The Denali fault system of Alaska
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The Denali fault system: an interior strike-slip structure responding to dextral and sinistral shear coupling
Denali (Mt. McKinley), 6000m, seen from near the
Eileson Visitor Center. Photo: T. F. Redfield.
This paper was spawned by an earlier paper we published in Science, documenting and interpreting the exhumation of the Denali (Mt. McKinley) massif (Fitzgerald et al., 1993). Denali (left) is the highest peak in the central Alaska Range and occupies a significant tectonic site: the apex of the arc formed by the Denali fault system (DFS). The apatite fission track data indicating Denali underwent rapid exhumation at ~5.5 Ma demanded a tectonic interpretation. In posing our answer we hypothesized that tectonic uplift was a result a change in motion of the Pacific plate relative to North America at ~5.5 Ma which focused stress in the DFS apex, forcing intervening crustal blocks upward against what was effectively a rigid backstop. Our hypothesis inspired a follow up study to investigate the predicted relative plate motions along the entire DFS throughout the Cenozoic and their effect upon the geotectonic evolution of southern Alaska.
Due to our Antarctic commitments, we were never able to present the follow up study as an abstract at a GSA or AGU annual meeting. However, we had intended to present it, and therefore created nice color figures to be used in a poster session. Now, the figures, captions, and abstract appear below as a virtual poster session on the Internet. Click on each figure to obtain a larger version for scrutiny, and form your own opinion about Denali, Cenozoic southern Alaska, and the DFS. Please refer to the original journal text for references and answers to specific questions, or send email to redfield@rmi.net or to p.fitzgerald@antarcticanz.govt.nz (Sorry, Sport, but you are an author too and must therefore also take some of the heat!!!).
At right: the two perpetrators. Photo: R. Sorkhabi.
Abstract
The Denali fault system (DFS) extends for ~1200 km, from southeast to southcentral Alaska. The DFS has been generally regarded as a right-lateral strike-slip fault, along which post late Mesozoic offsets of up to 400 km have been suggested. The offset history of the DFS is relatively unconstrained, particularly at its western end. For this study we calculated relative motion vectors at discrete points along the length of the DFS, based on the well-understood kinematic interaction between the North American, Pacific, and Kula plates, and the following assumptions: [1] The arcuate geometry of the DFS has existed essentially unchanged since the Late Cretaceous. [2] The Yukon-Tanana terrane and other terranes north of the DFS were fixed, in situ, prior to the accretion of the southern Alaskan terranes. [3] Tangential and normal relative motion vector components calculated for points along the DFS using the plate model of Kelley (1993) describe the plate kinematics of the DFS since the Late Cretaceous. The consequent kinematic model for the DFS predicts that left-lateral stresss have acted upon the western end of the DFS for much of its history, and conflicting senses of shear exist between the eastern and western ends of the system. The offset history of the western end of the Denali fault system should be significantly different than the history of the central and eastern sections; consequently, individual crustal blocks in southeast and southwest Alaska may have undergone, respectively, clockwise and anticlockwise rotations. The sense of rotation predicted by our model is in agreement with rotations determined by paleomagnetic studies and provides an alternative model to the Alaskan orocline hypotheses.
Alaska, Paleomagnetism, and the Great Alaskan Orocline
Physical and geological maps of Alaska show consistent convex patterns formed by mountain ranges and terrane boundaries. In a first explanation of the phenomenon, Carey (1955) suggested that most of Alaska pivoted 28 anti-clockwise degrees about a point north of the Gulf of Alaska, opening the Canada Basin north of Alaska and bending the central Alaska Range (and presumably the trace of the DFS) into their present arcuate geometries. Unlike his Expanding Earth hypothesis, Careys orocline became a fixation with many Alaskan tectonocists. The strongest evidence in support of oroclinal bending comes from the paleomagnetic record: with few exceptions, Tertiary rocks of south central and south west Alaska record anti-clockwise rotation. Because anti-clockwise small block rotation is associated with right lateral strike slip shear couples (e.g. Beck, 1989), and because the great faults of Alaska have been consistently described in the literature as right lateral strike slip structures, many authors have called upon some variant of the Orocline to explain the anomolous Alaskan Tertiary paleomagnetic declinations.
However, after reviewing the Alaskan literature, we are convinced that it is premature to assign a dextral offset history to the western end of the DFS. Data are limited to non-existent: in part due to a lack of exposure, the faults are poorly studied. In this paper we present plate tectonic relative motion data developed from the model of Engebretson (1985) that predict a sinistral strike slip offset history with consequent anti-clockwise paleomagnetic rotations for the western DFS, and a dextral strike slip, clockwise rotation history from the McKinley strand to the Chatham Strait fault in British Columbia. We believe that in any discussion of the geotectonic development of southern Alaska, this data cannot be ignored. While it is probably too much to expect that the Orocline hypothesis will meet its demise through our heretical hands, we hope to stimulate renewed interest in the problem of the Denali fault system by proposing an alternative model to accomodate 28 degrees of anti-clockwise paleomagnetic rotation in southwestern Alaska.
