Tectonic processes during oblique collision: Insights from the St. Elias orogen, northern North American Cordillera

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1 TECTONICS, VOL. 23,, doi: /2003tc001557, 2004 Tectonic processes during oblique collision: Insights from the St. Elias orogen, northern North American Cordillera Terry L. Pavlis, Carlos Picornell, 1 and Laura Serpa Department of Geology and Geophysics, University of New Orleans, New Orleans, Louisiana, USA Ronald L. Bruhn Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah, USA George Plafker U.S. Geological Survey, Menlo Park, California, USA Received 30 June 2003; revised 24 December 2003; accepted 30 January 2004; published 4 May [1] Oblique convergence in the St. Elias orogen of southern Alaska and northwestern Canada has constructed the world s highest coastal mountain range and is the principal driver constructing all of the high topography in northern North America. The orogen originated when the Yakutat terrane was excised from the Cordilleran margin and was transported along margin-parallel strike-slip faults into the subduction-transform transition at the eastern end of the Aleutian trench. We examine the last 3 m.y. of this collision through an analysis of Euler poles for motion of the Yakutat microplate with respect to North America and the Pacific. This analysis indicates a Yakutat-Pacific pole near the present southern triple junction of the microplate and predicts convergence to dextral-oblique convergence across the offshore Transition fault, onland structures adjacent to the Yakutat foreland, or both, with plate speeds increasing from 10 to 30 mm/yr from southeast to northwest. Reconstructions based on these poles show that NNW transport of the collided block into the NE trending subduction zone forced contraction of EW line elements as the collided block was driven into the subduction-transform transition. This suggests the collided block was constricted as it was driven into the transition. Constriction provides an explanation for observed vertical axis refolding of both earlier formed fold-thrust systems and the collisional suture at the top of the fold-thrust stack. We also suggest that this motion was partially accommodated by lateral extrusion of the western portion of the orogen toward the Aleutian trench. Important questions remain regarding which structures accommodated parts of this motion. The Transition fault may have accommodated much of 1 Now at ExxonMobil Production Company, Houston, Texas, USA. Copyright 2004 by the American Geophysical Union /04/2003TC the Yakutat-Pacific convergence on the basis of our analysis and previous interpretations of GPS-based geodetic data. Nonetheless, it is locally overlapped by up to 800 m of undeformed sediment, yet elsewhere shows evidence of young deformation. This contradiction could be produced if the overlapping sediments are too young to have accumulated significant deformation, or GPS motions may be deflected by transient strains or strains from poorly understood fault interactions. In either case, more data are needed to resolve the paradox. INDEX TERMS: 8102 Tectonophysics: Continental contractional orogenic belts; 8107 Tectonophysics: Continental neotectonics; 8150 Tectonophysics: Plate boundary general (3040); 8155 Tectonophysics: Plate motions general; KEYWORDS: oblique collision, transpression, microplate motion, polyphase folding, escape tectonics, erosiontectonic interactions. Citation: Pavlis, T. L., C. Picornell, L. Serpa, R. L. Bruhn, and G. Plafker (2004), Tectonic processes during oblique collision: Insights from the St. Elias orogen, northern North American Cordillera, Tectonics, 23,, doi: /2003tc Introduction [2] The St. Elias orogen of southern Alaska and northwestern Canada (Figure 1) is an active, transpressional orogen driven by oblique collision of the Yakutat microplate and North America [Plafker et al., 1994; Bruhn et al., 2004]. Both geologic [e.g., Plafker et al., 1978, 1994] and geodetic data [Sauber et al., 1993, 1997; Fletcher, 2002; Fletcher and Freymueller, 1999, 2003] indicate that mm/yr of oblique convergence is absorbed within the orogen through a combination of dextral strike-slip and thrust faulting [Bruhn et al., 2004]. These rates are among the highest continental convergence rate operating today. This rapid convergence has constructed an orogen that is approximately the size of the state of Nevada, contains most of North America s >4500 m peaks, and is the highest coastal mountain range on earth. [3] The St. Elias orogen is also the most heavily glaciated active orogenic belt. Glacial troughs up to 150 km long and 1of14

2 Figure 1. Regional tectonic map of southern Alaska showing the Yakutat terrane and major structures related to the collision of the Yakutat terrane with North America. 15 km wide feed large piedmont glaciers that spill onto a coastal plain and continental shelf (Figure 2) constructed largely by glacial marine sedimentation that built a continental terrace from the erosional products of the orogen. Recent studies of sedimentation rates offshore [e.g., Hallet et al., 1996; Jaeger et al., 1998; Sheaf et al., 2003] indicate that these glacial systems are eroding the orogen at some of the highest, if not the highest, rates on earth. In light of increasing evidence that feedbacks between tectonics and erosion may contribute directly to strain localizations in the crust [e.g., Pinter and Brandon, 1997; Pavlis et al., 1997; Koons et al., 2002] the St. Elias orogen is a superb natural laboratory to investigate this process. [4] Despite these superlatives, surprisingly little is known about the tectonic processes that generated this orogen. Major advances have been made during the past two decades in understanding the basic tectonic framework [e.g., Plafker et al., 1994; Bird, 1996; Doser and Lomas, 2000; Doser et al., 1997; Sauber et al., 1993, 1997; Fletcher and Freymueller, 1999, 2003], but by continental U.S. standards this region remains poorly understood. Little is known about the architecture of the deformed lithosphere, and major questions remain 2of14

