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A suite of geophysical data obtained along the Richardson Highway crosses the eastern Alaska Range and Denali fault and reveals the crustal structure of the orogen. Strong seismic reflections from within the orogen north of the Denali fault dip as steeply as 25° north and extend downward to depths between 20 and 25 km. These reflections reveal what is probably a shear zone that transects most of the crust and is part of a crustal-scale duplex structure that probably formed during the Late Cretaceous. These structures, however, appear to be relict because over the past 20 years, they have produced little or no seismicity despite the nearby Mw = 7.9 Denali fault earthquake that struck in 2002. The Denali fault is nonreflective, but we interpret modeled magnetotelluric (MT), gravity, and magnetic data to propose that the fault dips steeply to vertically. Modeling of MT data shows that aftershocks of the 2002 Denali fault earthquake occurred above a rock body that has low electrical resistivity (>10 ohm-m), which might signify the presence of fluids in the middle and lower crust.

INTRODUCTION

On November 3, 2002, earthquake activity in south-central Alaska culminated in an Mw = 7.9 shock that ruptured along the right-slip Denali fault and other faults (Eberhart-Phillips et al., 2003; Fig. 1). This earthquake underscored the need to better understand the tectonics and geologic structure of the Denali fault and of the Alaska Range orogen, where the fault has its clearest geomorphic expression. The Denali fault extends ∼ 1500 km from western Canada to near the shore of the Bering Sea (St. Amand, 1957; Grantz, 1966; Brogan et al., 1975; Wahrhaftig et al., 1975). In addition to its role in earthquake generation, the Denali fault facilitated the late Mesozoic and early Cenozoic northward translation and amalgamation of tectonostratigraphic terranes that form most of the Alaskan continental mass (Stout and Chase, 1980; Csejtey et al., 1982; Plafker et al., 1989; Nokleberg et al., 1994; Ridgway et al., 2002).

Figure 1. Geology of the study area and seismicity associated with the 2002 Denali fault earthquake. Geology simplified from Ridgway et al. (2002). Geophysical data shown herein were collected along the Richardson Highway, ∼ 100 km east of the main shock of the Denali fault earthquake.

Figure 1. Geology of the study area and seismicity associated with the 2002 Denali fault earthquake. Geology simplified from Ridgway et al. (2002). Geophysical data shown herein were collected along the Richardson Highway, ∼ 100 km east of the main shock of the Denali fault earthquake.

In this report we summarize the main findings from previous work (Fisher et al., 2004a, 2004b), which included a detailed investigation of the crustal structure of the Denali fault and Alaska Range orogen, using outcrop geology and seismicity as well as deep-crustal seismic-reflection, magnetotelluric (MT), gravity, and magnetic data. Geophysical data form a transect along the Richardson Highway (Fig. 1).

GEOLOGIC SETTING

The Alaska Range orogen near the Richardson Highway includes numerous tectonostratigraphic terranes and major fault zones (e.g., Nokleberg et al., 1994; Ridgway et al., 2002), many of which converge near the intersection between the geophysical transect and the Denali fault (Figs. 1 and 2). We discuss these terranes and the main faults from south to north.

Figure 2. Detailed geology and vibrator-point (VP) locations for the Denali seismic-reflection line.

Figure 2. Detailed geology and vibrator-point (VP) locations for the Denali seismic-reflection line.

Terranes South of the Denali Fault

The extensive Wrangellia terrane lies south of the Denali fault (Figs. 1 and 2). The oldest units of this terrane are upper Paleozoic volcanic and sedimentary rocks (Plafker et al., 1989; Nokleberg et al., 1994; Ridgway et al., 2002; Trop et al., 2002) that are disconformably overlain by Upper Triassic basalt (Nokleberg et al., 1985, 1994) and limestone as well as an overlap assemblage of Upper Jurassic and Lower Cretaceous island-arc volcanic rocks and flysch (Bond, 1973, 1976; Nokleberg et al., 1982, 1985). The Wrangellia terrane is interpreted to be a late Paleozoic island arc.