This Internet offering presents the highlights of our analysis. For a detailed description of our methods, assumptions, and geological constraints, please see the original journal submission Tectonics, v. 12, 5, 1195-1280, 1993).
Figure 1: Location Map
A simplified, schematic terrane map of Alaska after Coe et. al. (1985) and Howell et al. (1985) shows the locations of major tectonostratigraphic terranes, structures, Relative Motion Vector (RMV) analysis sites, and other localities mentioned in the journal text. (Click the map to load a larger version). The trace of the DFS defines two tectonic provinces of soutern Alaska, separating well-traveled allochthonous terranes to the south from less well-traveled, autochthonous (Cenozoically speaking) terranes to the north. Note that the Yakutat terrane extends offshore, actively colliding with North America even as you read this! RMV sites on the DFS are marked with a solid dot. Site 1 of the DFS is on the Togiak/Tikchik Fault. Sites 2 and 3 of the DFS are on the Holitna Fault. Sites 4-7 of the DFS are on the Farewell Fault. Sites 8-10 of the DFS are on the McKinley strand of the Denali fault. Sites 11 and 12 of the DFS are on the Shakwak Fault. Site 13 of the DFS is on the Dalton Fault. Site 14 of the DFS is on the Chatham Strait Fault. Together these faults make up the DFS, which has long been viewed as a right lateral (dextral) strike slip fault of continental proportions. RMV sites along other faults are marked with different symbols. We assume that the present-day traces of the great Alaskan fault systems approximate their traces of the geological past. As paleomagnetic studies have shown that terranes north of the DFS have not traveled great distances during the Cenozoic (e.g. Hillhouse and Gromme, 1982), we hypothesize that they formed an unyielding backstop for tectonic accretion. The premise of this paper is that with these assumptions we can use the shape of the DFS as a smoothed and averaged approximation of the Early Cenozoic southern Alaskan plate margin. Like soft-nosed dum dum bullets slamming into the pockmarked adobe walls of an early twentieth century Mexican hacienda, we conclude that the incoming allochthanous terranes borne by the ancestral Pacific Ocean basin plates were met by that unyielding margin, to be arrested, deformed, rotated, and translated during accretion.
Figure 2: On Plate Tectonic Convergence
With mathematics, plate motion models, and imagination one lithospheric plate may be held fixed relative to another and a Relative Motion Vector (RMV) calculated. The RMV resultants, broken into components, describes the normal and tangential tectonic stresses at the common plate margin. Mechanical coupling will cause deformation at the fixed plate margin.; substantial tangential components are known to drive strike slip faults (e.g. Jarrard, 1986). Diagram (a) shows (schematically!!! See Cobbold et al., 1989, for a sandbox example of fault block shear couple rotation.) how oblique plate tectonic convergence might produce a situation where both dextral and sinistral senses of shear could be present along a common plate margin as a result of convergence. Space problems are alleviated by intervening structures, such as the thrust fault shown (e,g, the Broxson Gulch thrust). Note the different senses of the tangential components of the two vector diagrams. (b) A simple block diagram (Beck's Ball Bearings, modified after Beck, 1980; 1989) shows conceptually how both clockwise and anticlockwise rotations of individual blocks could be expected along a single strike-slip fault as a function of fault geometry and the angle of convergence. (c) A sequence showing how anti-clockwise rotation of a block trapped within a left-lateral shear couple and the continued excavation of pre-existing drainages might create right-lateral offsets, possibly explaining observed drainage patterns of the Muldrow and Peters Glaciers north of Denali (Mt. McKinley).
Figure 3: Relative Motion Vectors
A schematic diagram showing RMV resultants for the major curved faults of Alaska between 5.6 Ma and the present. RMVs were calculated using map trace fault azimuths as backstop orientations; site numbers correspond to those on the location map. RMV source data are from plate tectonic models by Kelley (1993) and Engebretson et al. (1985). Color coding reflects both polarity and the tangential/normal component ratio: cool shades denote dextral regimes while warmer colors identify sinistral resultants. In the model, left-lateral strike-slip motion is predicted for the western ends of the DFS and the other great curved fault systems of Alaska. Note that tangential slip velocities are greatest at either end of a given system, and that convergence becomes dominately compressional within the armpit region of south central Alaska. This is a function of backstop geometry. Opposing forces predict that major tectonic structures should be present to relieve space problems. The Broxson Gulch thrust (see location map) is an example of one such structure present on south central Alaska. Other faults in southern Alaska may also be a response to crustal shortening caused by opposing convergence.
Figure 4: More Relative Motion Vectors
A block diagram synthesizing RMV data for the Denali fault system in space and time. Source data and color coding are the same as the preceeding diagram. These data suggest that the DFS can be broken into three regions, each having experienced different tectonic histories. The model predicts that the western end of the DFS has experienced entirely sinistral tangential components while the eastern end has always been characterized by dextral components. The partitioning of the middle regions into sinistral and dextral regimes is a function of fault/backstop geometry (see Figure 6). Complex thrust faulting and the Cenozoic (?) emplacement of many mini-terranes (e.g. Jones et al., 1982) characterize the central Alaska Range south of the DFS, possible consequences of the tremendous crustal shortening our model would seem to imply.
