-20]. This episodicity is reflected in both plutonic [9,16] and volcanic [ 19,20] terranes. Geochronologic studies of southern Alaskan plutonic rocks, reported by Burk [21 ], Reed and Lanphere [9,16] and others (reviewed recently by Hudson [22]), reveal an episodicity of plutonism over the

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Fig. 1. Chronology of plutonic (P), metamorphic (M), and sedimentation (S) events in southwestm:n Alaska. U = Uyak Complex, V = Valdez-Shumagin and equivalent units, O = Orca Group and equivalent units, and C = Middle/Late Cenozoic accretionaxy wedge. Time ranges are based on data referenced and discussed in text. Time scales from Berggren [39] and van Hinte [40,41].

past 200 m.y. Analysis of K-Ar mineral ages, combined with reconnaissance mapping, led Reed and Lanphere [9,16] to the conclusion that at least:three intrusive epochs occurred during the Mesozoic and Cenozoic Eras (Fig. 1). Concordant ages on coexisting mineral pairs were used to define the beginning and , ending of.each epoch. An Early/Middle Jurassic intrusive epoch ( 1 7 6 - 1 5 4 m.y.) was followed, after a long hiatus, by a ,Late Cretaceous/Early Paleogene epoch ( 8 3 - 5 8 m:y.) and a Late Paleogene epoch ( 3 8 - 2 6 m.y.). A possible'fourth plutonic epoch was added b y Carden et al. [23] who reported 193- to 183-m.'y. dates for the'Afognak pluton o f ~ e Kodiak Islands (Fig. 1).'In a~:review of a~ailable'da~a, Hudson'[22] revised the boundaries of the three oldest epochs. R6cks of the,four plutonic et~chs have'been

Fig. 2. Sketch map of southwestern Alaska showing positions of plutonic belts of Jurassic (Jp), Cretaceous/Paleogene (KPp), and Late Paleogene (Pp) age and sedimentary wedges accreted during the Middle Cretaceous (MK), the Cretaceous/ Paleogene (KP), the Middle Paleogene (P), and the Neogene (N). The Early Jurassic plutonic belt and the Triassic(?)/ Jurassic accretionary terrane overlap and are figured together as the unit ~PT. Plutonic belts arc based on Reed and Lanphere [9], Moore and Connelly [2], and Hudson [22]. The positions of the accretionary terranes follow Plafker et al. [ 31, Connelly [8 ], and Moore and Connelly [ 1 ].

assigned to a series of overlapping belts [1,2,10,22]. The three older belts occur in a sequence with each successively younger belt lying to the north of the preceding one. The older belts are overprinted by the Late Paleogene belt (Fig. 2).

3. Episodic accretion Raymond and Swanson [6] proposed that accretion events in the arc-trench gap environment of convergent plate margins are episodic rather than continuous. Because sedimentation is more or less continuous in that environment, accretion events are dated by K-Ar dates and youngest fossil ages on structural packages in the accretionary wedge [24,25].

31 In southwestern Alaska, only one accretionary package, the Seldovia Schist terrane, has been dated radiometrically (Fig. 1). Forbes and Lanphere [26] and Carden et al. [23] reported K-Ar ages averaging 188 m.y. on blueschist and greenschist facies rocks of the Kenai Peninsula and Kodiak Islands. PlaNer et al. [3] argue that these dates have been reset through argon loss resulting from Mesozoic intrusive events, e.g. the Early Jurassic intrusive epoch that produced the nearby Afognak pluton. Carden et al. [23] and Moore and Connelly [2] argue against this interpretation on the basis of "petrologic relationships" and the absence of plutons adjacent to the schist on the Kenai Peninsula. Yet it cannot be fortuitous that the Kenai and Kodiak dates are identical. If the Afognak pluton intruded the Seldovia schist on Kodiak Island, the intrusive event must have reset the K-Ar dates of the schist. The identical dates, therefore suggest that plutonism of the same age (about 188 m.y.) affected the Kenai Peninsula. Consequently, I have included the 188-m.y. data in Fig. 1, but have queried its position. Youngest fossil ages for the ocean floor-trench rocks of accretionary packages provide a lower limit to the age of accretion. Four accreted packages (Fig. 1), recognized in southwestern Alaska on the basis of stratigraphic-structural mapping and offshore drilling, have been dated by paleontological methods. The oldest of these accreted units is the Uyak Complex. Connelly [8] recently reviewed data on the Uyak and he and his co-workers renamed the unit, previously designated the Uyak Formation [27], the Uyak Complex. The Uyak Complex and its equivalents the McHugh Complex and Seldovia Bay Complex - have been described and correlated by several authors [ 1 - 3 , 8 , 2 8 - 3 0 ] . The unit is a m61ange consisting of pieces of wacke, greenstone, chert, gabbro, and ultramafic rocks enclosed in a sheared matrix of chert and argillite [8]. Fossils from the Uyak Complex range from Permian to Middle Cretaceous (Aptian) in age. Thus, if the fossils reflect the age limits of sedimentation, the Uyak Complex was probably accreted following the Aptian (i.e. after 108 m.y.B.P.) (Fig. 1). Two gaps. in the paleontologic record, one of Early/Middle Triassic age and another of Early/Middle Jurassic age [8] suggest the possibility that the Uyak Complex may be a tectonic melange consisting of three mixed accretionary units.

