Wesley K. Wallace

Abstracts

Thrust-truncated detachment folds vs. fault-propagation folds in the northern Brooks Range

Wesley K. Wallace and Thomas X. Homza

In fold-and-thrust belts, anticlines that have steep to overturned forelimbs and are underlain by thrust faults are commonly interpreted to be fault-propagation folds. This interpretation is generally based only on a geometric similarity of these folds to popular models for fault-propagation folds, although geometry alone does not demonstrate that the folds meet the requirement that they formed during and as a result of propagation of a thrust fault up a ramp. A similar geometry can also result by growth of a detachment fold, which forms by buckling of a competent unit over an incompetent unit, followed by its truncation by a thrust fault. Distinction between thrust-truncated detachment folds and fault-propagation folds is necessary to interpret correctly the structural geometry and evolution of a fold-and-thrust belt. The two interpretations differ in how the folds evolve and in the detailed geometry of the fold core, a part of the fold that commonly is unknown and must be reconstructed based on the interpretation chosen. This distinction is important for petroleum traps because the two fold types have different implications for the amount and distribution of reservoir and seal within a fold trap, and for the development and preservation of porosity and permeability.

A regional structural transition provides insights into the correct interpretation of an important set of thrust-related folds in the Brooks Range. The structural boundary between the northeastern Brooks Range and the main axis of the northern Brooks Range is characterized by a transition from unfaulted detachment folds to detachment folds that have been truncated and displaced by thrust faults. North of this boundary in the northeastern Brooks Range, detachment folds have formed in the competent Lisburne Limestone above the incompetent Kayak Shale. These folds range from open to isoclinal, are generally upright, don't display a preferred sense of asymmetry, and only rarely are cut by thrust faults. Small-scale structures indicate that these folds originated as buckles and grew by rotation of limbs between fixed hinges. Folds within the same units south of the boundary display a consistent north-vergent asymmetry with steep to overturned anticlinal forelimbs, and are underlain by thrust faults. These folds commonly are assumed to be fault-propagation folds based on their geometry. However, the transition from detachment folds to the north suggests that they formed instead when thrust faults truncated steep anticlinal forelimbs as detachment folds tightened with increasing shortening. This interpretation differs from conventional fault-propagation fold models in that folding occurs over a flat without propagation of a fault up a ramp, fold-growth is by limb-rotation about fixed hinges, and thrust faulting after folding results not only in a hangingwall anticline, but also a footwall syncline.

Thrust-related folds with comparable geometry also characterize a stratigraphically lower competent-incompetent pair, the Kanayut Conglomerate and Hunt Fork Shale. The geometry and mechanical stratigraphy suggest that these folds also are thrust-truncated detachment folds, although unfaulted detachment folds are rarely preserved in the Kanayut.

In: Proceedings of the Alaska Geological Society Technical Conference, Anchorage, Alaska, April, 1998.

Multi-level duplex structures: A new structural interpretation of the Brooks Range along the Dalton Highway

Wesley K. Wallace (University of Alaska Fairbanks) and Thomas E. Moore (U.S. Geological Survey)

The northern Brooks Range along the Dalton Highway consists of a stack of duplexes separated by regionally north-dipping thrust faults. These duplexes compose two northward-tapered, internally deformed wedges, the far-displaced Endicott Mountains allochthon above and the North Slope parautochthon below. Within each wedge, competent stratigraphic units have shortened independently by folding and thrust faulting between zones of detachment that are located mainly in incompetent units. The basal detachment of each wedge cuts up-section in a forward direction (northward) to define a wedge geometry within which units dip regionally forward. This forward dip is steeper than the present erosion surface so that progressively younger units are exposed northward within each wedge. This differs from previous interpretations in which regional northward decreases in the age of exposed rocks were interpreted to coincide with large-displacement thrust faults, as in the Rockies of Alberta. An important implication of this new interpretation is that Cretaceous deposits of the foreland basin have not been thrust any significant distance beneath the Endicott Mountains allochthon, a relation that is supported by down-plunge projection of exposures in the Brooks Range.

