Subduction erosion of the Jurassic Talkeetna-Bonanza arc and the Mesozoic accretionary tectonics of western North America Peter D. Clift School of Geosciences, University of Aberdeen, Aberdeen AB24 3UE, UK Terry Pavlis Department of Geology and Geophysics, University of New Orleans, New Orleans, Louisiana 70148, USA Susan M. DeBari Department of Geology, Western Washington University, Bellingham, Washington 98225, USA Amy E. Draut University of California–Santa Cruz, Santa Cruz, California 95060,USA, and U.S. Geological Survey, 400 Natural Bridges Drive, Santa Cruz, California 95060, USA
Matthew Rioux Department of Geological Sciences, University of California–Santa Barbara, Santa Barbara, California 93106, USA
Peter B. Kelemen Lamont-Doherty Earth Observatory, Columbia University, P.O. Box 1000, 61 Route 9W, Palisades, New York 10964, USA ABSTRACT The Jurassic Talkeetna volcanic arc of south-central Alaska is an oceanic island arc that formed far from the North American margin. Geochronological, geochemical, and structural data indicate that the arc formed above a north-dipping subduction zone after ca. 201 Ma. Magmatism migrated northward into the region of the Talkeetna Mountains ca. 180 Ma. We interpret this magmatism as the product of removal of the original forearc while the arc was active, mainly by tectonic erosion. Rapid exhumation of the arc after ca. 160 Ma coincided with the sedimentation of the coarse clastic Naknek Formation. This exhumation event is interpreted to reflect collision of the Talkeetna arc with either the active margin of North America or the Wrangellia composite terrane to the north along a second north-dipping subduction zone. The juxtaposition of accreted trench sedimentary rocks (Chugach terrane) against the base of the Talkeetna arc sequence requires a change from a state of tectonic erosion to accretion, probably during the Late Jurassic (before 150 Ma), and definitely before the Early Cretaceous (ca. 125 Ma). The change from erosion to accretion probably reflects increasing sediment flux to the trench due to collision ca. 160 Ma. Keywords: Alaska, terrane accretion, subduction, tectonic erosion, collision. INTRODUCTION The Talkeetna arc of south-central Alaska, United States, is one of the most complete oceanic arc crustal sequences known worldwide (DeBari and Sleep, 1991). Although its origin within an oceanic setting is well established (e.g., Burns, 1985; DeBari and Coleman, 1989), the exact nature and timing of its tectonic development, including its accretion to North America, continue to be debated. The arc forms part of the composite WrangelliaPeninsular terrane, which was one of the last major blocks to be added to the active western margin of North America (e.g., Coney et al., 1980). New geochemical and age data from the type localities in the Talkeetna Mountains and adjacent Chugach Mountains now allow us to better constrain its evolution and possibly link it to arc units of the Bonanza arc, exposed on Vancouver Island, British Columbia (Fig. 1). Limited age data do not allow an unequivocal correlation of the Bonanza and Talkeetna arcs, yet there is a broad overlap in the timing of activity; the Talkeetna arc is dated by U-Pb zircon geochronology as 205–156 Ma (61 Ma; Rioux et al., 2003a, 2003b) and the Bonanza arc as 202–165 Ma (Friedman and Nixon, 1995; DeBari et al., 1999).