The Kahiltna and Maclaren terranes (Figs. 1 and 2) consist of Jurassic and Cretaceous sedimentary and volcanic rocks and their variably metamorphosed equivalents (Nokleberg et al., 1982, 1985). The Kahiltna terrane includes extensively exposed Upper Jurassic and Lower Cretaceous flysch that is interpreted to be a remnant of a Jurassic and Cretaceous basin that originally extended for several thousand kilometers along the margin of the North American Cordillera. These flysch deposits constitute one of the major stratigraphic units in southern and southeastern Alaska (Berg et al., 1972; Richter and Jones, 1973; Richter, 1976; Monger and Berg, 1987; Wallace et al., 1989; Ridgway et al., 2002).

The Denali and Hines Creek Faults

Nokleberg et al. (1994) summarized evidence for substantial late Mesozoic and early Cenozoic dextral displacement along the Denali fault. During the Cretaceous, oblique underthrusting may have occurred along the ancestral Denali fault, as interpreted partly from the history of metamorphism of rocks that make up the Yukon-Tanana terrane along the fault's north side (Nokleberg et al., 1994; Ridgway et al., 2002). GPS measurements made before the 2002 earthquake indicate 6–8 mm/yr of right-lateral strike-slip motion across the Alaska Range and Denali fault (Freymueller et al., 2003).

The Hines Creek fault is a zone of intense shearing and penetrative deformation that extends ∼ 100 km westward from where it branches to the north from the Denali fault, near the Richardson Highway (Wahrhaftig et al., 1975; Figs. 1 and 2). This fault has a complex history of strike-slip and thrust movement that occurred from the middle Cretaceous into the Cenozoic.

Terranes North of the Denali Fault

The Aurora Peak terrane (Aleinikoff, 1984; Nokleberg et al., 1985) (Fig. 2) consists of probable Paleozoic or early Mesozoic sedimentary rocks that were metamorphosed to calc-schist, marble, quartzite, and pelitic schist, all of which were intruded by Late Cretaceous and early Tertiary plutonic rocks that are regionally metamorphosed and penetratively deformed.

Multiply deformed and metamorphosed rocks of the Yukon-Tanana terrane underlie most of the Denali seismic-reflection line and many of the field stations for the other types of geophysical data (Figs. 1 and 2; Nokleberg et al., 1989; Dusel-Bacon, 1991; McClelland et al., 1992; Ridgway et al., 2002). Intense Mesozoic metamorphism and ductile deformation affected Devonian and older sedimentary and subordinate volcanic rocks as well as Devonian metaplutonic rocks, all of which comprise the southern Yukon-Tanana terrane. Isotopic data indicate that metamorphism of the terrane occurred during the Early Jurassic and again during the middle to Late Cretaceous, from 115 to 102 Ma (Turner and Smith, 1974; Aleinikoff and Nokleberg, 1985; Nokleberg et al., 1986; Aleinikoff et al., 1986). Just after the intense middle and Late Cretaceous metamorphism, the Yukon-Tanana terrane was intruded by Late Cretaceous granitic plutons, and along the south margin of the terrane near the Denali fault, rocks of this terrane underwent retrograde greenschist facies metamorphism during ductile deformation (Nokleberg et al., 1986). This metamorphism caused an intense mylonitic schistosity and south-vergent fold trains to form. During retrograde metamorphism of the Yukon-Tanana terrane ca. 105 Ma ago, a major antiform developed that follows the north side of the Denali and Hines Creek faults for more than 200 km (axis shown in Figures 1 and 2) (Nokleberg et al., 1986). This fold deforms the metamorphic schistosity, which is flat, except in the fold, where it locally steepens to nearly vertical. This fault-parallel antiform figures prominently in the interpretation of seismic reflection data.

Cenozoic Rocks and Structures

The Denali seismic reflection line crosses thin accumulations of early Cenozoic sedimentary rocks. These rocks fill a broad syncline that strikes east-west, ∼ 20 km north of the Denali fault (Fig. 2). The syncline is expressed not only in the early Cenozoic rocks but also in the concordantly folded schistosity of the underlying metamorphic rocks.

GEOPHYSICAL INFORMATION

Seismic-Reflection Data

Seismic-reflection data were collected in 1986 for the U.S. Geological Survey using a Vibroseis source (Fisher et al., 2004a, 2004b). These 128-fold data were migrated after stack and converted from time to depth using a crustal velocity model (Brocher et al., 1991) combined with stacking-velocity information.