The multi-level duplex interpretation can be extended to depth and to the southern Brooks Range. Seismic reflection data reveal shingled reflectors at depth that suggest duplex structures exist within the North Slope parautochthon to a depth of about 15 km beneath the northern Brooks Range and to at least 30 km south of the Doonerak window. South of the Doonerak window and at shallow depth, the Hammond subterrane may represent a southward continuation of the duplex wedge of the Endicott Mountains allochthon within a lower part of the stratigraphic section. The Hammond subterrane structurally overlies the Coldfoot subterrane (Schist Belt) on a north-dipping fault, but Coldfoot cannot represent depositional basement because it includes protolith that is younger than strata in much of the Hammond. The fault between the two may be either north- or south-directed depending on whether Coldfoot is restored paleogeographically north or south of the Hammond/Endicott Mountains subterrane.

These interpretations are based on mapping and geophysical data collected as part of the U.S. Geological Survey's Trans-Alaska Crustal Transect (TACT) project. The data and interpretations that are most related to this presentation are published in the following papers in the TACT special section in Journal of Geophysical Research (September, 1997, v. 102, no. B9):

References cited:

Wallace, W.K., Moore, T.E., and Plafker, G., Multistory duplexes with forward dipping roofs, north central Brooks Range, Alaska: p. 20,773-20,796.

Moore, T.E., Wallace, W.K., Mull, C.G., Adams, K.E., Plafker, G., and Nokleberg, W.J., 1997, Crustal implications of bedrock geology along the Trans-Alaska Crustal Transect in the Brooks Range, northern Alaska: p. 20,645-20,684.

Fuis, G.S., J.M. Murphy, W.J. Lutter, T.E. Moore, K.J. Bird, and N.I. Christensen, Seismic imaging and interpretation of the Brooks Range, arctic Alaska: Crustal-scale duplexing with deformation extending into the upper mantle: p. 20,873-20,896.

Moore, T.E., Aleinikoff, J.N., and Harris, A.G., Stratigraphic and structural implications of conodont and detrital zircon U-Pb ages from metamorphic rocks of the Coldfoot terrane, Brooks Range, Alaska: p. 20,797-20,820.

Alaska Geological Society newsletter, v. 27, no. 7 (presentation for March, 1998 meeting).

Detachment folds with fixed hinges and variable detachment depth, northeastern Brooks Range, Alaska

Thomas X. Homza and Wesley K. Wallace

Detachment anticlines in the northeastern Brooks Range accommodated displacement above a detachment by buckling of a competent unit over an incompetent unit. Meso- and microstructures in hinges and a lack of relict hinge structures in limbs suggest that these folds grew with fixed hinges. The structural thickness of the incompetent unit beneath the folds (detachment depth) varies from less than to greater than the stratigraphic thickness. A model in which incompetent unit thickness varies with fold area better approximates the geometry of the folds than does a more conventional constant-depth model. Additional discrepancies between modelled and observed incompetent unit thickness and field observations suggest non-plane strain and/or transport of material through the boundaries of the fold in the plane of the cross section.

The results of this study suggest a typical evolutionary sequence for detachment folds in the northeastern Brooks Range, which may be applicable elsewhere. Anticlines initiate as fixed-hinge buckle folds. Rapid initial increase in anticlinal cross-sectional area results in a decrease in incompetent unit thickness. Fold area begins to decrease with tightening beyond an interlimb angle of 90°. Decreasing fold area is accommodated through some combination of structural thickening of the incompetent unit, transport of solid or dissolved material out of the plane of section, transport of material through the boundaries of the fold in the plane of the cross section, and/or truncation by thrust faults.

1997, Journal of Structural Geology, v. 19, nos. 3-4 (special issue on fault-related folding), p. 337-354.

Multistory duplexes with forward dipping roofs, north central Brooks Range, Alaska

Wesley K. Wallace, Thomas E. Moore, and George Plafker

The Endicott Mountains allochthon has been thrust far northward over the North Slope parautochthon in the northern Brooks Range. Progressively younger units are exposed northward within the allochthon. To the south, the incompetent Hunt Fork Shale has thickened internally by asymmetric folds and thrust faults. Northward, the competent Kanayut Conglomerate forms a duplex between a floor thrust in Hunt Fork and a roof thrust in the Kayak Shale. To the north, the competent Lisburne Group forms a duplex between a floor thrust in Kayak and a roof thrust in the Siksikpuk Formation. Both duplexes formed from north-vergent detachment folds whose steep limbs were later truncated by south-dipping thrust faults that only locally breach immediately overlying roof thrusts. Within the parautochthon, the Kayak, Lisburne, and Siksikpuk-equivalent Echooka Formation form a duplex identical to that in the allochthon. This duplex is succeeded abruptly northward by detachment folds in Lisburne. These folds are parasitic to an anticlinorium interpreted to reflect a fault-bend folded horse in North Slope "basement", with a roof thrust in Kayak and a floor thrust at depth.