3 Figure 2. Tectonic map of the Gulf of Alaska region, modified from Plafker [1987], showing contemporary motions determined from GPS and model motions for Yakutat-Pacific motion using the pole determined in this study. See color version of this figure at back of this issue. concerning which structures are absorbing the bulk of the deformation. [5] In this paper we examine some of these questions through an assessment of the plate tectonic evolution of the orogen. We begin with a review of the major tectonic features of the orogen and the Cenozoic history that led to the present actively deforming system. We then analyze the present-day motion of the colliding block by determining new Euler poles for relative motion of the Yakutat microplate with respect to North America and with respect to the Pacific. We use this conclusion to develop first-order map-scale reconstructions for the orogen. This analysis points to an important paradox of the role of the offshore Transition fault in the active tectonics, and this problem is discussed in the context of the regional evolution. Finally, we present a preliminary scenario for the tectonic evolution of the orogen emphasizing the role of constriction during oblique convergence 3of14

4 into the subduction-transform transition and the potential role of lateral escape in this process. 2. Tectonics of the St. Elias Orogen 2.1. Active Tectonics [6] The St. Elias orogen was produced by interaction between the Yakutat microplate and the North American plate (Figure 1). We use the term Yakutat microplate as broadly synonymous with the term Yakutat Block used by many previous studies. We distinguish the Yakutat microplate from the Yakutat terrane [e.g., Plafker et al., 1994] with the former defined by limits of active structures that bound the microplate and the latter defined on terrane maps by the distinctive Cenozoic cover sequence. The microplate is broadly equivalent to the Yakutat terrane, but as defined here, we include elements of the Prince William and Chugach terrane in the microplate because these assemblages are caught up in the active deformation between the Yakutat terrane suture and Fairweather-Bagley strike-slip system. The Yakutat microplate originated in mid-cenozoic time when the Yakutat terrane was excised from western Canada [Plafker et al., 1994] or the Pacific northwest [Bruns, 1983] and transported northward along the Queen Charlotte transform system (Figure 1). As this interaction began, the Wrangell magmatic arc (Figure 1) developed from eastern Alaska to northern British Columbia, but the southern portion of this arc shut down from south to north as the Yakutat terrane was transported northward [Plafker et al., 1994]. Presently, the active arc is restricted to an 250 km long segment of the Wrangell Mountains marked by large, active stratovolcanos located above a short subducting slab segment (Figure 1) that is separate from the main Aleutian magmatic arc [e.g., Page et al., 1989]. [7] Between 5 and 10 Ma, the leading edge of the Yakutat terrane encountered the Aleutian trench, and in the last 5 m.y. the Yakutat terrane has been jammed into the subduction corner, constructing the present high topography [e.g., Plafker et al., 1994; Jaeger et al., 2001]. The effects of this interaction are not limited to the St Elias Mountains, however, and it is likely that Yakutat-North American interaction is ultimately responsible for most of the active deformation in southern Alaska including the ongoing uplift of the Alaska Range along the Denali fault (Figure 1) as well as the coastal ranges to the west of St. Elias Mountains [Lahr and Plafker, 1980]. [8] The Yakutat microplate is triangular shaped in map view with an undeformed interior and with broad zones of deformation on all three sides (Figures 1 and 2). To the east, earthquakes during the last century as well as surface geology indicate that highly oblique convergence is slip partitioned from the southern edge of the orogen to at least Yakutat Bay [Doser and Lomas, 2000; Bruhn et al., 2004]. In this interpretation dextral strike-slip is accommodated along the Fairweather fault and at least part of the convergence taken up along thrust systems beneath the Yakutat foothills (Figure 2) forming a transpressional flower structure [Bruhn et al., 2004]. [9] To the north of Yakutat Bay (Figure 2) structural trends turn sharply to approximately EW and deformation is dispersed through a broad fold and thrust belt. This fold and thrust belt is morphologically unusual, however, in that the active deformation front of the fold and thrust belt is not a topographic slope break. Instead, many structures cut across the topographic grain, particularly in the offshore segments of the orogen [e.g., Bruns and Schwab, 1983] where the Pamplona and Kayak Island zones (Figure 2) link up with the Aleutian trench [Bird, 1996]. Other important complications within this segment of the orogen include the following. [10] 1. A prominent curvilinear valley continues north and west from the northernmost known trace of the Fairweather fault into the interior of the orogen. This deep valley is occupied by the upper Seward Glacier and the Bagley Icefield and parallels a major fault that separates the Chugach and Prince Williams terranes (Figure 2). Locally, this fault trace is called the Bagley fault [Plafker, 1987], but elsewhere in southern Alaska this lithologic boundary is mapped as the Contact fault which represents a major thrust along most of its trace [Plafker et al., 1994]. Available evidence suggests, however, that the Bagley fault is a strike-slip fault representing an active, westward extension of the Fairweather system. This evidence includes geodetic data [Savage and Lisowski, 1986; Sauber et al., 1997] showing an oblique displacement field and displacement gradients suggestive of dextral shear across the structure and minor dextral strike-slip faults recognized in rocks adjacent to the valley [Bruhn et al., 2004]. More data are needed to analyze this conclusion. Nonetheless, this relationship is important because it implies slip partitioning continues well into the core of the orogen [Bruhn et al., 2004], and the trace of the fault can be used to constrain the motion of the microplate. [11] 2. Complex re-folding of older fold-thrust belt structures is observed in the extreme western edge of the orogen between the Copper River and Bering Glacier [Bruhn et al., 2004]. Bruhn et al. [2004] interpreted these structures as young structures accommodating part, or all, of the obliqueslip motion transferred to the western part of the orogen along the Fairweather-Bagley transform system. [12] The manner in which these structures link up to convergence in the Aleutian trench is poorly resolved, but presumably is connected in part through the Kayak Island zone (Figures 1 and 2). The Kayak Island zone is recognized as a zone of active contraction [e.g., Plafker, 1987] but is not well imaged in seismic data and may include components of oblique slip. [13] The trailing edge of the Yakutat microplate is marked by the Transition fault, which is the most enigmatic structure of the entire orogen (Figures 1 and 2). This fault parallels the base of the continental slope and forms the plate boundary between the Yakutat microplate and the Pacific plate. The structure has been variably described as a rejuvenated left-lateral fault with only minor Plio-Pleistocene motion [Bruns, 1983; Bruns and Carlson, 1987], a dextral-oblique fault [Lahr and Plafker, 1980; Plafker, 1987], and a low-angle thrust fault [Bayer et al., 1978; 4of14