Seismic-reflection data collected south of the Denali fault reveal little about the structure of the igneous rocks that make up the Wrangellia terrane or the metasedimentary and plutonic rocks comprising the Kahiltna and Maclaren terranes (Fig. 3). Shallow (< 2 km) reflections below VP 1050 and VP 1400 are probably from Cenozoic sedimentary rock. No fault-plane reflections reveal the locations of either the Denali or Hines Creek faults, and the appearance of reflections across these faults' surface traces does not change systematically to indicate any breaks in the crust.

Figure 3. Assembled geophysical information over the Denali fault, including gravity and magnetic, seismic-reflection, crustal-velocity, and magnetotelluic data, as well as earthquake seismicity. Horizontal and vertical scales are equal. The plane of section used in making Figure 3 parallels the Richardson Highway, which is not perpendicular to the Denali fault (Figs. 1 and 2). Hypocenters shown lie within 10 km of the plane of section, measured along the strike of the Denali fault, and the hypocenters were projected parallel to this fault's strike onto the section. The letters A, B, and C indicate the main reflection bands. R1, R2, R3, and R4 show the resistivity bodies described in the text. The Denali bright spot is included within the white box ∼ 8 km below VP 2700.

Figure 3. Assembled geophysical information over the Denali fault, including gravity and magnetic, seismic-reflection, crustal-velocity, and magnetotelluic data, as well as earthquake seismicity. Horizontal and vertical scales are equal. The plane of section used in making Figure 3 parallels the Richardson Highway, which is not perpendicular to the Denali fault (Figs. 1 and 2). Hypocenters shown lie within 10 km of the plane of section, measured along the strike of the Denali fault, and the hypocenters were projected parallel to this fault's strike onto the section. The letters A, B, and C indicate the main reflection bands. R1, R2, R3, and R4 show the resistivity bodies described in the text. The Denali bright spot is included within the white box ∼ 8 km below VP 2700.

Coherent seismic reflections begin abruptly at about VP2400, where the Denali seismic-reflection line begins to diverge from the Hines Creek fault (Figs. 2 and 3). We identify three main reflection bands, labeled A, B, and C (Figs. 3 and 4). The prominent reflection band A dips north from near the surface trace of the Hines Creek fault, and belowVP3000 this band reaches its greatest travel time of ∼ 3 s (Fig. 4) and depth of ∼ 7 km (Fig. 3). Reflection band A and shallower reflections reveal a broad synform with an axis below VP 2950. This axis lies ∼ 5 km south of the axis of an outcropping east-west syncline that deforms lower Tertiary sedimentary rocks and the compositional layering of underlying Yukon-Tanana terrane metamorphic rocks (axes shown in Fig. 2). The close spatial association of the two axes suggests that the early Tertiary or younger age of folding evident in surface rocks also dates the synformal folding of rocks that caused reflection band A.

Figure 4. Line drawing of part of migrated data (versus travel time) collected north of the Denali fault (Figs. 2 and 3). The drawing shows the three main reflection bands and the axis of the subsurface antiform that projects upward to the south to correlate in location with the axis of an antiform that developed during the Cretaceous. (DBS = Denali bright spot)

Figure 4. Line drawing of part of migrated data (versus travel time) collected north of the Denali fault (Figs. 2 and 3). The drawing shows the three main reflection bands and the axis of the subsurface antiform that projects upward to the south to correlate in location with the axis of an antiform that developed during the Cretaceous. (DBS = Denali bright spot)

North of VP 3000, linear reflections begin at the bottom of reflection band A, splay upward to the south, and terminate at a shallower reflection (Fig. 4). We interpret these reflections to mean that thrust faults are confined within an upper-crustal shear zone. This inferred shear zone is folded by the synform described in the preceding paragraph.

Below VP 2800 an asymmetric antiform is evident below reflection band A, between ∼ 2 s and 4 s (Fig. 4). The axis of this antiform dips and flattens northward. The south limb of the antiform is steeper, and this limb ends downward and southward against reflection band B. The axis of the antiform can be projected upward and southward along reflection band B to coincide in location with the axis of the antiform that follows the north side of the Denali fault (axis shown in Figs. 1 and 2). This coincidence suggests that the subsurface antiform developed during the Cretaceous retrograde metamorphism that produced the surface feature.

The Denali bright spot (DBS on Figures 3 and 4), at ∼ 3 s below VP2750, is a flat event that occurs within the arched reflections from the antiform (Fig. 4). The bright spot is made up of the strongest reflections recorded along the Denali seismic line.