These structures constitute two northward-tapered, internally deformed wedges that are juxtaposed at the base of the allochthon. Within each wedge, competent units have been shortened independently between detachments, located mainly in incompetent units. The basal detachment of each wedge cuts up-section forward (northward) to define a wedge geometry within which units dip regionally forward. The forward dip reflects forward decrease in internal structural thickening by forward-vergent folds and hindward-dipping thrust faults.

1997, Journal of Geophysical Research, v. 102, no. B9 (special section on the USGS Trans-Alaska Crustal Transect), p. 20,773-20,796.

Style, controls, and timing of fold-and-thrust deformation of the Jago stock, northeastern Brooks Range, Alaska

Paige R. Peapples, Wesley K. Wallace, Catherine L. Hanks, Paul B. O'Sullivan, and Paul W. Layer

Involvement of the Devonian Jago stock in Cenozoic fold-and-thrust deformation of the northeastern Brooks Range illustrates the influence of a relatively small, isolated crystalline body on the mechanical stratigraphy and subsequent deformational behavior of an otherwise layered sedimentary package. The small size of the stock allowed it and the structurally coupled overlying Mississippian Kekiktuk Conglomerate to deform non-penetratively as a horse in a regional duplex, in contrast to the semi-ductile behavior of the nearby but much larger Okpilak batholith. Shear was localized in the upper part of the stock and the conglomerate due to partial detachment of the overlying Carboniferous Lisburne Group. North-vergent thrust-related folds formed in the mechanically layered Lisburne Group carbonates instead of the symmetrical, unfaulted detachment folds more typical of the region because an underlying regional detachment horizon in the Mississippian Kayak Shale is depositionally absent over the stock. Unusually competent contact-metamorphosed pre-Mississippian metasedimentary rocks were thrust over the stock and its cover because a ramp formed at the edge of the stock and cut up-section through the Lisburne Group due to the absence of Kayak Shale. A40Ar/39Ar age of foliated white mica indicates thrusting of the stock by 61 Ma; fission-track ages indicate cooling at ~44 Ma and ~28 Ma. These ages indicate a cooling history that implies ~11 km of unroofing since ~61 Ma, only ~1.5 km of which can be explained by the inferred duplex structure. The remaining ~9.5 km of unroofing is most likely due to sub-duplex structural thickening above a deep regional detachment.

1997, Canadian Journal of Earth Sciences, v. 34, no. 7, p. 992-1007.

Thrust-truncated detachment folds and their distinction from fault-propagation folds: Examples from the Brooks Range

Wesley K. Wallace and Thomas X. Homza

Workers in fold-and-thrust belts commonly presume that anticlines with steep to overturned forelimbs that are underlain by thrust faults are fault-propagation folds. This presumption frequently is based only on a geometric similarity to popular models for fault-propagation folds, rather than kinematic evidence that the fold actually formed during and as a result of fault propagation. A much older model for the evolution of fault-related folds, the "break-thrust" model, commonly is overlooked as an alternative. According to this model, the fold forms first as a buckle, and then is truncated by a thrust fault. Such folds differ fundamentally from fault-propagation folds in origin and kinematics.

Thrust-truncated detachment folds common in the northern Brooks Range of Alaska exemplify the "break-thrust" model. These folds have originated as buckles in a competent unit over an incompetent unit, without propagation of a fault up a ramp. The anticlines have grown by limb rotation, with a fixed hinge. The steep limb of the anticline is eventually truncated by a thrust fault. A fault-bend fold geometry can be superposed on the fold if the anticline is displaced over a ramp. These folds differ from the popular fault-propagation fold models in that folding is above a flat, non-propagating fault, fold-growth is by limb-rotation about a fixed hinge, and deformation in the footwall generally is reflected by a footwall syncline.