5 Figure 3. Part of USGS seismic line 915, and 1:1 cross sections derived from interpretation of line 915 and nearby line (909). Note the overlap of the Transition fault by the thick package of undeformed sediments in both lines. See Figure 2 for location of the cross sections. Griscom and Sauer, 1990; Perez and Jacob, 1980; Plafker et al., 1994; Fletcher and Freymueller, 1999]. [14] The inference that the Transition fault is no longer active is supported by seismic reflection data (Figure 3) that show undisturbed sediments overlapping the trace of the fault. In contradiction to this observation, Fletcher and Freymueller [1999] concluded that 20 mm/yr of convergence must be occurring across the Transition fault based on inferences from geodetic observations on shore. We address this problem below through an analysis of the regional plate motions. [15] The nature of the Transition fault is also important for the driving process in the orogenic belt. If the Transition fault is inactive, the Yakutat microplate is essentially a part of the Pacific plate and orogenesis is Pacific plate driven. If it is active, however, the orogenesis may be driven largely, if not entirely, by the small slab-remnant beneath the Wrangell Mountains (Figure 1) Pre-Quaternary Geology and Tectonics [16] The collided block comprises the Yakutat terrane of Jones and Silberling [1979]. The Yakutat terrane (Figure 2) is characterized by a distinctive Eocene to Quaternary sedimentary cover that is absent from adjacent terranes that formed the trailing edge of North America [Plafker et al., 1994]. These adjacent terranes include the Chugach and Prince William terranes which represent a subduction complex accreted to the western margin of North America from early Mesozoic to early Eocene time [Plafker et al., 1994]. This subduction complex was essentially welded into a coherent slab of continental crust during Eocene time when the Kula-Farallon ridge was subducted beneath the margin and heated these forearc assemblages to low-p, high-t metamorphic conditions that locally reached upper amphibolite facies conditions [e.g., Hudson et al., 1977a, 1977b; Hudson and Plafker, 1982; Sisson et al., 1989; Pavlis and Sisson, 1995, 2003]. These high-t metamorphic assemblages are now exposed throughout the core of the orogen from the Copper River in the west to well south of Cross Sound (Figure 2) and are referred to regionally as the Chugach metamorphic complex [Hudson and Plafker, 1982; Dusel- Bacon, 1994; Pavlis and Sisson, 1995]. This assemblage is important for tectonic models of the orogen because this continentalized forearc assemblage is the backstop against which the Yakutat terrane was accreted. Moreover, part of the basement of the Yakutat terrane was also affected by this metamorphic event [e.g., Plafker, 1987]. [17] This metamorphic backstop and the distinctive cover sequence of the Yakutat terrane allow a clear definition of the suture throughout most of the orogen [Plafker, 1987]. To the west of Icy Bay (Figure 2) the suture is mapped as a single fault zone, the Chugach-St. Elias fault. This fault system has an unusual geometry on a regional scale, however, in that it trends EW from Icy Bay to just east of the Copper River where the fault trace makes a >90 bend and can be traced southward to Wingham Island where it disappears offshore. This bend in the trace of the suture is coincident with the location of refolded fold-thrust systems in Tertiary rocks west of the Bering Glacier and is interpreted by Bruhn et al. [2004] as a regional, steeply plunging fold in the suture. This interpretation is important to our regional reconstructions (see below) that use this fold as a marker. [18] Offshore the cover sequence of the Yakutat terrane thins dramatically from west to east across the NS trending Dangerous River zone (Figures 1 and 2), a basement fault system that separates oceanic basement on the west from transitional continental basement (Chugach terrane) on the east [Plafker, 1987]. The Dangerous River zone comes 5of14

6 onshore just east of Icy Bay (Figure 2) in the Samovar Hills [Plafker, 1987] and is coincident with a major change in structural style along the strike of the orogen. To the west, where sedimentary cover is thick to the south of the suture, thin-skinned thrusting dominates. To the east and south, however, basement-involved thrusting accommodates the contraction (Figure 2). In the basement-involved thrust systems Tertiary cover is either absent or restricted to localized fault slices along thrusts, which handicaps unequivocal definition of the suture. Here we infer that the suture is coincident with the St. Elias fault (Figure 2) which can be traced from the Chugach-St. Elias fault eastward across the south face of Mount St. Elias to Mount Augusta where the trace of the north dipping fault is deflected northward by intersection with steep terrain and disappears beneath the Seward Glacier [Campbell and Dodds, 1982; T. Pavlis, V. Sisson, and K. Stuwe, unpublished field observations, 1998]. This definition of the suture is supported by (1) abrupt juxtaposition of upper amphibolite facies rocks atop lower greenschist facies assemblages along the St. Elias fault [Dusel-Bason, 1994; Sisson et al., 2003; T. Pavlis, V. Sisson, and K. Stuwe, unpublished field observations, 1998]; and (2) scattered occurrences of Tertiary sedimentary assemblages in the footwall of the St. Elias fault which indicates rocks structurally beneath the St. Elias fault are part of the Yakutat terrane. This definition of the suture is important because it implies the suture is truncated to the east by the dextral Fairweather transform fault system. That is, to the east of the Seward Glacier, the Fairweather fault is the also the suture, which has important implications for reconstruction of the orogen (see below). 