Downloaded from geology.gsapubs.org on February 1, 2015 Escape tectonics and the extrusion of Alaska: Past, present, and future T. F. Redfi eld Geological Survey of Norway, Leiv Eirikssens vei 39, 7491 Trondheim, Norway David W. Scholl Department of Geophysics, Stanford University, Stanford, California 94035, USA, and U.S. Geological Survey, Menlo Park, California 94025, USA Paul G. Fitzgerald Department of Earth Sciences, Syracuse University, Syracuse, New York 13244, USA Myrl E. Beck, Jr. Department of Geology, Western Washington University, Bellingham, Washington 98225, USA ABSTRACT as a laterally moving crustal raft or orogenic fl oat (e.g., Oldow et al., The North Pacifi c Rim is a tectonically active plate boundary 1990; Mazzotti and Hyndman, 2002). zone, parts of which may be characterized as a laterally moving oro- We refer to this mobilization of crustal blocks as the North Pacifi c genic stream. Crustal blocks are transported along large-magnitude Rim orogenic stream (NPRS). We postulate that, since the Eocene, NPRS strike-slip faults in western Canada and central Alaska toward the crust has ascended the British Columbia margin, entered the apex of the Aleutian–Bering Sea subduction zones. Throughout much of the Alaska orocline, encountered a buttress or backstop preventing further Cenozoic, at and west of its Alaskan nexus, the North Pacifi c Rim northward displacement, and escaped southwestward toward the Aleu- orogenic Stream (NPRS) has undergone tectonic escape. During tian and Bering Sea subduction zones (Beck, 1986; Scholl and Stevenson, transport, relatively rigid blocks acquired paleomagnetic rotations 1991; Dumitru et al., 1995; Mackey et al., 1997; Fujita et al., 2002). and fault-juxtaposed boundaries while fl owing differentially through the system, from their original point of accretion and entrainment ANATOLIA: AN INSTRUCTIVE ANALOGY toward the free face defi ned by the Aleutian–Bering Sea subduction Anatolia, whose crust is extruded toward the Hellenic subduction zones. Built upon classical terrane tectonics, the NPRS model pro- zone in response to the northward impingement of the Arabian block vides a new framework with which to view the mobilistic nature of the (Nyst and Thatcher, 2004), provides a tectonic analog. The escaping western North American plate boundary zone. Anatolian crustal panel (Fig. 1A) is a structurally complex and internally deforming region, comparable to southern Alaska. The Eurasian Plate to Keywords: Alaska, Anatolia, orocline, extrusion, escape, Denali Tintina, the north acts as a rigid backstop: the dextral North Anatolian fault sys- fault, Bering Sea. tem accommodates westward escape. The north-dipping Hellenic sub- duction zone to the west and southwest makes room for extruded crust. INTRODUCTION Global positioning system (GPS) data have resolved complicated rela- For more than half a century, the geotectonic evolution of Alaska tive motions between individual Anatolian crustal domains (McClusky has been viewed from the context of its arcuate internal structure. Under et al., 2000; Nyst and Thatcher, 2004). the Alaskan Orocline paradigm of Carey (1955), counterclockwise The Anatolian model provides a useful kinematic and dynamic (CCW) vertical-axis paleomagnetic rotations have been interpreted as framework with which to analyze the tectonics of southern and south- small block rotations related in some way to tectonic reshaping of Alaska east Alaska. Structures in Canada, Alaska, and the Bering Sea region have (e.g., Coe et al., 1989). However, simple bending of Alaskan crust con- direct counterparts in Anatolia (Figs. 1A, 1B, 1C). The role of the Arabian fl icts with the lack of extensional and compressional structures outside indenter is played by PAC-NAM dextral oblique to strike-slip coupling, and inside the curvature of the arc (Schultz and Aydin, 1990). Although which drives the NPRS northward along the eastern Pacifi c Rim. consistent with orocline models, the southern Alaskan CCW rotations require another explanation (e.g., Glen, 2004). KINEMATICS AND STRUCTURE IN CENTRAL ALASKA Alaska has long been known as a tectonic collage. Packer and Fitch (1972) proposed that oblique subduction could drive continen- Stone (1972, 1974) suggested southern Alaska had moved north and tal-scale transform faulting. Plate kinematic models for at least the past rotated clockwise (CW) to achieve its present position. Packer et al. 50 m.y. predict dominantly transpressional convergence between Cas- (1975) defi ned three distinct regions, bounded by the Tintina-Rocky cadia and southeast Alaska (Tarduno et al., 2003). Subduction zone– Mountain Trench and Denali fault systems, and made up of multiple driven oblique shear along the western North