The origin of these folds is documented in the Brooks Range by the regional transition from unfaulted detachment folds to thrust-truncated detachment folds. Our observations indicate that geometry alone may not distinguish between thrust-truncated detachment folds and fault-propagation folds. The most reliable clues to origin as thrust-truncated detachment folds are the mechanical stratigraphy and the presence of footwall synclines. Other clues are a greater range in fold geometry than predicted by fault-propagation fold models, lack of preserved fault tips, and displacement-distance relations.

1996, Geological Society of America Abstracts with programs, v. 28, no. 7, p. A-239.

Differences between fault-propagation folds and detachment folds and their subsurface implications

Wesley K. Wallace and Thomas X. Homza

Fault-related anticlines with steep to overturned forelimbs commonly are assumed to be fault-propagation folds based on their geometric similarity to popular models. Examples in the northern Brooks Range suggest that some such folds may not have originated as a result of fault propagation but instead as detachment folds. The Brooks Range folds originated as buckles in a competent unit over an incompetent unit. Anticline growth was by rotation of the limbs about fixed hinges in the competent unit and by thickening of the incompetent unit, but without propagation of a fault up a ramp. Commonly, the steep forelimbs of these anticlines were subsequently truncated by thrust faults as shortening continued.

Folds formed as detachment folds differ fundamentally from fault-propagation folds. A detachment anticline is cored by an incompetent unit and, if the anticline has been truncated by a thrust fault, a syncline is commonly present in the footwall ramp. In contrast, a fault-propagation fold is more likely to form in dominantly competent rocks and to have a relatively little-deformed footwall ramp. The timing, distribution, and amount of strain also differs between a thrust-truncated detachment fold with rotating limbs and fixed hinges and a fault-propagation fold with fixed interlimb angles and some migrating hinges. Distinguishing between these two fold mechanisms thus has important implications for interpreting the amount and distribution of reservoir, seal, and trap, and the development and preservation of porosity and permeability.

1997, 1997 American Association of Petroleum Geologists Annual Convention, Dallas, Official Program, v. 6, p. A122.

Crustal implications of bedrock geology along the Trans-Alaska Crustal Transect in the Brooks Range, northern Alaska

Thomas E. Moore, Wesley K. Wallace, Charles G. Mull, Karen E. Adams, George Plafker, and Warren J. Nokleberg

Geologic mapping during the Trans-Alaska Crustal Transect (TACT) project along the Dalton Highway in northern Alaska provides new geologic data that require significant modification to existing stratigraphic, structural, and tectonic models for the north-vergent contractional Brooks Range orogen. These data indicate that the Endicott Mountains allochthon and an overlying higher allochthon (De Long Mountains terrane) of the northern Brooks Range form a northward-tapered wedge that terminates north of the range front in a triangle zone beneath Lower Cretaceous foreland-basin strata of the Colville basin. The basal thrust of the allochthonous rocks cuts down section in the hanging wall from Lower Cretaceous strata in the north to beneath Upper Devonian rocks on the northern limb of the Mt. Doonerak fenster (Amawk thrust) in the south. Footwall rocks everywhere along this distance are Permian and Triassic shale of the North Slope terrane, rather than Jurassic and Lower Cretaceous strata of the Colville basin as is shown in most models. Thickening of the Hammond terrane across the Mt. Doonerak antiform above the Amawk thrust and new age and stratigraphic data from the Hammond terrane indicate that the basal thrust continues to cut downward to the south beneath the Hammond terrane and probably daylights along its southern margin as a ductile shear zone. These relations indicate that the Hammond and Endicott Mountain allochthon together compose a single composite allochthonous succession that has been thrust northward a minimum of 90 km in the Early Cretaceous. The presence of parautochthonous rocks beneath the combined allochthon at least as far south as the Mt. Doonerak fenster and the character of the basal thrust as a brittle fault to the north and a ductile structure to the south suggest that the thrust represents part of the basal decollement that linked Early Cretaceous thrusting in the northern Brooks Range with coeval ductile deformation in the southern Brooks Range. The combined allochthon is folded and faulted by younger structures such as the Mt. Doonerak antiform that fission-track data indicate were formed in the latest Cretaceous and Tertiary. Seismic reflection/refraction data show that the younger structures root into a second, deeper decollement that climbs from a depth of about 30 km beneath the southern Brooks Range to less than 10 km beneath the Tertiary northeastern salient of the Brooks Range. Thus, the Brooks Range exposes (1) an Early Cretaceous thin-skinned deformational belt developed during arc-continent collision and (2) a mainly Tertiary thick-skinned orogen that likely represents the northward continuation of the Rocky Mountains orogen. A down-to-the-south zone of both ductile and brittle normal faulting along the southern margin of the Brooks Range probably formed in the mid-Cretaceous by extensional exhumation of the Early Cretaceous contractional deformation.