3. Present-Day Motion of the Yakutat Microplate [19] Although the Yakutat microplate is relatively small, its motion with respect to the North American and Pacific plates cannot be treated as a linear velocity problem in Cartesian coordinates. The trace of the Fairweather and Bagley-Seward Glacier fault system is curved and concave toward the southwest in map view (Figure 4). This curvature is in the opposite sense of the trace of the North American- Pacific transform (Figure 4) to the south of the Saint Elias orogen (Queen Charlottes system) and implies that an Euler pole for Yakutat-North American relative motion is located offshore in the Gulf of Alaska. This conclusion is further supported by GPS data [Sauber et al., 1997; Fletcher and Freymueller, 1999] that show consistent counterclockwise rotations of velocities for Yakutat microplate with respect to North American when compared to predicted Pacific-North American velocity (Figure 2). [20] We used the trace of the Fairweather transform system together with the azimuths of published GPS vectors to estimate the position of the Euler pole for motion of the Yakutat microplate (Y) with respect to North America (NA). We determined transform fault azimuths at 0.5 degree intervals along the trace of the Fairweather fault (17 points) and assigned equal weight to both fault azimuth and azimuth of GPS motion vectors for sites on the Yakutat microplate (6 points). We then calculated theoretical linear velocities for the entire data set using trial Euler poles picked on a half degree spaced grid, and contoured the error (S(az observed az theoretical ) 2 ) to determine the Euler pole that minimized the error. This produces the well-known elliptical error envelop [e.g., see Cox and Hart, 1986] that is typical of poles determined from azimuthal data only (Figure 4) with a best fit pole at 53.5N, 150W. To determine an angular velocity rate we used only the GPS-based linear velocity at Yakutat [from Fletcher and Freymueller, 1999] because it is the farthest from known active faults and lies within the undeformed interior of the Yakutat microplate. This assumption is subject to error because of poorly resolved effects of elastic loading from several nearby structures as well as viscoelastic effects from glacial rebound. Nonetheless, the GPS velocity is within 10 mm/yr of geologically estimated slip rates on the Fairweather fault [Plafker et al., 1978] and the effects on the calculated poles (see below) from assuming the GPS velocity versus the higher geologic rates are not large. [21] To determine the motion of the Yakutat microplate with respect to the Pacific plate (P) we summed rotation about the Y-NA pole with the NA-P pole of Argus and Gordon [2001] using the vector sum: Pw Y ¼ P w NA þ NA w Y : [22] The result (Figure 4) defines a Y-P Euler pole that lies close to the Transition fault near the southern Y-P-NA triple junction (57 N, W, 3.2 /m.y.). This location is robust because the Y-NA pole is close to the Yakutat microplate. Specifically, because of its close proximity, the Y-NA pole has an angular velocity that is nearly ten times greater than the magnitude of the NA-P pole. Thus, the Y-NA pole always dominates in the vector sum that determines the Y-P pole, and small errors in the position of the Y-NA pole produce even smaller changes in the calculated positions for Y-P poles. For example, Y-P poles derived from the extremes of the Y-NA error ellipse do not significantly change the position of the inferred Y-P pole (pts A and B, Figure 4). [23] Similarly, the angular velocity of the Y-P pole is also relatively high because of the high Y-NA velocity it is derived from. Because of these high angular velocities and the proximity of the pole, the calculated linear velocities along the Transition fault increase from 10 mm/yr in the southeast to >30 mm/yr at the northwest end of the fault with nearly pure convergence predicted along the entire fault (Figure 2). Thus this analysis is consistent with the conclusion of Plafker et al. [1994] and Fletcher and Freymueller [1999] that the Transition fault is an active thrust system. [24] To further evaluate the effects of errors in the location of the Y-P pole we considered two alternative microplate models (Figure 4): (1) a model including complications from the motion of a southern-alaska microplate defined by the region between the Denali fault system, Aleutian trench, and Yakutat microplate; and (2) a model that ignores the GPS data and assumes the Fairweather 6of14

7 Figure 4. Tectonic map of the north Pacific region showing the position of the Yakutat terrane, the Fairweather fault system used to derive Euler poles for Yakutat-North American motion, and the position of Euler poles determined in this study. Contour lines are lines of equal error and show the elliptical error envelop of these data. strike-slip system surrounds the Yakutat microplate and connects to the Aleutian trench through the Kayak Island zone (Model 2, Figures 4 and 5). We consider these end members in a family of motion models that are allowable from existing data. [25] For the southern Alaska microplate model, we used the pole determined by Stout and Chase [1980] for motion of south central Alaska relative to North American and linear rates from Lahr and Plafker [1980] and summed that pole with our Y-P pole. The results of these calculations are virtually indistinguishable from Model 1 (Figure 4) because the NA-S. Alaska microplate pole is very close to the Y-S. Alaska pole. Thus, aside from a difference in magnitude the sum of the poles is not significantly different from a Y-NA pole that ignores the existence of a southern Alaska microplate. We also analyzed a slightly different pole from Fletcher [2002] that used the fault segment ruptured in the 2002 Denali earthquake (not shown), and that result was similar to that shown in Figure 4 for the same reasons of proximity and relative velocities. [26] The second model used the same fault azimuth data as Model 1 but included two measurements along the structure that follows the Bering Glacier and assumes that structure is a strike-slip fault. This model predicts a Y-P pole 7of14