Deformed Cretaceous turbidites of the ShumaginValdez complex [ 1,2] and deformed clastic rocks and m61ange of the Paleocene Ghost Rocks Formation [8,27] form the second accreted unit. The ShumaginValdez complex - formerly referred to as the Valdez Group [30], Shumagin Formation [21], and the Kodiak Formation [8,27] - was underthrust beneath the Uyak Complex [1,2]. Because the youngest fossils found in the Shumagin-Valdez complex are Maestrichtian in age [5,31 ], the date of accretion is here considered to be about 65 m.y. This event was short-lived (which perhaps explains the absence of extensive shearing in the Shumagin-Valdez complex) and was followed by deformation and accretion of the Ghost Rocks Formation. Ghost Rocks accretion was also short-lived, explaining the lack of a significant break in plutonism in the arc. Clastic sedimentary rocks interpreted as submarine fan facies with accompanying oceanic tholeiites, greenstones, and melanges of the Lower Paleogene (Paleocene/Eocene) Orca Group [3,12] and equivalent units, e.g. the Tolstoi Formation [21], comprise the third paleontologically dated accretionary package. These rocks are considered to have accreted during the period 5 0 - 4 0 m.y.B.P. The youngest of the accreted terranes of southwestern Alaska lies predominantly offshore. The oldest of the formations in this youngest accreted wedge may be the Sitkalidak Formation [27], a sequence of deformed Eocene/Oligocene turbidites faulted against the Ghost Rocks Formations although this unit may be a deformed trench slope basin sequence. Additional deep marine sediments, deposited on the Pacific Plate, have been scraped off and accreted to the southern Alaska margin [32-34]. Deformed Late Neogene (Plio-Pleistocene) sediments were encountered in the cores at DSDP Site 181 south of the Kenai Peninsula [34] and deformed Early Neogeue (Late Miocene) rocks are present along the Aleutian Ridge to the east [35]. If deformation of all of these rocks is related to an accretion episode, that episode extends essentially to the present. 4. Discussion and conclusion Data from southwestern Alaska give insight into the intricacies of orogeny along Andean-type convergent plate margins. Episodes of accretion may have

32 occurred during the Triassic ( 2 2 0 - 1 9 5 m.y.) and Early Jurassic(?) ( 1 8 4 - 1 7 6 m.y.) and clearly took place during the Middle Cretaceous ( 1 0 8 - 8 3 m.y.), Early Paleogene ( 6 5 - 6 0 m.y.), Middle Paleogene ( 5 0 40 m.y.), and Neogene ( 2 5 ? - 0 m.y.). The episodes o f accretion alternated with periods o f plutonism during the Early Jurassic ( 1 9 3 - 1 8 4 m.y.), Middle/Late Jurassic ( 1 7 6 - 1 4 5 m.y.), Late Cretaceous/Early Paleogene ( 8 3 - 5 0 m.y.), and Late Paleogene ( 3 8 26 m.y.). Clearly, plutonism and accretion were not coeval. The implications of the Episodic AccretionPlutonism Model for the stratigraphic record are twofold. First, the model explains the incomplete nature of the stratigraphic record in the accreted wedge. Scholl and Marlow [36] and Scholl et al. [37] have pointed out that circum-Pacific foldbelts contain insufficient volumes of deformed deep marine deposits to account for the masses o f sediment that should have been accreted given the long history of Mesozoic/Cenozoic convergence. The Episodic AccretionPlutonism Model requires that most sedimentary materials be subducted during plutonic episodes. Thus, accretionary wedges will contain only those sedimentary rocks accreted to the margin during accretion episodes plus those rocks deposited in trench slope basins. A combination of episodic accretion, selective subduction [38], and deformation of trench slope deposits [37] may account for the nature o f the stratigraphic record in accretionary prisms. The second implication o f the model for the stratigraphic record is that forearc sections may be incomplete as a result of episodic accretion. Uplift o f the forearc region, with attendant erosion or non-deposition, would be expected whenever (1) accretion was vigorous, (2) decoupling between the subducting plate (with its oceanic sediments) and the accreted wedge was pronounced; or (3) subduction ceased. In southwestern Alaska, Burk [21 ] and Moore and Connelly [2] report an Early to Late Cretaceous hiatus in the forearc that corresponds to the Middle Cretaceous accretion event. Burk [21, fig. 17] and PlaNer et al. [ 12] report Late Oligocene/Miocene to Pleistocene unconformities in the forearc basin, perhaps related to uplift during the most recent accretion event. Forearc sedimentation, as reflected by the stratigraphic record, is not as severely affected by episodic accre-

tion as is sedimentation in the accretionary wedge. However, significant unconformities in the forearc may be developed during some accretion events.

Acknowledgements I wish to thank G. Plafker and S.E. Swanson for stimulating discussions, J.C. Moore for his comments on an earlier version of the manuscript, S.E. Swanson and F.K. McKinney for their reviews, and C. Muirhead for assistance in preparing the manuscript. Research was supported by Appalachian State University and Division of Earth Sciences National Science Foundation, NSF grant EAR 76-06062.

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