Wesley K. Wallace

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

Abstract 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 bedlength, 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 one fold geometry can have a range of possible initial and final detachment depths. Kinematic paths for folds with fixed hinges (fixed arc-lengths), migrating hinges (variable arc-lengths), and selfsimilar 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 the geometry and kinematics of ideal detachment folds, particularly for the variable-depth model, than is the case 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 sufficient area and evaluating strain may compensate for these limitations.

Structural provinces of the northeastern Brooks Range, Arctic National Wildlife Refuge, Alaska

The dominant Cenozoic structures of the northeastern Brooks Range are anticlinoria with cores of sub-Mississippian rocks, reflecting a regional north-vergent duplex with a floor thrust in the sub-Mississippian sequence and a roof thrust in the Mississippian Kayak Shale. The number of horses forming each anticlinorium and the structural style of the overlying Mississippian and younger cover sequence varies regionally, providing a basis for dividing the northeastern Brooks Range into structural provinces. In the western province, each anticlinorium contains a single horse, and shortening above the Kayak Shale was accommodated mainly by detachment folds. To the north in the Sadlerochit Mountains, the Kayak Shale is depositionally discontinuous and rocks elsewhere separated b this detachment deformed together. In the eastern province, each anticlinorium contains multiple horses, and shortening above the Kayak Shale was accommodated largely by thrust duplication of Mississippian through Triassic rocks. In the narrow central province, the Devonian Okpilak batholith was detached from its roots, internally shortened along shear zones and by penetrative strain, and transported northward. Because the Kayak Shale is locally absent, the Mississippian and younger cover sequence deformed in part penetratively along with the batholith. East-northeast trends formed where sub-Mississippian rocks were not involved in deformation, and probably are normal to the direction of Cenozoic tectonic transport. East trends formed where sub-Mississippian rocks were involved in deformation, and probably reflect a pre-Mississippian structural grain. At any given location, east trends generally post-date east-northeast trends, reflecting a drop over time of the basal detachment into sub-Mississippian rocks. INTRODUCTlON Well-exposed structures in the northeastern Brooks Range fold and thrust belt (Figure l) may provide insights into the evolution of similar structures elsewhere in the world, as well as offering clues to the factors that control their geometry. In addition, the northeastern Brooks Range includes the nearest well- exposed analogs to structures that may underlie the Arctic coastal plain immediately to the north, the most promising area for onshore hydrocarbon exploration remaining in North America. The stratigraphy of the northeastern Brooks Range has had a significant influence on the geometry of structures formed during deformation, as is true in many other fold and thrust belts (Woodward and Rutherford, 1989). The interlayering of strata of differing thicknesses, lithologies, and structural competencies has resulted in a structural stratigraphy in which particular stratigraphic intervals display a specific structural style. Several different structural provinces can be defined in the northeastern Brooks Range based upon lateral variations in structural style (Figure 2). These lateral variations commonly correspond with lateral variations in stratigraphy. Recent discussions of the structural geometry and evolution of the northeastern Brooks Range have dealt mainly with the western part of the region (for example, Kelley and Foland, 1987; Leiggi, 1987; Oldow et al., 1987a). In this paper, we illustrate the variations in structural geometry that exist over a much larger region, and argue that lateral changes in stratigraphy influence the style of deformation. Our objective is to provide a regional overview of the structure of the northeastern Brooks Range, and to interpret the influence of variations in stratigraphy on the structural geometry of the fold and thrust belt. This overview and interpretation are based primarily on our own detailed geologic studies throughout the northeastern Brooks Range, complemented by studies by graduate s udents at the University of Alaska on specific structural problems. However, End_Page 1100------------------------------ we do not intend to do more than summarize the results of these studies here. Rather, we seek in this paper to provide a conceptual and testable regional structural interpretation that will serve as a framework for future, more detailed papers and for further detailed structural and stratigraphic studies.

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

Abstract 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 dg. 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.

Crustal implications of bedrock geology along the Trans‐Alaska Crustal Transect (TACT) in the Brooks Range, northern Alaska

Geologic mapping of the Trans-Alaska Crustal Transect (TACT) project along the Dalton Highway in northern Alaska indicates that the Endicott Mountains allochthon and the Hammond terrane compose a combined allochthon that was thrust northward at least 90 km in the Early Cretaceous. The basal thrust of the combined allochthon climbs up section in the hanging wall from a ductile shear zone in the south through lower Paleozoic rocks of the Hammond terrane and into Upper Devonian rocks of the Endicott Mountains allochthon at the Mount Doonerak antiform, culminating in Early Cretaceous shale in the northern foothills of the Brooks Range. Footwall rocks north of the Mount Doonerak antiform are everywhere parautochthonous Permian and Triassic shale of the North Slope terrane rather than Jurassic and Lower Cretaceous strata of the Colville Basin as shown in most other tectonic models of the central Brooks Range. Stratigraphic and structural relations suggest that this thrust was the basal detachment for Early Cretaceous deformation. Younger structures, such as the Tertiary Mount Doonerak antiform, deform the Early Cretaceous structures and are cored by thrusts that root at a depth of about 10 to 30 km along a deeper detachment than the Early Cretaceous detachment. The Brooks Range, therefore, exposes (1) an Early Cretaceous thin-skinned deformational belt developed during arc-continent collision and (2) a mainly Tertiary thick-skinned orogen that is probably the northward continuation of the Rocky Mountains orogenic belt. 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.