8 Figure 5. Evaluation of the microplate motion model. (a) Histograms of angular dispersion of observed versus theoretical azimuths of Y-NA motion from the two main models discussed in this paper. Note that the outliers in both models are either fault traces that are not necessarily pure strike-slip or GPS data. (b) Flow lines for Yakutat-North American motion derived from Model 1 (left, Fairweather-Bagley fault trend with GPS data) and Model 2 (right, using fault trends only from the Fairweather, Bagley, and Bering Glacier systems). at the extreme end of a linear (great circle) array of Y-P poles defined by the error ellipse in the original model, which is consistent with the Y-NA array from which they are calculated. The effect of these alternatives is seen primarily in the azimuths, and azimuthal variation, of predicted linear velocities along the Transition fault (Figure 2); i.e., the model velocities range from oblique (dextral thrust) motion to nearly pure convergence. [27] Analysis of the dispersion of the data relative to the models (Figure 5a) is informative when placed in a 8of14

9 spatial context. The dispersion is significantly greater in Model 1 than Model 2 (Figure 5a) but the outliers in Model 1 are fault azimuth data from the Bagley fault (25 counterclockwise from the model) and the GPS data (5 29 clockwise). In Model 1 these discrepancies cancel out in the calculation of a pole and calculated flow lines (Figure 5b) closely match the trace of the Fairweather fault up to the Seward Glacier where the fault trace diverges from the model flow lines, which is equivalent to the counterclockwise dispersion shown in the histograms (Figure 5a). In Model 2, dispersion is less than in Model 1 and would have been even less had we eliminated the two data points from the Bering Glacier segment of the inferred fault (Figure 5a). Nonetheless, Model 2 slightly overcompensates to fit fault trace data in the core of the orogen, and the flow line track diverges from the Fairweather fault trace before rejoining it along the Bagley fault (Figure 5b). [28] From this analysis we conclude that neither model is likely to be exactly correct but the models constitute reasonable end members. One possibility is that our assumption of significant strike-slip on the Bagley fault is in error, and all of the Fairweather motion is transferred into the thrust belt. We believe this is unlikely, however, based on both the geologic evidence described above, and the peculiarities of slip partitioning. In Model 1 the largest dispersion aside from the trace of the Bagley fault (Figure 5) are the GPS data from within the fold and thrust belt of the collision zone (Figure 2) and the discrepancy in the GPS data from Yakutat is small (5 ). Had we plotted GPS data in Figure 5a, the discrepancy would have been even larger in Model 2, but rotated in the same sense. In light of our field studies that suggest significant slip partitioning in the orogen [Bruhn et al., 2004] these discrepancies cannot be evaluated without consideration of the effects of slip partitioning. McCaffrey [1992] showed that in slip-partitioned oblique convergence, the displacement vector in the hanging wall of the frontal thrust system is rotated between the convergence vector and orthogonal convergence with the amount of rotation depending on coupling. The more northerly (clockwise) azimuths of the GPS displacements relative to the fault azimuths or model azimuths (either model) match this hypothesis closely. Note, however, that in reference to Pacific-North American plate motion (flow line in Figure 5b and linear velocity shown in Figure 2) the GPS displacements are more westerly (counterclockwise) than Pacific flow lines, which is inconsistent with predictions from McCaffrey s [1992] slip-partitioning models. This basic observation is critical because it implies that it is unlikely that the Yakutat microplate is moving with the Pacific plate, at least at supracrustal levels where faults and GPS displacements provide constraints on the motion. Thus we conclude that slip partitioning probably accounts for the data dispersion shown in Figure 5a, and the models represent a reasonable approximation of the present motion of the Yakutat microplate. The relative merits of Model 1 versus Model 2 remain unclear in the absence of more data. Nonetheless, the distinctions are sufficiently small that they do not significantly influence the main results of this paper on the reconstruction of Plio-Pleistocene interactions in the orogen. 4. Large-Scale Kinematics of Plio-Pleistocene Deformation in the Orogen [29] The inferred Euler pole positions for motion of the Yakutat microplate with respect to the North American and Pacific plates allow a preliminary assessment of the longterm affects of this interaction. Thus we extrapolated the present day rates back in time to 0.5 Ma and 3 Ma to evaluate the kinematic affects of the microplate interaction (Figure 6). [30] In these reconstructions we used a simple cut and paste technique to restore the map-view positions of the Pacific plate and the undeformed part of the Yakutat terrane. Given uncertainties of the nature of the Transition fault we assumed the Transition fault has taken up only half of the Y-P motion, and the remainder of the motion is taken up along thrusts beneath the Yakutat foothills (Figure 2); a reasonable assumption given that part of this thrust system moved during the 1899 earthquake sequence [e.g., Tarr and Martin, 1912; Plafker and Thatcher, 1982; Nishenko and Jacob, 1990]. We then split the deformed portions of the Yakutat terrane into four pieces (Figure 6): (1) the Yakutat foothills block; (2) the Mount Cook block; (3) the onland portion of the fold and thrust belt between the Malaspina and Bering Glaciers; and (4) the offshore portion of the fold and thrust belt. The boundaries between these blocks are based on surface geology [Bruhn et al., 2004] except the boundary between the onshore and offshore portions of the fold and thrust belt which is arbitrary. The latter was made to illustrate an important kinematic consequence of the microplate interaction. [31] The positions of the Yakutat foothills block and Mount Cook block were restored by assuming that these blocks moved northward by pure strike-slip along the Fairweather fault at present rates of mm/yr. For the two fold and thrust belt blocks (onshore and offshore), however, the restored locations are somewhat arbitrary because gaps in the reconstruction represent shortening accommodated on several structures within the belt. Nonetheless, this reconstruction illustrates the general features of oblique convergence, particularly the large-scale effects on the Yakutat microplate as it was driven into the subduction-transform corner. [32] For the first phase of the reconstruction to 0.5 Mawe assumed that most of the westward component of motion of the Yakutat microplate was absorbed by second phase folding in the western part of the orogen, by oblique slip across a cryptic structure along the Bering Glacier, or both. Thus we restored the northern half of the thrust belt by strike-slip along the Bagley fault, opening a gap along what is now the Bering Glacier. Similarly, strike-slip restoration of the Yakutat foothills and Mount Cook blocks opens a significant gap along the foothills belt, which increases dramatically toward the Mount Cook block because of restoration around a sharp restraining bend in the Fairweather strike-slip system. Note that the inferred increase in contraction at this restraining 9of14