Reflection band B begins at ∼ 3.2 s (Fig. 4) or 10 km (Fig. 3) below VP 2400. Locally this band dips as steeply as 25° north, and the dip decreases progressively as it soles out northward. At ∼ 10 km depth, band B includes strong events that have four or five reflection peaks, but north of VP 3000 this band widens and its amplitude decreases as it flattens progressively into the middle crust within the depth range of 17–20 km (Fig. 3).

Reflection band C diverges to the south from beneath reflection band B (below VP 2800 on Figures 3 and 4). This band extends south to where the signal from deep rocks is lost below VP 2500. Numerous subhorizontal and discontinuous seismic events are apparent below reflection band C; however, no convincing events from the Moho are evident anywhere along the Denali seismic-reflection section.

Earthquake Data

The Mw = 7.9 Denali fault earthquake generated thousands of aftershocks (Eberhart-Phillips et al., 2003; Ratchkovski et al., 2003); those having magnitudes greater than or equal to 2 were relocated (Ratchkovski et al., 2003) using a double-difference algorithm (Waldhauser and Ellsworth, 2000; Fig. 1). Aftershocks occurred mainly within the upper 10 km of the crust, and few fell along the main reflection bands described previously. Also we used the double-difference technique to relocate the sparse regional seismicity that occurred between 1975 and the 2002 Denali fault earthquake. The epicenters of aftershock events of the 2002 earthquake and the earlier regional seismicity are concentrated close to the surface trace of the Denali fault (Fig. 3).

Magnetotelluric Data

MT data can be modeled to show the distribution of electrical resistivity in Earth's crust (Vozoff, 1991). Details about the acquisition and processing of MT across the Denali fault are described in Fisher et al. (2004b). MT stations were located along the Richardson Highway, across the Denali fault (Fig. 1). The temporal and spatial sampling of the MT survey described here is suitable to determine deep, large-scale crustal structure, not to characterize the Denali fault in detail. In particular, impedance values within 3–5 km of Earth's surface are poorly constrained.

Previous interpretations of MT data collected along five transects near and across the Alaska Range (Stanley, 1986, 1989; Stanley et al., 1990) indicate a thick (20 km), very-low-resistivity (1–3 Ω-m) rock body making up much of the middle and lower crust north of the Denali fault. The most likely source of the low-resistivity body was inferred to be carbon-rich Jurassic and Cretaceous flysch. However, the temporal recording range used to obtain the earlier data was about an order of magnitude less than used for this study. Hence earlier data focused on the shallow crust. Furthermore, the earlier 2-D model was constructed using a series of 1-D models stitched together, and data interpreted with 1-D models can be severely distorted by multidimensional effects.

Our preferred MT model (Fig. 3) reveals four main bodies (R1 through R4) defined on the basis of their electrical resistivity. South of the Denali fault, body R1 includes high-resistivity (> 1000 Ω -m) rocks. The north boundary of this body dips ∼ 60° south, and the body extends downward to depths as great as 25 km. The high-resistivity values that characterize R1 are consistent with unweathered igneous or metamorphic rock like those exposed in the Talkeetna Mountains east of the Richardson Highway. Igneous rocks of the Wrangellia terrane are exposed where the body R1 is close to the surface.

Body R2 is characterized by very low resistivities (∼ 10 Ω-m) and underlies the Denali and Hines Creek faults. This body is nearly vertical and apparently extends downward to great depth. Body R3 is moderately resistive (∼ 300 Ω -m) and extends downward through much of the middle and lower crust. Rocks at the surface above body R3 are metamorphic rocks of the Yukon-Tanana terrane. Body R4 (< 30 Ω-m) dips north from near the surface into the middle crust. Metamorphic rocks of the terrane crop out where body R4 is shallow.

Gravity and Magnetic Data

The Denali fault is characterized in both gravity and magnetic data by regional gradients that decrease northward from high and irregular values south of the fault. The long-wavelength regional gradients across the Denali fault amount to 15 mGal and 300nT over a distance of ∼10 km, suggesting to us that the causative features extend to relatively deep levels in the crust. South of the Denali Fault, several large-amplitude, short-wavelength features in potential-field data precluded our modeling these data to understand the deep crust, but we interpret the modeling to mean that the Denali fault dips steeply to vertically down to a depth of at least 10 km.