TECTONIC SETTING OF THE TALKEETNA ARC The Talkeetna arc in south-central Alaska forms the Peninsular terrane of Plafker and Berg (1994). The Talkeetna arc is a remarkably well preserved magmatic arc complex with the entire crustal sequence from sedimentary through volcanic and intrusive to mantle rocks exposed in an E-W–trending, north-dipping belt north of the Border Ranges fault (Burns, 1985; DeBari and Sleep, 1991; Fig. 1). This fault, which we interpret here as a paleo–subduction-zone thrust, was reactivated in the Cenozoic as a right-lateral strike-slip fault, possibly responsible for hundreds of kilometers of motion along the western North American margin (Roeske et al., 2003). The Talkeetna arc does not have any known basement of either continental or oceanic character that predates subduction (Pavlis et al., 1988). We failed to confirm the presence of midocean-ridge–type lavas or the gabbros proposed by DeBari and Sleep (1991), but found remarkable homogeneity among Talkeetna subduction-related gabbronorites over the 200-km-long, ;20-km-thick outcrop area in the northern Chugach Mountains (Kelemen et
al., 2003). Because the crustal section includes the mantle, and because zircon U/Pb systematics show little or no sign of inheritance (Rioux et al., 2003a, 2003b), it is reasonable to assume that the arc section in the northern Chugach Mountains is entirely the product of suprasubduction magmatism. Bonanza arc volcanic and plutonic rocks of Early Jurassic age are exposed on Vancouver Island, and these are intruded into and emplaced over Wrangellia terrane basement. There is no preserved continental basement to Wrangellia, but Wrangellian sequences in the St. Elias Mountains overlie the Alexander terrane, which includes a sedimentary cover as old as Late Proterozoic–Cambrian (Gardner, 1985) and sedimentary zircons with ages as old as 1.8 Ga (Butler et al., 1997). Thus either the Bonanza arc represents a second coeval arc, or the Talkeetna and Bonanza arcs composed a continuous magmatic province associated with a single subduction zone that straddles both oceanic and continental crust. An oceanic subduction setting origin for the Talkeetna arc is suggested by geochemical and isotopic data, which indicate little involvement of continental sediment in its petrogenesis (Kelemen et al., 2003; Clift et al., 2005). The ranges of Nd isotope, La/Sm, and Nb/Zr values are comparable to those in the Mariana and Tonga arcs of the western Pacific (Kelemen et al., 2003; Clift et al., 2005). The polarity of subduction has been debated (e.g., Reed et al., 1983; Trop et al., 2002). We infer it to have been north dipping on the basis of lavas that have geochemical signatures suggestive of reduced subduction influence and a more enriched mantle source exposed north of the main arc outcrop (Clift et al., 2005). These northern lavas are interpreted as having been erupted in a backarc setting, which is consistent with evidence for Early Jurassic blueschists to the south of the main arc outcrop (e.g., 185–179 Ma and 190–204 Ma, respectively, for Iceberg Lake schists near Talkeetna and Raspberry schists of Kodiak Island; Sis-
q 2005 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or
[email protected]. Geology; November 2005; v. 33; no. 11; p. 881–884; doi: 10.1130/G21822.1; 2 figures.
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Figure 1. Tectonic map of Alaska and northwestern Canada showing major terranes accreted to cratonic North America (modified after Trop et al., 2002).
son and Onstott, 1986; Roeske et al., 1989). Although these blueschists have been used as evidence for accretion during this period, in detail their structural association is more consistent with later reincorporation into a younger accretionary complex. The negligible influence that sediment subduction had on chemistry of the Talkeetna arc suggests that the trench must have contained very little sediment, a condition associated with subduction erosion and long-term net crustal loss in modern arcs (von Huene and Scholl, 1991; Clift and Vannucchi, 2004). This prediction is consistent with the lack of a major preserved accretionary complex the
Figure 2. Proposed tectonic evolution of Talkeetna arc in south-central Alaska; collision is with active margin to north. 882
age of the Talkeetna arc. Radiometric age data (Rioux et al., 2003a, 2003b) indicate that the Talkeetna arc was active at 201–156 Ma, during which time we infer a state of active subduction, tectonic erosion, and magmatism. Although the Talkeetna arc preserves a complete crustal section, it is noteworthy that the exposures comprise rocks of the arc volcanic front with no evidence of a forearc or trench slope. In modern arcs, ;75–100 km of crust, spanning the forearc and trench slope, are between the volcanic front and the trench; this material is missing in Alaska. The lack of a forearc assemblage has been recognized for some time (e.g., Hudson, 1979) and has been ascribed to exhumation (e.g., Pavlis et al., 1988), strike-slip truncation, subduction erosion, or some combination of the three (e.g., Roeske et al., 1992). NATURE OF JURASSIC SUBDUCTION EROSION Tectonically erosive modern arcs show rates of trench retreat of 1–3 km/m.y. over geologic time scales (e.g., Clift and Vannucchi, 2004). Thus, in the Talkeetna arc, 45–135 km of crust might have been lost if similar rates of subduction erosion had operated between 201 and 156 Ma, when the arc was active. Based on modern rates, there was adequate time to remove the entire forearc by subduction erosion. Consistent with this rate estimate, arc magmatism migrated northward during the Jurassic at rates comparable to the apparent trench retreat (Fig. 2). While the ages of plutons within exposures close to the Border Ranges fault range from 201 to 181 Ma, the Peninsular terrane plutons in the Talkeetna Moun-