10 Figure 6. Map view reconstruction for the Yakutat terrane at 0.5 Ma (left) and 3 Ma (right) using constant motion model from Euler poles determined in this study. See text for discussion. bend is consistent with the observed increase in topographic relief north of Yakutat Bay. Finally, following Picornell [2001] and Picornell et al. s [2001] conclusions that the Pamplona zone absorbs only a small fraction of convergence we assume that the Pamplona zone is primarily the product of counterclockwise oroclinal bending in the fold-thrust belt and most of the late Pleistocene convergence is taken up on the Kayak Island zone and the interior of the orogen. An important consequence of this reconstruction is that a gap appears between the two arbitrarily defined pieces of the fold and thrust belt. This result is robust even without our assumptions for the Pamplona zone because this is a firstorder geometrical feature that forces east-west trending lines to be shortened as the Yakutat microplate is carried westward into the bend in the North American plate margin. Specifically, the microplate is being driven northwestward into a corner between northeast trending structures of the Aleutian megathrust, and northwest trending thrusts of the Yakutat foothills system; thus EW line elements carried in to this corner must be shortened. [33] This corner effect is dramatically illustrated in the 3 Ma reconstruction (Figure 6). Here we used the same crustal blocks and motions as in the 0.5 Ma restoration but added an additional restoration component by unfolding the suture at the western end of the orogen. The suture was unfolded by line-length restoration about vertical axis folds which restores the suture to an originally straight configuration. Although there is no direct evidence that the suture was originally straight, the restored geometry is consistent with the transferal of strike-slip motion along the Bagley- Seward Glacier fault into steeply plunging fold systems that warp the suture. Moreover, this restoration is consistent with the observed cutoff of the suture by the Fairweather fault, which implies the suture has been displaced westward by strike-slip. As in the 0.5 Ma reconstruction, gaps in the reconstruction indicate significant convergence by contraction within the fold-thrust belt. More importantly, however, the corner effect is exemplified here where large gaps appear not only in the direction of convergence, but also between and along the edges of the two fold and thrust belt blocks. Based on this geometry we infer that the fold and thrust belt developed not only by NS directed thrusting, but also experienced a component of EW directed shortening strain. This EW shortening presumably was partially absorbed by the second phase folding in the western part of the orogen, but the reconstruction illustrates that indentation of the suture, and second phase folding alone are not sufficient to absorb all of this convergence. We suggest that lateral, westward extrusion of the fold and thrust belt and development of NE trending structures of the Pamplona zone provides the best explanation for the accommodation of this secondary component of the deformation. 5. Discussion 5.1. Nature of the Transition Fault and Implications for Erosional Denudation [34] The results of our plate motion analysis predict that the Yakutat-Pacific pole lies just southeast of the southern triple junction of the P-NA-Y system for all of the models we considered. These poles produce two predictions: (1) convergence to dextral oblique convergence across the Transition fault, the Yakutat foothills fault system, or both; and (2) increase in relative plate speed from southeast to northwest. 10 of 14

11 [35] In light of conclusions based on GPS data [Fletcher and Freymueller, 1999] these predictions seemingly contradict the observation that the Transition fault is overlapped by undeformed sediments (Figure 3). We suggest that two, as yet unresolvable alternatives, could resolve this paradox: (1) the GPS data are recording a short-term displacement field that does not reflect the long-term motion; or (2) the sediments that overlap the Transition fault are too young to have accumulated sufficient strain to produce structures detectable by seismic images Transient Displacement Hypothesis [36] The Fairweather fault lies in a deep topographic trough within the interior of the orogen and is located landward of a range of foothills (Figure 2) that are bounded on their seaward side by a linear, 1500 m high escarpment. This morphology together with evidence from great earthquakes in the area implies a positive flower-structure produced by concurrent strike-slip on the Fairweather fault and west directed thrusting beneath the Yakutat foothills [Bruhn et al., 2004]. Thus surface morphology is consistent with the hypothesis that the thrusting along the Yakutat foothills could accommodate most, if not all, of the convergent component. Nonetheless, the GPS data from Yakutat [Fletcher and Freymueller, 1999, 2003] show present day motions parallel to the Fairweather fault, which implies the convergent component is transferred offshore to the Transition fault. [37] Discrepancies between short-term strain accumulations measured by GPS and long-term strain accumulation estimated by geology are common and ultimately are critical for evaluating seismic strain accumulation. We suggest that one explanation for the GPS data in this area is that the present velocity measured by GPS is disturbed by transient strain accumulations from nearby, or