1997, Journal of Geophysical Research, v. 102, no. B9 (special section on the USGS Trans-Alaska Crustal Transect), p. 20,645-20,684

Origin of arcuate structural trends at the boundary between the Siberian platform and accreted terranes, Chersky Range, northeastern Yakutia

Wesley K. Wallace, Vladimir Oxman, Andrei Prokopiev, and Leonid Parfenov

A regional westward-convex bend in structural trends in the Chersky Range of northeastern Yakutia is defined by northwest-trending structures to the southwest and northeast-trending structures to the northeast. In this region, lower Paleozoic carbonate platform terranes have been juxtaposed with upper Paleozoic-lower Mesozoic basinal deposits along the outer edge of the collapsed passive continental margin of the Siberian platform in a complex polyphase deformation history. The bend corresponds with a major reentrant in the continental margin, but it is unclear to what extent structures formed in their present orientation or were oroclinally bent. Field work during the summers of 1993 and 1994 built on previous work by Russian workers to provide some new insights into the origin of structures of different orientations.

Two major structural events are recorded in the Selennyakh Range, in the northeast-striking limb of the bend. A northwest-vergent fold-and-thrust belt formed first, with penetrative deformation, metamorphic grade, and generations of structures increasing toward the southeast. Later northeast-southwest shortening refolded the earlier structures, and probably resulted in strike-slip reactivation of appropriately oriented faults. The two different shortening directions are most obviously reflected by widespread steeply plunging folds that reflect fold interference.

A similar two-stage history is observed in the Tas Khayakhtakh, in the northwest-striking limb of the bend and near its hinge. Steeply plunging folds also are the most distinctive feature of the deformation here, but the shortening directions responsible for their origin are not so clear.

These observations suggest that multiple events with very different shortening directions, coupled with the irregular shape of the Siberian continental margin, probably are the major cause of the regional arcuate structural trends. Northwest-vergent early structures probably resulted from oblique collision of terranes with the irregular Siberian continental margin. Later northeast-southwest shortening may represent collision of Arctic Alaska with the Siberian continental margin and the terranes accreted to it, which resulted in formation of the south Anyui suture.

1995, Geological Society of America Abstracts with Programs, v. 27, no. 5, p. 82-83.

Plate tectonic model for the neotectonics of northern Alaska

John W. Whitney and Wesley K. Wallace

Several lines of geologic evidence indicate a slow rate of tectonic activity in northern Alaska. Significant seismicity, mainly extensional, has occurred in the Seward Peninsula of NW Alaska and just to the east, in the Yukon-Kuskokwim basin. Young basaltic volcanism and normal faulting have occurred on the Seward Peninsula and high heat flow was measured in wells in Norton Sound. Diffuse seismicity occurs in and north of the northeastern Brooks Range in Alaska and adjacent Yukon Territory. Young folds and faults are present in the coastal plain and offshore north of the northeastern Brooks Range. High elevations and large volumes of young sediment derived by erosion reflect continuing tectonic activity in the northeastern Brooks Range, in contrast with the rest of the range.

Two, possibly related, plate-tectonic phenomena may cause this neotectonism. The intersection of the Pacific, North American, and Eurasian plates is complex and likely is a zone of other, much smaller plates, such as the hypothesized Okhotsk and Bering plates. Triple-junction analysis suggests that the Seward Peninsula neotectonics could reflect incipient or failed rifting that defines the boundary between the Bering and North American plates.