DISCUSSION

Intriguing results from the geophysical investigation of the Alaska Range orogen include the reflection bands and the low-resistivity body R2, which is located beneath the surface trace of the Denali fault. The rock reflectivity and electrical resistivity shed light on tectonic processes that were involved in the development of the Alaska Range orogen. In this section, we explain possible origins for the crustal-resistivity variation and describe a model for the structure of the Alaska Range.

The vertical resistivity body R2 conforms, in general, with results obtained from modeling MT data along other active strike-slip faults. For example, the Altyn Tagh fault of Tibet (Bedrosian et al., 2001), the Alpine fault in New Zealand (Wannamaker et al., 2002), strike-slip faults in the Basin and Range province of eastern California (Park and Wernicke, 2003), and the San Andreas fault in California (Unsworth et al., 1997, 1999, 2000; Bedrosian et al., 2002) are all associated with steeply dipping or vertical zones having low resistivity and considerable depth extent. Although some of these studies indicate that carbon films in metasedimentary rocks are responsible for the low resistivities along these strike-slip faults, most studies highlight the role of aqueous fluids.

From previous research aimed at understanding the Alaska Range, the most likely causes for the low resistivity of body R2 are carbon films (Stanley, 1989; Stanley et al., 1990) and aqueous fluids (Fisher et al., 2004b). Near the Denali fault, carbon-rich rocks include the Upper Jurassic and Lower Cretaceous flysch of the Kahiltna and Gravina overlap assemblages. When subjected to low-grade metamorphic conditions, carbon particles in these rocks can be smeared into films that conduct electricity. In addition, high-grade metamorphic rocks in theYukon-Tanana terrane contain carbon films (Mathez et al., 1995), which might have precipitated from carbon-rich metamorphic fluids (a process described in Wannamaker, 1986, and Wannamaker et al., 2002). The possibility that carbon films can form from mobile carbon-rich fluids implies that such films are not restricted to any particular lithology.

Aqueous fluids can also produce the low-electrical resistivity of body R2, and the Denali bright spot may be evidence for such fluids in the middle crust of the Alaska Range. Hyndman and Shearer (1989) indicate that midcrustal bright spots can be caused by magma, acoustic tuning in rock layers, and fluids. The Denali bright spot is flat and discordant to the antiform outlined by other nearby reflections, so the bright spot is unlikely to be caused by rock layering. Therefore, we prefer the interpretation that aqueous fluids cause the bright spot and the low resistivity. Also, aqueous fluids are commonly invoked to explain findings from other MT studies of strike-slip faults.

We propose a crustal-structure model of the Alaska Range orogen by assuming a two-phase geologic history that is simplified from discussion in Nokleberg et al. (1994) and Ridgway et al. (2002). The first phase occurred during the mid-Cretaceous, when an oblique-convergent plate boundary lay near what would later become the strike-slip Denali fault. The second phase involved Cenozoic strike-slip offset along this fault.

During the Late Cretaceous and early Cenozoic phase of deformation, metamorphic rocks of the Yukon-Tanana terrane formed the middle and lower crust of an oblique-convergent continental margin (Nokleberg et al., 1994; Ridgway et al., 2002). Terrane assembly along this margin to form much of the Alaskan continental mass ended before or during the early Cenozoic (ca. 57 Ma), when granite plutons were intruded across major tectonic boundaries in the northern and central terrane (Foster et al., 1987; Nokleberg et al., 1989).

The metamorphism and ductile deformation of Yukon-Tanana terrane rocks likely produced reflective ductile-strain features, such as shear zones, compositional layering, and mineral anisotropy. Given the high metamorphic grade of surface rocks, the reflection bands most likely reveal ductile shear zones, perhaps ones that developed according to kinematic models of oblique-convergent margins (e.g., Fitch, 1972; Beck, 1983; Chemenda et al., 2000). Metamorphic structures can cause reflections like those within reflection band A, where subsidiary reflections extend upward to the south from the base of the band and end at the top of the band (Fig. 4). This reflection geometry suggests it is a shear zone.

We propose the features that cause reflection bands C, B, and probably A date from the mid-Cretaceous and early Cenozoic phase of deformation along the oblique-convergent margin. Reflection band B may have formed as a thrust fault and/or a ductile shear zone within the upper plate of the subduction zone, as Wrangellia and other terranes collided with Alaska. This age is inferred from the seismic reflections between reflection bands A and B that outline the asymmetric antiform (Fig. 4). This antiform at the surface developed during the Late Cretaceous (ca. 105 Ma) retrograde metamorphism (Nokleberg et al., 1994; Ridgway et al., 2002).