underlying, faults that are not taken into account by models presented by Fletcher and Freymueller [1999]. Alternatively, the earthquake sequence in the Gulf of Alaska was a series of strikeslip earthquakes outboard of the Transition fault on the Pacific plate [Lahr et al., 1988] and it possible that deformation of the Pacific plate is as important as the Transition fault. Finally, this area was affected in 1899 by an earthquake sequence that produced the largest coseismic uplift ever recorded [e.g., Plafker and Thatcher, 1982], and effects of this earthquake may still be disturbing the local strain field. For example, post-seismic relaxations are still affecting the area of the 1964 Great Alaskan and 1960 Chilean earthquakes [Freymueller et al., 2000; Klotz et al., 2001]. In any case, the basic geologic framework and the evidence from the 1899 Yakutat Bay earthquake(s) clearly indicate that a significant component of Yakutat-NA convergence is occurring beneath the Yakutat foothills, and longer GPS time series or more geodetic sites are needed to clarify the meaning of the available GPS data in the context of the motion on the Transition fault Young Sediment Hypothesis [38] The seismic line (USGS 915) shown in Figure 3 crosses the Transition fault at the base of the Yakutat submarine canyon and includes the head of a large submarine fan complex developed at the base of the slope [Bruns and Carlson, 1987]. This section shows 800 m of virtually undeformed sediment overlapping the Transition fault. Other seismic lines that cross the Transition fault show variable affects of syndepositional deformation, consistent with continuation of significant motion on the Transition fault, but their significance depends on the age of the overlapping sediments. [39] It is unlikely that the deposits are <12 Ka (Holocene) because during the Holocene virtually all sediment was trapped on the shelf or carried westward by longshore currents [Jaeger et al., 1998]. Thus, although Holocene high sedimentation rates are recognized on the shelf (e.g., 6 10 mm/yr according to Jaeger et al. [1998] and Sheaf et al. [2003]), little of this sediment reaches the deep sea fan imaged in Figure 3. If we assume these overlapping sediments date from the last major glacial maximum (15 Ka to 30 Ka [Ruddiman, 2001]), however, the time period is short enough that a relatively modest amount of plate convergence (600 m at 20 mm/yr) would have occurred and large, finite structures may not yet have developed in these unconsolidated sediments. Older ages are probably not allowable if the Transition fault is active. For example, if these sediments were deposited during the entire last glacial (12 Ka to 120 Ka [Ruddiman, 2001]) as much as 2.4 km of convergence is predicted at 20 mm/yr; a sufficient magnitude that major structures should be recognizable. Deposition exclusively during the last glacial maximum is certainly possible given the paleogeographic setting where large Pleistocene glaciers spread across the shelf during sea level low stand and carried their sediment loads directly to the deep sea fans [e.g., Mann and Hamilton, 1995]. Nonetheless, if these sediments date exclusively from the last glacial maximum, they record astonishing sedimentation rates; i.e., 800 m maximum sediment thickness in 15 Ka is equivalent to a sedimentation rate of 50 mm/yr. Admittedly this rate is a maximum, but even using the more modest thicknesses of m of undeformed sediment on other seismic lines (e.g., line 909) sedimentation rates greater than 10 mm/yr are indicated. [40] If this interpretation is correct, it ultimately has an important bearing on erosion rates within the St. Elias orogen. Work offshore [e.g., Jaeger et al., 1998; Sheaf et al., 2003], as well as fiord studies [Hallet et al., 1996; Koppes and Hallet, 2002], shows that Holocene sediment yields from the St. Elias orogen imply spectacularly high erosion rates of 5 30 mm/yr. An open question, however, has been if these erosion rates are representative of long-term rates or are peculiar to deglaciation periods [e.g., see Koppes and Hallet, 2002]. Powell and Cooper [2002] used high-resolution seismic stratigraphy to infer sedimentation rates at the shelf edge comparable to Holocene rates, implying large volumes of sediment may have been carried off the shelf into the fan. Thus this hypothesis is allowable from available data, although it can only be tested by obtaining new data on the deep sea fan Large-Scale Kinematics of Plio-Pleistocene Deformation in the Orogen [41] Using the tectonic reconstructions in Figure 6, we developed a conceptual, working model for the kinematic 11 of 14

12 Figure 7. Tectonic summary of the evolution of the orogen inferred for 3 Ma (right) to 0.5 Ma (left). History is inferred from surface geology and map-scale reconstruction shown in Figure 6. Note the consequence of EW contraction and contemporaneous indentation as the Yakutat microplate is driven into the constrictional corner. See text for more details. evolution of the orogen over the last 3 Ma (Figure 7). In this model the Yakutat microplate begins to produce contraction along the orogen by 5 10 Ma, and by 3 Ma (Figure 7) a wide fold and thrust belt had developed outboard of the suture. By this time the coastal mountains were high enough for large alpine glaciers and these glacial systems were shedding sediments onto the Yakutat terrane as the lower part of the Yakataga Formation [Plafker, 1987; Lagoe et al., 1993]. The thick sedimentary cover, as well as the transitional basement east of the Dangerous River zone, apparently choked the subduction channel and crustal thickening coupled with subduction of basement was unable to keep pace with the large volume of sediment and basement entering the subduction zone. Alternatively, the Yakutat terrane may have carried an oceanic plateau [Ferris et al., 2003] as well as a continental fragment, and the arrival of this thickened oceanic basement may have been the principal driver of the deformation. Indeed, the arrival of a subducting plateau could have produced increased contraction that developed not only much of the present topography, but also far-field effects such as rejuvenation of the Denali fault and the associated uplift of the Alaska Range. [42] Although some convergence apparently was taken up outboard of the suture along the Transition fault, this convergence was not sufficient to nucleate a new subduction zone. Thus, in a small-scale analog of the