Since mid-Mesozoic time, numerous allochthonous terranes have collided with and accreted to southern Alaska, including the presently colliding Yakutat block. Convergence of this continental block with Alaska has led to the high mountains of the Chugach-St. Elias Range, as well as active fold-and-thrust structures and considerable seismicity. Partial coupling between the Pacific and North American plates in Alaska, facilitated by collision of terranes such as the Yakutat block, has led to accommodation of a small part of the convergence within the continent. This convergence has been transferred northward in blocks bounded to the east by the Fairweather, Denali, Tintina, and Richardson right-lateral strike-slip fault systems, and to the north by complex systems of thrust and strike-slip faults. At its northern limit, this convergence has led to continued northward propagation of the fold-and-thrust belt in the northeastern Brooks Range into the Arctic Ocean basin. This pattern of Cenozoic deformation in Alaska appears to be a smaller-scale, mirror image of the indenter tectonics resulting from the India-Eurasia collision.

1995, Geological Society of America Abstracts with Programs, v. 27, no. 5, p. 84.

Geometric and kinematic models for detachment folds with fixed and variable detachment depths

Thomas X. Homza and Wesley K. Wallace

Detachment folds are defined by competent rock units and are cored by incompetent units deformed internally above a detachment horizon. We have developed two geometric models to constrain possible geometries and kinematic paths for ideal detachment folds. The models each independently relate fold geometry to shortening and to detachment depth. Model assumptions include plane-strain, constant competent bed-length, constant cross-sectional area, chevron fold geometry, and no bed-parallel shear outside the fold. Detachment depth is constant in one model but may vary in the other, thus allowing evaluation of the implications for fold geometry and kinematics of fixed vs. variable detachment depth.

Detachment folds formed above a detachment unit of constant thickness (constant detachment depth) must be initially symmetrical and cannot grow with fixed hinges (fixed arc-length) or a self-similar geometry. Detachment folds formed above a detachment unit of variable thickness (variable detachment depth) must also be initially symmetrical, but any given fold geometry can have a wide range of possible initial and final detachment depths. Kinematic paths for folds with fixed hinges (fixed arc-length), migrating hinges (variable arc-length), and self-similar geometries are all possible if detachment depth varies. The change in detachment depth during deformation can be determined using the variable detachment depth model if either initial or final detachment depth is known.

The models demonstrate a wider range of variability in ideal detachment fold geometry and kinematics, particularly for the variable-depth model, than for ideal fault-bend and fault-propagation folds. This variability limits the usefulness of simple geometric models for reconstructing the geometry of natural detachment folds. Balancing cross sections over a sufficiently large area and evaluating strain may compensate for these limitations. Geometric models are very useful to assess the geometric and kinematic implications of specific assumptions, and can be used to test the validity of those assumptions for natural detachment folds whose geometry is well constrained.

1995, Journal of Structural Geology, v. 17, no. 4, p. 475-588.

Multiple episodes of Cenozoic denudation in the northeastern Brooks Range: Fission track data from the Okpilak batholith

Paul B. O'Sullivan, Catherine L. Hanks, Wesley K. Wallace, and Paul F. Green

The northeastern Brooks Range of Alaska is a complex Mesozoic to Cenozoic northward-verging fold and thrust belt. In response to regional compression, shortening in the upper crust has occurred through the duplexing of thrust sheets and formation of associated fault-bend folds. Apatite and zircon fission-track data from the Okpilak batholith and adjacent sedimentary rocks exposed within the northeastern Brooks Range provide new constraints on the timing, magnitude, and rate of cooling of these thrust sheets as they were rapidly denuded in response to uplift during Cenozoic time. Fission-track results indicate that a previously recognized episode of Paleocene cooling was followed by at least two younger episodes of rapid cooling during Middle Eocene and Late Oligocene time. The two younger episodes of rapid cooling are interpreted to reflect denudation in response to uplift resulting from Cenozoic thrusting and related folding. As a result of structural thickening, up to 8 km of material was eroded from the top of the batholith between ~41-45 Ma (Middle Eocene). Renewed shortening and emplacement of an underlying thrust sheet at ~25 Ma (Late Oligocene) resulted in at least 2 km of uplift and erosion of sedimentary rocks immediately north of the batholith. These results suggest that, even though Paleocene uplift and erosion may have occurred across the northeastern Brooks Range, the major episode of thrust faulting, responsible for structural emplacement of the batholith into its present position and kilometer-scale denudation, most likely occurred during Middle Eocene time.

1995, Canadian Journal of Earth Sciences, v. 32, no. 8, p. 1106-1118.

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