Fracture paragenesis and microthermometry in Lisburne Group detachment folds: Implications for the thermal and structural evolution of the northeastern Brooks Range, Alaska

The distribution, character, and relative age of fractures in detachment folded Mississippian–Pennsylvanian Lisburne Group carbonates and overlying Permian–Triassic clastic rocks in the northeastern Brooks Range of northern Alaska provide important clues to the thermal and deformational sequence experienced by these rocks. Although paleothermal indices in the host rock limit the conditions of folding to temperatures equal to or less than 280C, field and petrographic relationships suggest that different fracture sets formed at different times during the deformational history of the rocks, providing a record of deformation under changing temperature and pressure conditions. These rocks probably initially entered the oil-generation window (80–140C) during the Early Cretaceous formation of the Colville basin via thrust loading by the Brooks Range to the south. Regional fractures formed during this time as a result of high pore pressures and low in-situ differential stresses. Shortening in these rocks related to the advancing northeastern Brooks Range fold and thrust belt began during the Late Cretaceous to early Tertiary. Early phases of detachment folding were via flexural slip, with associated fracturing. With continued shortening and growth of detachment folds, structural thickening resulted in deeper burial of the bottom part of the deforming wedge. Early fold-related fractures were subsequently overprinted by penetrative strain during peak folding at temperatures of approximately 280C. Continued shortening resulted in uplift and erosional unroofing at approximately 60 Ma. Late fold-related fractures formed at about 150C. Subsequent uplift of the thickened wedge through 60C occurred after about 25 Ma. Late pervasive extension fractures related to unroofing and/or regional stresses formed at relatively shallow depths and low temperatures, overprinting all the earlier fractures and penetrative structures.

Competent unit thickness variation in detachment folds in the Northeastern Brooks Range, Alaska: geometric analysis and a conceptual model

Detailed measurements of six map-scale detachment folds in the northeastern Brooks Range, Alaska, document significant variations in structural thickness of the competent Lisburne Limestone. Thickness variations occur mainly by parasitic folding and penetrative strain, and may be controlled by differences in mechanical stratigraphy, relative thicknesses of the competent and incompetent units, and structural relief of the underlying basement. The geometry of these detachment folds is not consistent with key assumptions of existing geometric and kinematic models, such as constant competent unit thickness or constant detachment depth. We propose a new model that allows both competent and incompetent unit thicknesses to vary throughout the fold. This model allows a more realistic geometric description of some natural detachment folds than previous models, but the number of variables makes unique reconstruction of specific fold geometries or kinematics difficult. Comparison of models with natural folds demonstrates that significant error can result if shortening estimates are based on models that incorrectly assume constant competent unit thickness or constant detachment depth. Use of surveying techniques to quantify map-scale fold geometry can provide better reconstructions of fold geometry, better shortening estimates, and information to constrain kinematic reconstructions.

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

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 upsection forward (northward) to define a wedge geometry within which units dip regionally forward. These dips reflect forward decrease in internal structural thickening by forward vergent folds and hindward dipping thrust faults.

Out-of-sequence, basement-involved structures in the Sadlerochit Mountains region of the Arctic National Wildlife Refuge, Alaska: Evidence and implications from fission-track thermochronology

Fission-track thermochronology and structural analysis set limits on the timing and nature of structural development of the Sadlerochit Mountains, along the southern edge of the coastal plain in the Arctic National Wildlife Refuge (ANWR) of northeastern Alaska. The Sadlerochit Mountains are the northernmost part of the north-vergent Brooks Range fold-and-thrust belt and lie close to the Arctic continental margin. Thermochronology results indicate that sedimentary rocks exposed within Ignek Valley, south of the Sadlerochit Mountains, were subjected to two episodes of rapid cooling from elevated paleotemperatures at ca. 45 Ma and at some time since ca. 31 Ma, whereas similar-aged rocks exposed along the northern flank of the Sadlerochit Mountains cooled rapidly at ca. 45 Ma and ca. 27 Ma. Combined with five additional analyses from the Beli Unit #1 well, located northwest of the Sadlerochit Mountains, the thermochronology results indicate that the Sadlerochit Mountains region was progressively heated during Late Cretaceous through middle Eocene time, after which two major episodes of rapid cooling occurred in the middle Eocene at ca. 45 ± 3 Ma (±2σ) and in the late Oligocene at ca. 27 ± 2 Ma (±2σ). These episodes of rapid cooling are interpreted to have occurred in response to kilometer-scale erosional denudation resulting from rapid (≤5 m.y.) uplift due to structural thickening during the emplacement of thrust sheets in a basement-involved duplex. Initially, at least one thrust sheet was probably emplaced to the north of the Sadlerochit Mountains at ca. 45 Ma. Subsequently, at ca. 27 Ma, (1) the Sadlerochit Mountains thrust sheet was probably emplaced out of sequence behind the earlier-emplaced thrust sheet(s), and (2) basement-involved deformation formed structures beneath the coastal plain to the north. Both of these events occurred far within the continent, >1200 km from the southern Alaska convergent plate boundary. These results indicate that maximum burial, and hence peak hydrocarbon generation, occurred prior to middle Eocene time, before the formation of potential traps in and immediately north of the Sadlerochit Mountains. However, (1) hydrocarbons generated from these rocks could have migrated updip into existing stratigraphic traps prior to structural deformation, and (2) hydrocarbons generated later in more distal parts of the basin could have migrated updip into subsurface structures formed in middle Eocene and late Oligocene time north of the Sadlerochit Mountains and along strike to the east and west.