Indian-Asia interaction, it appears that it was easier to deform the continental interior than to nucleate a new subduction zone. That is, the microplate began to act as an indentor that either generated, or reactivated strike-slip faulting in what is now the Bagley Icefield, and began to drive the thrust belt westward, presumably escaping toward the Aleutian trench (right, Figure 7). [43] We infer that as this system evolved the suture was warped into a large S-shaped fold in map view, the western end of the thrust belt was refolded about steeply plunging fold axes, and part of the offshore fold and thrust belt was extruded across the Transition fault, forming the complex zone of structures along the base of the slope that connects the Kayak Island zone to the Pamplona zone (left, Figure 7). Simultaneously, the western end of the thrust belt was probably also rotated counterclockwise, about a steep axis. [44] It is not yet clear if the folding of the suture was driven by the indentor effect of the microplate, or if it is an edge effect of a larger scale extrusion of the entire fold and thrust belt toward the west. In either case, we infer that much of the complex overprinting was driven by the development of a geometric constriction between NW trending strike slip zones and the NE trending Aleutian megathrust. This constriction forced an along-strike contraction that converted the fold-thrust belt from a typical stacked fold-thrust sequence to a fold-thrust system with vertical axis refolding, and young structures that crosscut the regional structural trends. Note that in this interpretation, the Pamplona zone, which is shown on many regional maps as the NA-P plate boundary, is interpreted as a minor, young cross structure generated by EW contraction and oroclinal bending related to the partial extrusion of the fold-thrust belt toward the trench. [45] It is also unclear if the apparent near surface deformation pattern represented by Figure 7 is directly connected to basement (lithospheric) motion or if basement motion is distinct from the movement of cover. The latter is possible because most of the structures on the Yakutat terrane are detached from basement along a regional decollement [e.g., Plafker, 1987] implying that these structures could interact 12 of 14

13 to form a different displacement field than the lithospheric motion that drives them. In this case the Euler pole analysis presented here may not be a good representation of the actual motion of the Yakutat microplate, and other information would be needed to evaluate the large-scale kinematics at depth. [46] Finally we note that important questions remain on the fundamental driving process for the construction of the local orogenic topography as well as far field effects of the collision. Recent passive seismic imaging of the subducted oceanic slab in central Alaska indicates that this slab carries an unusually thick (12 24 km) crustal section [Ferris et al., 2003]. Brocher et al. [1994] had previously recognized similar crustal thicknesses beneath Prince William Sound in the subducted basement of Yakutat terrane, but their data were close to the projected trace of the subducted Transition fault and the thick crustal section was attributed to duplication by thrusting. Ferris et al. s [2003] profile, however, lies well north of the westward projection of the Transition fault suggesting that the basement of the Yakutat terrane is actually km thick. If true, this implies that the oceanic basement west of the Dangerous River zone on the Yakutat microplate may not be normal oceanic crust, but instead may represent an oceanic plateau. This result could be critical to geodynamic models of the St. Elias orogen because subduction of an oceanic plateau would represent a much larger indentor in the margin than the small microcontinental block represented by Yakutat terrane east of the Dangerous River zone. Thus resolution of this issue should be an important goal of future studies in the orogen. 6. Conclusions [47] Analysis of plate motions associated with the Yakutat microplate collision in southern Alaska indicates that the Transition fault, the Yakutat foothills structures, or both take up from 10 to 30 mm/yr of convergence related to the independent motion of the Yakutat microplate. The Euler pole for Y-P motion lies near the Y-P-NA triple junction in the vicinity of Cross Sound and thus, linear P-Y convergence rates increase rapidly toward the northwest. This analysis, as well as conclusions from GPS data [Fletcher and Freymueller, 1999], contradict the observation that the offshore Transition fault is overlapped by undeformed sediments. We propose that this contradiction can only be explained if (1) the Transition fault is not active and Y-P convergence is concentrated along the deformation front of the St. Elias Mountains; or (2) the sediments overlapping the Transition fault were all deposited during the last glacial maximum and there has been insufficient convergence to produce recognizable structures. The second hypothesis can only be tested by dating of the sediments overlapping the Transition fault. Extrapolation of present day tectonic rates and Euler poles deduced from them allow a preliminary reconstruction of the development of the orogen during the last 3 m.y. (Figure 7). We infer that the present warping of the NA-Yakutat suture developed when the Yakutat terrane was jammed into the corner between the NE trending Aleutian megathrust and the NW trending strike-slip system. Transport into this corner produced not only a NW directed contraction, but also an EW component of shortening as the terrane was driven into the constriction. We infer that this process forced the western portion of the orogen to redeform about vertical axis folds, and part of the fold and thrust belt has been forced to escape westward, toward the Aleutian trench. [48] Acknowledgments. This work was supported by NSF grants EAR to T. Pavlis and L. Serpa and EAR to R. L. Bruhn. We thank Bernard Guest and Scott Richards for assistance in the field. Thorough reviews for Tectonics by Jeanne Sauber and Jeff Freymueller provided critical input that helped improve the manuscript. References Argus, D. F., and R. G. Gordon (2001), Present tectonic motion across the Coast Ranges and San Andreas fault system in central California, Geol. Soc. Am. Bull., 113, Bayer, K. C., R. E. Mattick, G. Plafker, and T. R. 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