Figure 5: Still More Relative Motion Vectors
A block diagram synthesizing RMV data for the Kaltag/Tintina, Iditarod-Nixon Fork, Castle Mt./Fairweather faults, and the present day NA/Pacific plate boundary, in time and space. Color coding and source data are the same as in the preceeding diagrams. In general, extending the model to the other curved faults of Alaska and to the present day North America/Pacific plate boundary shows that western Alaska has been largely characterized by sinistral strike slip components. However, RMV analysis suggests that the late Cenozoic Kaltag/Tintina and Iditarod/Nixon Fork fault systems would have experienced dextral stress. Coupling between the curved faults would result in a complex internal structure (e.g. Cobbold et al., 1989). While it is less certain that the stress fields generated at todays margin should be resolved against the Interior faults, the hypothesis we advance is that, prior to the Cenozoic accretion of south central Alaska, the northern curved faults may have focused terrane accretion and created a geological room problem.
Figure 6: A Caveat ~ Limits to the Model
A schematic geologic map of a portion of the Farewell fault (after Beikman, 1980) shows the fault azimuth dependency of this study. Because net convergence in western Alaska is largely normal, small variations in backstop azimuths result in conflicting tangential components. RMVs calculated for Sites 4, 5, and 6 show, respectively, dextral, sinistral, and dextral tangential components between 33 Ma and 5.6 Ma. While todays fault traces can be measured accurately, constraining the Early Tertiary backstop azimuths is necessarily more speculative. However, after reviewing the geologic and paleomagnetic literature, we believe our backstop assumptions to be reasonably well founded. Consequently, we present this analysis as a provocative argument supporting a net sinistral strike slip history for the western end of the DFS and the rejection of the Orocline hypothesis in southwestern Alaska.
Conclusions
The plate tectonic model of Engebretson et al. (1985) and its direct successor (Kelley, 1993) can be used to place significant constraints upon the evolution of the Denali fault system (DFS) if certain assumptions are made. These assumptions include: [1] the geometry of the DFS has remained essentially unchanged since the Late Cretaceous; [2] terranes north of the DFS provided an existing backstop; [3] Relative Motion Vectors (RMVs) calculated along the DFS are representative of the driving stresses generated by interaction of the Kula or Pacific plates with continental North America. The resulting kinematic model suggests that, while the eastern end of the DFS has probably been characterized by right-lateral shear, the western end of the system has experienced dominantly left-lateral or compressive regimes. Absolute offsets across the DFS may be variable between the southwestern and the southeastern ends of the system, possibly explaining some of the conflicting estimates of strike slip displacements across the DFS.
In western Alaska, along the Togiak/Tikchik, Holitna, and Farewell faults, local supra-crustal tectonics may have been controlled by locally conflicting slip vectors. In southcentral Alaska, near the apex of the DFS arc, opposing senses of shear would have caused space problems. As a result of the approximately 5 Ma change in motion of the Pacific Plate (Cox and Engebretson, 1985; Kelley, 1993), increased mean stress at the arc apex (Schultz and Aydin, 1990) may have resulted in the uplift of the present day Denali massif (Plafker et al., 1992; Fitzgerald et al., 1993).
The sense of shear calculated using RMVs at any one point along the DFS is dependent upon the azimuth of the fault. Points of convergence and divergence (where right-lateral shear switches to left-lateral shear, or vice versa) are a function of changing backstop geometry as well as varying direction of plate convergence that existed along the DFS in southwest and southcentral Alaska. As the southern terranes were accreted to the Alaskan margin, their paleomagnetic rotations may have been at least partially determined by the prevailing regime: right-lateral or left-lateral. Paleomagnetic clockwise or anticlockwise rotations would therefore be, in part, a function of where a given block was accreted. In our model, the observed paleomagnetic data are interpreted by local accretion or passive rotation (an apparent rotation accquired during latitudinal translation of a given terrrane) rather than invoking a 40 degree oroclinal bend of much of southwestern Alaska.
In conclusion, we note again that ours is a model-dependant hypothesis, built upon a number of assumptions. However, the simplicity of the idea intrigues us. While unequivocal proof linking the anticlockwise block rotations of western Alaska with paleosinistral strike-slip shear coupling may be difficult to obtain, we speculate that future field studies along the western ends of the curved fault systems may help clarify the geotectonic development of western Alaska throughout the Cenozoic.
Acknowledgements
This paper greatly benefitted from discussion with Bob Grimm. Clem Chase, Peter Coney, Simon Peacock, and Myrl Beck. David Stone and Dave Scholl are thanked for thorough and supportive reviews. Kevin Kelley and Dave Engebretson kindly provided new stage poles before their work saw publication. Sue Selkirk and the Planetary Group at ASU are thanked for providing expertise and for the use of a color printer that yielded these snazzy diagrams. Support was from NSF grant DPP 8821937 and DPP-900995.
At Windy Point, Mt. Lemmon
Photo: M. Genenatti.
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