Life and death of the Resurrection plate: Evidence for its existence and subduction in the northeastern Paci®c in Paleocene±Eocene time Peter J. Haeussler² Dwight C. Bradley U.S. Geological Survey, 4200 University Drive, Anchorage, Alaska 99508, USA Ray E. Wells U.S. Geological Survey, 345 Middle®eld Drive, Menlo Park, California 95064, USA Marti L. Miller U.S. Geological Survey, 4200 University Drive, Anchorage, Alaska 99508, USA ABSTRACT zone consumption of the last of the Resur- multaneous changes in Paci®c-Farallon rection plate. and Paci®c-Kula plate motions concurrent Onshore evidence suggests that a plate is The existence and subsequent subduction with demise of the Kula-Resurrection missing from published reconstructions of the of the Resurrection plate explains (1) north- Ridge. northeastern Paci®c Ocean in Paleocene± ward terrane transport along the south- Eocene time. The Resurrection plate, eastern Alaska±British Columbia margin Keywords: tectonics, Eocene, Kula plate, named for the Resurrection Peninsula between 70 and 50 Ma, synchronous with Farallon plate, North America, magmatism. ophiolite near Seward, Alaska, was located an eastward-migrating triple junction in east of the Kula plate and north of the Far- southern Alaska; (2) rapid uplift and vo- INTRODUCTION allon plate. We interpret coeval near-trench luminous magmatism in the Coast Moun- magmatism in southern Alaska and the tains of British Columbia prior to 50 Ma Marine magnetic anomalies in the northern Cascadia margin as evidence for two slab related to subduction of buoyant, young Paci®c provide evidence for the existence of windows associated with trench-ridge- oceanic crust of the Resurrection plate; (3) trench (TRT) triple junctions, which three plates and their associated spreading cessation of Coast Mountains magmatism ridges during the early Tertiary: the Kula, Far- formed the western and southern bound- at ca. 50 Ma due to cessation of subduction, aries of the Resurrection plate. In Alaska, allon, and Paci®c plates (Atwater, 1970; Grow (4) primitive ma®c magmatism in the Coast and Atwater, 1970). However, subduction of a the Sanak-Baranof belt of near-trench in- Mountains and Cascade Range just after 50 trusions records a west-to-east migration, critical part of the anomaly record beneath Ma, related to slab-window magmatism, (5) from 61 to 50 Ma, of the northern TRT tri- North America destroyed what would be the birth of the Queen Charlotte transform ple junction along a 2100-km-long section most straightforward evidence for (1) the ge- margin at ca. 50 Ma, (6) extensional exhu- of coastline. In Oregon, Washington, and ometry of the Kula-Farallon Ridge; (2) the lo- mation of high-grade metamorphic ter- southern Vancouver Island, voluminous ba- cation of its intersection with the continental ranes and development of core complexes saltic volcanism of the Siletz River Volca- margin, and (3) the possible existence of other in British Columbia, Idaho, and Washing- nics, Crescent Formation, and Metchosin ridges and plates in the region. The onshore ton, and extensional collapse of the Cordil- Volcanics occurred between ca. 66 and 48 geologic record is the sole remaining source leran foreland fold-and-thrust belt in Al- Ma. Lack of a clear age progression of of evidence for these features. berta, Montana, and Idaho after 50 Ma magmatism along the Cascadia margin sug- Along the northeast Paci®c margin, geolo- related to initiation of the transform mar- gests that this southern triple junction did gists have long suggested that interactions not migrate signi®cantly. Synchronous gin, (7) enigmatic 53±45 Ma magmatism as- with the Kula-Farallon Ridge could explain near-trench magmatism from southeastern sociated with extension from Montana to unusual near-trench Paleocene±Eocene mag- Alaska to Puget Sound at ca. 50 Ma docu- the Yukon Territory as related to slab matism both in southern Alaska and the Wash- ments the middle Eocene subduction of a breakup and the formation of a slab win- ington and Oregon coastal rangesÐregions spreading center, the crest of which was sub- dow, (8) right-lateral margin-parallel separated by .4000 km (Figs. 1, 2). In the parallel to the margin. We interpret this ca. strike-slip faulting in southern and western Cascadia margin of coastal Oregon, Washing- 50 Ma event as recording the subduction- Alaska during Late Cretaceous and Paleo- ton, and southern Vancouver Island, geologists cene time, which cannot be explained by proposed that intersection of the Kula-Farallon ²E-mail: [email protected]. Farallon convergence vectors, and (9) si- Ridge with the continental margin (Fig. 1A) GSA Bulletin; July 2003; v. 115; no. 7; p. 867±880; 7 ®gures. For permission to copy, contact [email protected] q 2003 Geological Society of America 867 HAEUSSLER et al. Figure 1. Plate geometries proposed to explain the latest Cretaceous to early Tertiary