Rocks that cause reflection band B thicken downward and sole out northward within the depth range of 17–20 km (Figs. 3 and 5). This shape probably reveals the ramp of a crustal-scale shear zone, as has been suggested for hinterland-dipping reflections from other contractional orogens (Ando et al., 1984; Allmendinger et al., 1987; Eisbacher et al., 1989; Gray et al., 1991; Cook and Varsek, 1994; Beaumont and Quinlan, 1994). This reflection band thickens with increasing depth, a common attribute of crustal-scale faults (Sibson, 1982; Smithson et al., 1986).

Figure 5. Structural model across the Alaska Range orogen and aftershock zone of the Denali fault earthquake. Opposed thrusting developed during Late Cretaceous terrane collision, involving Wrangellia and an oblique-convergent margin along what was then the south and west margin of Alaska. Structures revealed by seismic-reflection data are mostly aseismic, but the lower cutoff for seismicity is at the shallow depth of only 10 km, so some of these features could support ductile strain. Low-resistivity rocks in body R2 underlie the surface trace of the Denali fault and may indicate the fault's position in the deep subsurface. The cause for the low resistivity is unknown but likely results from carbon films or fluids.

Figure 5. Structural model across the Alaska Range orogen and aftershock zone of the Denali fault earthquake. Opposed thrusting developed during Late Cretaceous terrane collision, involving Wrangellia and an oblique-convergent margin along what was then the south and west margin of Alaska. Structures revealed by seismic-reflection data are mostly aseismic, but the lower cutoff for seismicity is at the shallow depth of only 10 km, so some of these features could support ductile strain. Low-resistivity rocks in body R2 underlie the surface trace of the Denali fault and may indicate the fault's position in the deep subsurface. The cause for the low resistivity is unknown but likely results from carbon films or fluids.

The sigmoid shape of reflection band B and the divergence between reflection bands B and C (Fig. 3) suggest they form a crustal-scale duplex structure (Iverson and Smithson, 1983a, 1983b; Smithson et al., 1986; Le Gall, 1990) that soles out northward into the middle crust at ∼ 20 km depth (Fig. 5). Reflection bands B and C partly outline resistivity body R3, and because this body underlies the surface antiform in Yukon-Tanana terrane rocks, we propose that this body's emplacement caused the antiform to develop. If so, then body R3 and its associated reflections reveal structures that are ca. 105 Ma old, the time of retrograde metamorphism of antiformal surface rocks. The rock types that make up this proposed duplex structure are unknown. However, body R3 (Fig. 5) does not include a flap of lower crustal or upper mantle material incorporated into the Alaska Range orogen because both gravity and magnetic values are low and smooth over this body (Figs. 3 and 5).

Few earthquake hypocenters fall along the reflection bands (Fig. 3), indicating that the structures causing these bands are currently aseismic. All of reflection band A and the upper part of band B are shallower than 10 km depth, and neither band produced seismicity after the 2002 Denali fault rupture. The 10-km depth is the regional maximum depth for earthquakes (Ratchkovski et al., 2003).

Conceptually, the first-phase structures were overprinted during the early Cenozoic, when the present strike-slip Denali fault began to form in response to a change in North Pacific plate motions (Nokleberg et al., 1994; Ridgway et al., 2002). We cannot confirm this overprinting because, in general, strike-slip faults, especially ones in crystalline terranes, produce only muted, indirect response involving poor data zones that are 5–20 km wide (Lemiszki and Brown, 1988).

South of the Denali fault, the shallow part of resistivity body R1 correlates well in location with volcanic and plutonic rocks of the Wrangellia terrane. MT modeling suggests that this resistivity body transects the crust to depths near 25 km (Fig. 5). However, modeling of the complicated gravity and magnetic anomalies south of the Denali fault does not reveal a similar transecting body (Fisher et al., 2004b). Even so, MT modeling indicates that the north boundary of body R1 dips steeply south, and we interpret this boundary as a thrust fault that formed when the Wrangellia terrane collided with Alaska.