tains to the northwest are consistently younger, 177–156 Ma, and young to the northwest (Rioux et al., 2003a, 2003b). Although such a migration of magmatism could represent a shallowing of the subduction angle, migration is approximately of the same dimension (;80 km between the Border Ranges fault and the younger arc plutons) as the arc-trench distance represented by the missing forearc basement, and thus the combination of northward arc migration after ca. 180 Ma and the missing forearc is most easily explained by subduction erosion. Structural relationships and cooling ages along the outboard edge of the Talkeetna arc are complex, but are also consistent with Jurassic subduction erosion. The Talkeetna arc is juxtaposed along its southern margin against the Chugach terrane, a large clasticdominated subduction accretionary complex (Fig. 1). This complex was accreted in the same north-directed subduction mode as the Talkeetna arc (e.g., Roeske et al., 1992; Plafker et al., 1994) but includes younger units. Specifically, the Chugach terrane consists of two distinct assemblages (e.g., Plafker et al., 1994): an older me´lange exposed intermittently along the Border Ranges fault and an outboard Upper Cretaceous assemblage composed predominantly of structurally coherent trench-fill turbidites, accreted well after magmatic activity in the Talkeetna arc. The evolution of the older me´lange relative to the cooling of the Talkeetna arc plutonic complex provides key constraints on the timing of tectonic erosion. First, the older me´lange is devoid of continental sedimentary sources, with a chertGEOLOGY, November 2005
argillite-tuff matrix surrounding blocks of mid-oceanic-ridge basalt and graywackes (e.g., Pavlis, 1982; Plafker et al., 1994). Fossil ages, primarily of radiolaria from cherts in the me´lange, range from Triassic to Late Cretaceous (e.g., Plafker et al., 1994), but these ages only demonstrate that part of the me´lange is younger than the Late Cretaceous. Nonetheless, combined with the lithology, they demonstrate that the Talkeetna arc had no significant continental sedimentary flux into its trench until Late Cretaceous time. Second, the me´lange and the adjacent Talkeetna arc were locally intruded by Early Cretaceous plutons (ca. 125 Ma, e.g., Barnett et al., 1994). During this event, deformation and plutonism were localized along the Border Ranges fault, with the low-grade Chugach me´lange structurally beneath the Cretaceous plutonic-metamorphic assemblage, suggesting that most of the me´lange is younger than ca. 125 Ma (Pavlis et al., 1988), consistent with fossil ages from Kodiak Island (e.g., Clendenen et al., 2003). Thus, the first major accretionary products along the margin are much younger than the Talkeetna arc magmatism. Third, the Chugach me´lange locally is sliced together with Jurassic blueschist, including the Iceberg Lake blueschist (Sisson and Onstott, 1986), and the Raspberry and Seldovia schists (Roeske et al., 1992). The Early Jurassic cooling ages of these rocks (186–179 Ma; Sisson and Onstott, 1986; Roeske et al., 1989) seemingly contradict the tectonic erosion hypothesis with an inference of an Early Jurassic accretionary system, yet all of these blueschists are in structural contact with the Chugach me´lange and are spatially limited. Recent thermochronology of the Raspberry schist and the Afognak pluton of Kodiak Island showed that these rocks carry virtually the same cooling history after ca. 150 Ma (Clendenen et al., 2003), which is consistent with completion of major tectonic erosion by that time. The blueschists probably formed deep in the subduction zone prior to ca. 179 Ma, when tectonic erosion was ongoing at shallower levels in the forearc. Modern margins show that tectonic erosion and accretion can occur at the same time at different depths in the subduction zone. Aside from the area of Early Cretaceous reheating, the entire southern margin of the Talkeetna arc cooled rapidly in the Early Jurassic. Hornblende K-Ar dates are generally older than 170 Ma, and biotite K-Ar ages range from 161 to 187 Ma (Burns et al., 1991), which suggests that by ca. 175 Ma, when magmatic activity had shifted to the north, the entire southern margin had already cooled to biotite closure temperatures (300 8C). Cooling occurred in the absence of clear evidence for erosional unroofing because the Early Jurassic GEOLOGY, November 2005