near-trench magmatic record of western North America at Chron 25 time (56.1 Ma). The orientation and geometry of spreading ridges in gray are speculative. (A) Kula-Farallon TRT triple junction would explain near-trench magmatism along the Cascadia margin, but not in southern Alaska. (B) Kula-Farallon TRT triple junction would explain near-trench magmatism in southern Alaska, but not along the Cascadia margin. (C) Two TRT triple junctions, one in southern Alaska and another along the Cascadia margin, indicate the presence of an additional oceanic plateÐthe Resurrection plate. This is the hypothesis we prefer and explore in this paper. produced the oceanic basalt basement of the intrusions are found along the entire 2100 km within the ophiolitic sequence (U-Pb zircon Coast Ranges (Fig. 2; e.g., Simpson and Cox, length of the accretionary complex rimming age, Nelson et al., 1989) was emplaced prior 1977; Duncan, 1982; Wells et al., 1984; En- south-central and southeastern Alaska (Fig. 2; to the intrusion of a 53.4 6 0.4 Ma near- gebretson et al., 1985; Davis and Plafker, Hudson et al., 1979; Bradley et al., 1993). trench granodiorite cutting the shear zone 1986; Thorkelson and Taylor, 1989; Babcock Geochemical studies indicate that they repre- along which the ophiolite was emplaced (40Ar/ et al., 1992). Other workers, however, pro- sent anatectic melts of the host turbidites 39Ar biotite age; Kusky and Young, 1999; posed that the Kula-Farallon Ridge intersected (Hudson et al., 1979) that in some areas also Bradley et al., 2000). Thus, according to Ku- the southern Alaska margin at the same time have a MORB (mid-oceanic-ridge basalt) sky and Young (1999), less than ;3.6 m.y. (Fig. 1B), where it was responsible for granitic component (Hill et al., 1981; Barker et al., elapsed between the ophiolite's birth and its near-trench intrusions and high-temperature, 1992; Lytwyn et al., 2001). According to the incorporation into the southern Alaska accre- low-pressure metamorphism (Marshak and most reliable U-Pb and 40Ar/39Ar ages, these tionary complex. Karig, 1977; Helwig and Emmett, 1981; Sis- intrusions range in age from 61 Ma in the west son et al., 1989; Bol et al., 1992; Bradley et to 50 Ma in the east (Fig. 3; Bradley et al., The Cascadia Margin al., 1993; Sisson and Pavlis, 1993; Haeussler 1993, 2000). Areas of high-temperature and et al., 1995; Pavlis and Sisson, 1995). low-pressure metamorphic rocks (Sisson et al., Between southern Vancouver Island and The same trench-ridge-trench (TRT) triple 1989; Sisson and Pavlis, 1993; Loney and southern Oregon, the basement rocks of the junction cannot explain the simultaneous near- Brew, 1987; Zumsteg et al., 2000) are coeval Cascadia forearc consist of ocean-crust±type trench magmatism of both areas. In this paper, with the ages of nearby near-trench intrusions. basalt erupted in Paleocene and Eocene time. we summarize the geologic evidence from Mesothermal lode-gold deposits in the accre- This basaltic basement, commonly known as coastal southern Alaska and the Paci®c North- tionary complex also record an unusual ther- the Siletz terrane or Siletzia, consists of the west for the presence of two coeval TRT triple mal event. Isotopic dates of these mineral oc- Metchosin Volcanics of southern Vancouver junctions. This circumstance implies the ex- currences have a west-to-east trend of Island, the Crescent Formation of western istence of an additional oceanic plate in the decreasing age and are coeval with nearby Washington, and the Siletz River Volcanics of Paci®c Ocean basin in Paleocene±Eocene time near-trench intrusions (Haeussler et al., 1995). western Oregon (e.g., Tabor and Cady, 1978; (Fig. 1C). Ophiolitic sequences are also present in the Snavely et al., 1968). The volcanic sequence accretionary complex. The most complete of is voluminous; a 16 km thickness is exposed RECORD OF NEAR-TRENCH these is on Resurrection Peninsula, near Sew- in the eastern Olympic Peninsula (Babcock et MAGMATISM ard, Alaska (Fig. 2). The pillow-lava section al., 1992), and seismic studies show that ma®c of this ophiolite (and some of the other crust is 20±35 km thick beneath coastal Wash- The Southern Alaska Margin ophiolitic sequences) has interbedded turbidite ington and Oregon (Trehu et al., 1994; Par- beds, which indicates that it formed in prox- sons et al., 1999). Near-trench (i.e., with respect to the present imity to a continental margin (Helwig and On the Olympic Peninsula, abundant trench and modern and coeval arcs) granitic Emmet, 1981). A 57 6 1 Ma plagiogranite pillow basalt consists of normal to en- 868 Geological Society of America Bulletin, July 2003 LIFE AND DEATH OF THE RESURRECTION PLATE riched mid-oceanic-ridge basalt (MORB) and oceanic-island tholeiite (OIB); it is locally overlain by subaerial tholeiitic and alkalic ba- salt ¯ows (Snavely et al., 1968; Massey, 1986; Babcock et al., 1992).