In our structural model of the Denali fault and the Alaska Range orogen, we assume the Denali fault is vertical (Fig. 5). Gravity and magnetic modeling indicates that the Denali fault dips steeply to vertically downward from below the fault's surface trace to depths of around 10 km. In addition, modeling of MT data indicates the vertically elongated body R2 below the fault's surface trace, and we propose that body R2 indicates the Denali fault's location down to depths as great as 30 km (Fig. 5). In our view, this fault is sandwiched within an old suture zone, where structural remnants of the opposed Late Cretaceous and early Cenozoic thrusting extend deeply (20 km) into the crust. Judging from sparse hypocenters that occurred more than 10 km perpendicularly away from the Denali fault (Fig. 3), we propose that the highly reflective crustal features north of this fault are relict and are not involved in modern seismicity. Similarly, the absence of seismicity along the north boundary of body R1 indicates that it, too, is relict.

We compare the preferred structure model (Fig. 5) to sections showing aftershock hypocenters to derive a possible configuration of seismogenic structures along the Denali fault (Fig. 6). Along this fault, alignments of aftershock hypocenters have inconsistent dips, and some of the alignments end below the surface trace of the Denali fault. These alignments are interpreted to reveal a positive flower structure (as defined in Harding, 1985) along the Denali fault. The hypocenter alignments are interpreted to fall along oblique-slip faults like the Hines Creek fault. Because many faults west of the Richardson Highway converge with the Denali fault, the faults interpreted to make up the flower structure west of this highway are not likely to be continuous with interpreted faults east of the highway. The distribution of aftershock events in map view (Fig. 1) indicates that the oblique-slip faults have limited extent (10–20 km) along strike.

Figure 6. Three hypocenter cross sections perpendicular to the Denali fault, showing the highly variable distribution of aftershock events along the Denali fault from the 2002 earthquake. Also shown are the main features interpreted from geophysical data (Fig. 5). In making this figure, aftershock hypocenters (Ratchkovski et al., 2003) were projected parallel to the Denali fault onto the cross section (Fig. 5) that obliquely crosses the fault (index map). The Denali fault is interpreted to lie within a flower structure as defined by the heavy black lines.

Figure 6. Three hypocenter cross sections perpendicular to the Denali fault, showing the highly variable distribution of aftershock events along the Denali fault from the 2002 earthquake. Also shown are the main features interpreted from geophysical data (Fig. 5). In making this figure, aftershock hypocenters (Ratchkovski et al., 2003) were projected parallel to the Denali fault onto the cross section (Fig. 5) that obliquely crosses the fault (index map). The Denali fault is interpreted to lie within a flower structure as defined by the heavy black lines.

CONCLUSIONS

The main findings from an integrated interpretation of geological and geophysical data are:

  1. The low-resistivity body R2 underlies the surface trace of the Denali fault. Gravity and magnetic data can be interpreted to show the Denali fault is vertical to depths of ∼ 10 km. These modeling results suggest that the Denali fault dips steeply to depths of around 30 km. Data are lacking to demonstrate whether a low-resistivity body like R2 extends for a long distance along the strike of the Denali fault, and given the strong variation in both seismicity and geologic structure along the fault, this issue must be resolved.

  2. Strong reflection bands outline resistivity body R3, which we propose is part of a crustal-scale duplex structure that formed during the Late Cretaceous terrane collision along the ancestral Denali fault. The inferred date of this body comes from projecting the axis of a subsurface antiform, imaged by seismic reflection data, to the surface, where an antiform deflects the schistosity of metamorphic rocks. The coincidence in location of subsurface and surface axes suggests that the subsurface antiform developed during the Late Cretaceous (ca. 105 Ma).

  3. The Denali bright spot indicates fluids in the crust; hence, the low resistivity of body R2 under the Denali fault may be due to fluids, which could have an important influence on earthquake seismicity by reducing rock friction in the middle and lower crust.

  4. In our preferred structural model, the present strike-slip Denali fault formed within a Late Cretaceous collision and suture zone that involved oppositely directed thrusting. Seismically reflective features formed as thrust faults or as a ductile shear zone, but these features now appear to be seismically inactive. The strike-slip Denali fault appears to dip steeply through the crust to depths as great as 30 km. This dip follows from our proposal that resistivity body R2 locates the Denali fault at depth. Rocks near the deeper reaches of this fault zone are weak, possibly owing to entrained fluids. One possibility is that deep fluids along the Denali fault weaken the crust and localize earthquake seismicity.

We thank Tom Parsons and Victor Labson (both USGS) for supporting this research. Terry Bruns and Shirley Baher made helpful comments on an early version of this report.

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Figures & Tables

Contents

References

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