Talkeetna volcanic rocks still cap the plutonic assemblages of the northern Chugach Mountains, and the oldest known unconformity in the section is beneath the Upper Jurassic Naknek Formation (Trop et al., 2005). Instead, structural unroofing by normal faults or cooling of lower-crustal arc rocks as they approached the trench as a result of ongoing subduction erosion, or both, probably produced the observed cooling. Because the global subduction system is now .50% tectonically erosive, the presence of this and other tectonically truncated arc sections should be anticipated in many ancient suture zones. The Dras-Kohistan oceanic arc of the western Himalaya, for example, has no associated accretionary complex and is directly tectonically juxtaposed against the telescoped passive margin of India (e.g., Garzanti et al., 1987), suggesting dominant tectonic erosion prior to that collisional event. LATE JURASSIC REGIONAL CHANGES There is evidence of significant tectonic change in the nature of subduction after ca. 160 Ma, which may account for the change in trench tectonics to accretion. In the Talkeetna Mountains (Fig. 1), relatively rapid exhumation of the Talkeetna arc is documented by the appearance of conglomerate clasts with zircon U/Pb ages of 168–156 Ma in sedimentary rocks of the Naknek Formation; biostratigraphic ages overlap that range (Trop et al., 2005). In both the Talkeetna and Chugach Mountains, a major change in tectonic regime is shown by the cessation of magmatism, tilting of the section, erosion and deposition of the sandy Tuxedni Formation, followed by the more conglomeratic Naknek Formation (Trop et al., 2002). The cause of this deformation and exhumation is not clear, but likely reflects a collision event. We suggest that the deformation, exhumation, sedimentation, and cessation of magmatism at 160–150 Ma reflect collision of the Talkeetna arc with a block to the north. If the Bonanza arc, which intrudes the Wrangellia terrane, and Talkeetna arc, which has no known basement, were separate entities then this collision could have been between the Talkeetna arc and Wrangellia and thus occurred far from the North American margin. This scenario is implied by reconstructions of a North American passive margin and separate south-dipping subduction zone at this time (Hansen and Dusel-Bacon, 1998). Alternatively, if the Talkeetna-Bonanza arc were a coherent feature, the collision would have been between this arc in the Talkeetna area and the North American margin (Trop et al., 2002), starting ca. 160 Ma. The lack of any geochemical evolution in
the Talkeetna volcanic stratigraphy to more enriched compositions during 201–156 Ma was interpreted by Clift et al. (2005) to indicate that the Talkeetna Trench did not collide with a passive margin of North America, consistent with the north-dipping subduction polarity implied from the magmatic chemistry. Had the Talkeetna Trench collided with North America, changes in arc magmatic chemistry caused by sediment subduction should have been preserved. Such geochemical progressions are observed in other ancient arc complexes, such as the Irish Caledonides, where gradually increasing continental sedimentary involvement in petrogenesis due to trench– passive margin collision has been documented (Draut et al., 2004). As a result, we infer that the subduction zone north of the Talkeetna arc (trench) must have been north dipping in order to permit collision. Hansen and Dusel-Bacon’s (1998) model for a passive North American margin would indicate that this subduction was outboard of the continental part of the Yukon composite terrane, possibly along the Wrangellian margin or on the southern edge of the Stikinia arc system, following southdipping subduction there. Collision of the Talkeetna arc and another terrane to the north would explain the deposition of the coarse sedimentary Naknek Formation and would have increased the flux of sediment into the trench south of the Talkeetna arc, resulting in a change from tectonic erosion to subduction accretion. Tectonic uplift following arc-arc collision would have increased erosion and sediment flux to the trench compared to the restricted, purely volcaniclastic supply from largely submarine arc volcanoes predating collision. CONCLUSIONS We propose that the intraoceanic Talkeetna arc originated above a north-dipping subduction zone within the Early Jurassic Pacific Ocean (ca. 205 Ma). It may have been continuous along strike with the oceanic Bonanza arc of British Columbia, which is of similar age. The magmatic chemistry and lack of a preserved forearc in the Talkeetna arc are consistent with intraoceanic subduction and tectonic erosion until ca. 160 Ma, when the arc collided with another tectonic block north of a second north-dipping subduction zone. This block could have been Wrangellia or another active continental margin, possibly North America. The resultant collision caused deformation, uplift, and erosion of the arc, together with sedimentation of the conglomeratic Naknek Formation. Increased sediment flux to the trench caused by arc-arc collision resulted in a change from tectonic erosion to subduction accretion sometime between ca. 160 and 125 Ma, after which the Chugach terrane was ac883
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