Yakataga Fold‐and‐Thrust Belt: Structural Geometry and Tectonic Implications of a Small Continental Collision Zone

Collision of the Yakutat terrane with southern Alaska created a collisional fold-and-thrust belt along the Pacific―North America plate boundary. This southvergent fold-and-thrust belt formed within continental sedimentary rocks but with the seaward vergence and tectonic position typical of an accretionary wedge. Northward exposure ofprogressively olderrocks reflects that the fold-and-thrust belt forms a southward-tapered orogenic wedge that increases northward in structural relief and depth of erosion. Narrow, sharp anticlines separate wider, flat-bottomed synclines. Relatively steep thrust faults commonly cut the forelimbs of anticlines. Fold shortening and fault displacement both generally increase northward, whereas fault dip generally decreases northward. The coal-bearing lower part of the sedimentary section serves as a detachment for both folds and thrust faults. The folded and faulted sedimentary section defines a regional south dip of about 8°. The structural relief combined with the low magnitude of shortening of the sedimentary section suggest that the underlying basement is structurally thickened. I propose a new interpretation in which this thickening was accommodated by a passive-roof duplex with basement horses that are separated from the overlying folded and thrust-faulted sedimentary cover by a roof thrust with a backthrust sense of motion. Basement horses are ∼7 km thick, based on the thickness between the inferred roof thrust and the top of the basement in offshore seismic reflection data. This thickness is consistent with the depth of the zone of seismicity onshore. The inferred zone of detachment and imbrication of basement corresponds with the area of surface exposure of the fold-and-thrust belt within the Yakutat terrane and with the Wrangell subduction zone and arc farther landward. By contrast, to the west, the crust of the Yakutat terrane has been carried down a subduction zone that extends far landward with a gentle dip, corresponding with a gap in arc magmatism, anomalous topography, and the rupture zone of the 1964 great southern Alaska earthquake. I suggest that, to the east, detachment and imbrication of basement combined with coupling in the fold-and-thrust belt allowed the delaminated dense mantle lithosphere to subduct with a steeper dip than to the west, where buoyant Yakutat terrane crust remains attached to the subducted lithosphere. According to this interpretation, the Wrangell subduction zone is lithosphere of the Yakutat terrane, not Pacific Ocean lithosphere subducted beneath the Yakutat terrane. The Pacific-North America plate boundary would be within the northern deformed part of the Yakutat terrane, not along the boundary between the undeformed southern part of the Yakutat terrane and oceanic crust of the Pacific Ocean. The plate boundary is an evolving zone of distributed deformation in which most of the convergent component has been accommodated within the fold-and-thrust belt south of the northern boundary of the Yakutat terrane, the Chugach-St. Elias thrust fault, and most of the right-lateral component likely has been accommodated on the Bagley Icefield fault just to the north.

Character, relative age and implications of fractures and other mesoscopic structures associated with detachment folds: an example from the Lisburne Group of the northeastern Brooks Range, Alaska

ABSTRACT Fractures and other mesoscopic structures formed at different times during the evolution of individual detachment folds in Lisburne Group carbonates of the northeastern Brooks Range. These structures provide clues to the mechanism of folding, the conditions under which folds evolved and the paragenesis of fractures in the fold-and-thrust belt as a whole. The earliest fractures strike NNW and probably represent orogen-normal extension fractures that developed in the foreland basin in advance of the fold-and-thrust belt. These rocks and fractures were later incorporated into the thrust belt, where they were thrust-faulted and folded. Later fractures, strained markers and dissolution cleavage developed during detachment folding as a result of flexural slip and homogeneous flattening. Fracturing associated with flexural slip occurred early in the development of folds. These early fractures were commonly overprinted or destroyed by ductile strain as later homogeneous flattening accommodated additional shortening. This penetrative strain was in turn overprinted by late extension fractures that formed during flexural slip in the waning phases of folding or after folding due to unroofing of the orogenic wedge. Early fracturing, overprinting by ductile structures and subsequent later fracturing in detachment-folded Lisburne Group emphasizes the importance of understanding the unique character and history of each fold-and-thrust belt in a successful hydrocarbon exploration effort. In particular, the mechanical stratigraphy and conditions of deformation play an important role in the type of fold that develops, the fold mechanisms that are active and the subsequent distribution and character of fractures and other mesoscopic structures. 1 Current address: National Park Service, Denali National Park, Alaska USA End_Page 121------------------------