American margin clearly blocks of conti nental lithosphere brought together in a progressive series drove inboard transform faulting (Beck, 1980, 1986; Gabrielse et al., of accretionary events. Later, Jones et al. (1977) identifi ed Wrangellia 2006). Piercing points along the Kaltag-Tintina–Rocky Mountain Trench as exotic to Alaska. Consolidating the terrane paradigm, Beck (1980), and Denali Fault System (DFS) require signifi cant net northward transla- Stone (1980), and Coney et al. (1980) suggested that the assembly of tion toward the interior of eastern Alaska since 55 Ma (Lanphere, 1978). much of western North American crust was driven by Pacifi c–North Approximately 430 km of dextral displacement occurred across the Tintina America (PAC-NAM) relative convergence. fault system in the Eocene (Gabrielse et al., 2006; Fig. 1D). Along the However, the conventional terrane model incompletely describes eastern segment of the DFS, the geologic record since the earliest Tertiary the mobility of the present-day margin. Western British Columbia and requires ~370 km of dextral offset (Eisbacher, 1976, Fig. 1D). The sum of southern Alaska constitute a diffuse plate boundary zone characterized these numbers (~800 km) multiplied by the width of the orogen (between by non-rigid deformation, mountain building, and block rotations (e.g., ~700 km in British Columbia and ~1200 km in Alaska south of the Kobuk Stein and Freymueller, 2002). Here, we suggest that the “Bering Block” fault) provides a sense of the length and width of the NPRS conveyor. of Mackey et al., (1997) and Fujita et al. (2002) is part of this mobi- The 2002 large-magnitude dextral rupture of the DFS (Haeussler lized zone (Fig. 1). We extend the zone eastward to include the 500- to et al., 2004) demonstrated that relative PAC-NAM plate convergence 1000-km-wide belt of diffusively deformed crustal material south of the continues to drive crust along the interior strike-slip faults of western Brooks Range and outboard of stable parts of the Canadian Cordillera. Canada and southeastern Alaska. Geologic, seismologic, and paleo- We suggest that non-rigid behavior characterizes much of this region, magnetic evidence for northward translation predicts that a signifi cant and that it has been tectonically active throughout much of the Cenozoic space problem should develop in central Alaska. In the apices of the © 2007 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, November November 2007; 2007 v. 35; no. 11; p. 1039–1042; doi: 10.1130/G23799A.1; 1 fi gure. 1039 Downloaded from geology.gsapubs.org on February 1, 2015 Figure 1. Inset maps (polar stereographic pro- jections) comparing doc- umented Anatolian extru- sion (A) with hypoth e sized escape of western Alaska (B). C: Polar stereo- graphic map showing principal physiography of North Pacific Rim region. Arrows schemati- cally show motion of North Pacifi c Rim oro- genic stream (NPRS) and Pacifi c Plate relative to fixed North America. Dashed lines and light plum color delineate the approximate boundaries of the stream. Dark plum colored overlay delin- eates the Bering Block as originally defi ned (Mackey et al., 1997). NPRS com- prises all the crustal ter- ranes of the Canadian Pacifi c, Alaska, and the Bering Sea that are undergoing lateral trans- port northward toward Alaska and west of the curving nexus of central Alaska’s “orocline,” south- westward toward the Aleutian subduction zone. D: Offset map illustrating the space problem in central Alaska associ- ated with varying offsets across the great curved fault systems of Alaska. great curved faults of the Alaskan orocline, the increasingly orthogonal Alaskan seismicity is dominated by Benioff zone events, interior Alaskan resolution of PAC-NAM relative convergence signifi cantly reduces the earthquake solutions support translation along strike-slip faults and escape tangential component driving right-lateral slip (Redfi eld and Fitzgerald, of crust to the southwest. Between the curved faults, thrust faults accom- 1993; Glen, 2004). Experiencing much greater tangential components, modate some of the relative motions of individual tracks of crustal ter- the northern end of the British Columbia sector of the orogen should ranes. The disparity of offset between east and west ends of the curved impinge upon crustal blocks already transported into the orocline. Lack- fault systems (Lanphere, 1978; Miller et al., 2002) also presents a space- ing a mechanism for tectonic escape, telescoping of crust and conse- motion problem that cannot be resolved by conventional models of terrane quent uplift, mountain building, erosion, and laterally extensive deep transport. However, velocity changes across strike-slip fault systems may exhumation