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Experimental clay modeling is used to study the geometry andevolution of deformation zones formed above basement faults.The nature of deformation is compared for reverse faults dipping30, 45, and 60°, vertical faults, and normal faults dipping60 and 75°. Basement faulting results in the formation of asheared triangular deformation zone that widens upward in thecover units. Most of the deformation is focused within a centralzone of secondary faults, which propagate upward and eventuallybreak through the entire section. Basal units exhibit steeperdips and higher fault densities, whereas stratigraphically higherunits exhibit gentler dips and lower fault densities. The widthof the deformation zone is generally greater for reverse faultswith low dips. The anticlinal axial surface typically dips in thesame direction as the synclinal axial surface for low- tomoderateanglereverse faults; however, the axial surfaces dip in oppositedirections for vertical faults and normal faults. Particle pathssuggest a convex upward rotation within the deformation zone,suggesting transfer of material across the extension of themasterfault. Reverse faults are associated with secondary faults withreverse and thrust separations. Vertical faults are associatedwithmostly vertical and reverse secondary faults, whereas normalfaults are generally associated with normal separation of mostsecondary faults. Therefore, the nature of secondary faults varieswith structural position, possibly within the same layer. Theseresults provide some key insights that are useful in the interpretationof macroscopic surface and subsurface basement–involved structures.
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  • Deformation and secondaryfaulting associated withbasement-involved compressional

    secondary faults. Therefore, the natureof secondary faults varieswith structural position, possibly within the same layer. These

    AUTHORS

    James F. Miller School of Geology andGeophysics, University of Oklahoma, Norman,Oklahoma; [email protected]

    James F. Miller received his B.S. degree in ge-ology from Pennsylvania State University and hisM.S. degree from the University of Oklahoma.He is currently a geoscientist at Conoco Phillips.

    Shankar Mitra School of Geology andGeophysics, University of Oklahoma, Norman,Oklahoma; [email protected]

    Shankar Mitra holds the Monnett Chair andProfessorship in Energy Resources at theUniversity of Oklahoma. He received his Ph.D.from Johns Hopkins University in 1977.

    ACKNOWLEDGEMENTS

    We thank David Stearns for providing part ofthe apparatus for the vertical uplift experiments.We also acknowledge the comments and sug-gestions of Rick Groshong, Julia Gale, an anon-ymous reviewer, and editor Gretchen Gillis,which improved the manuscript.The AAPG Editor thanks the following reviewersfor their work on this paper: Julia F. Gale,Richard H. Groshong, and an anonymousreviewer.results provide some key insights that are useful in the inter-pretation of macroscopic surface and subsurface basementinvolved structures.

    Copyright 2011. The American Association of Petroleum Geologists. All rights reserved.

    Manuscript received January 20, 2010; provisional acceptance April 6, 2010; revised manuscript receivedApril 22, 2010; final acceptance September 13, 2010.DOI:10.1306/09131010007and extensional structuresJames F. Miller and Shankar Mitra

    ABSTRACT

    Experimental clay modeling is used to study the geometry andevolution of deformation zones formed above basement faults.The nature of deformation is compared for reverse faults dip-ping 30, 45, and 60, vertical faults, and normal faults dipping60 and 75. Basement faulting results in the formation of asheared triangular deformation zone thatwidens upward in thecover units.Most of the deformation is focusedwithin a centralzone of secondary faults, which propagate upward and eventu-ally break through the entire section. Basal units exhibit steeperdips andhigher fault densities, whereas stratigraphically higherunits exhibit gentler dips and lower fault densities. The widthof the deformation zone is generally greater for reverse faultswith low dips. The anticlinal axial surface typically dips in thesame direction as the synclinal axial surface for low- tomoderate-angle reverse faults; however, the axial surfaces dip in oppositedirections for vertical faults and normal faults. Particle pathssuggest a convex upward rotationwithin the deformation zone,suggesting transfer ofmaterial across the extension of themasterfault. Reverse faults are associated with secondary faults withreverse and thrust separations.Vertical faults are associatedwithmostly vertical and reverse secondary faults, whereas normalfaults are generally associated with normal separation of mostAAPG Bulletin, v. 95, no. 4 (April 2011), pp. 675689 675

  • INTRODUCTION

    Basement-involved structures form chains of up-lifts in the frontal zones of many fold and thrustbelts (Rodgers, 1987). A number of structural mod-els (Figure 1) have been proposed for the geometryand evolution of these structures, including mod-els associated with reverse or thrust faults (Berg,1962; Spang et al., 1985; Blackstone, 1993; Brown,1993; Stone, 1993; Narr and Suppe, 1994), normalfaults and associated drape folds (Stearns, 1978),and vertical faults that decrease in dip within thesedimentary cover (Prucha et al., 1965).

    A general model that appears to explain thegeometry of the structures involves the dissipationof fault slip within a triangular deformation zone,also referred to as a trishear zone (Erslev, 1991;ErslevandRodgers, 1993), in the cover units. The detailedkinematics of deformation within the trishear zonehas also been modeled (Hardy and Ford, 1997;Allmendinger, 1998; Zehnder and Allmendinger,2000; Jin and Groshong, 2006). A number of vari-ations of this basic model have been developed anddemonstrated to apply to both surface and sub-surface structures (Schmidt et al., 1993; Mitra andMount, 1998; Johnson and Johnson, 2001). Thebasicmodel applies to fault-propagation folds formedin both extension and compression.

    However, the kinematicmodels do not addressthe details of the nature of deformation and thegeometry of secondary faults within the triangularzone. This information is important in interpretingsurface or subsurface structures with limited data.In addition to kinematic models, scaled experi-mental models provide useful information on theevolution of these structures. A number of exper-iments have been conducted in the past to studydifferent types of basement-involved structures.Clay and sandmodels have been used extensively tostudy basement structures associated with normalfaults (Sanford, 1959; Horsfield, 1977; Withjacket al., 1990;Withjack and Callaway, 2000; Jin andGroshong, 2006). Rock models deformed underrelatively high confining pressures (Logan et al.,1978; Stearns, 1978; Friedman et al., 1980) havebeen used to study the formation of folds related toreverse and normal faults. Although these studies676 Deformation and Secondary Faultinghave provided insights for the development ofstructures, they have used different materials andsetups for the normal- and reverse-fault experiments.

    In this article, we develop experimental mod-els to study the evolution of deformation zones forbasement faults with reverse, normal, and verticaldisplacements. To build on previous experimentalstudies, we have conducted our experiments forstructures involving reverse and normal faults withvarious dips and vertical faults using similar mate-rials and at the same scale. Because similarmaterialsand experimental approaches are used to study the

    Figure 1. Structural models of basement-involved structures.(A) Reverse fault model. (B) Normal fault model. (C) Upthrustmodel associated with vertical to very steeply dipping reversefaults.

  • ingmaterial because the fractures and faults formedin clay as a response to applied stress are similar to

    Table1.

    Summaryof

    Results

    ofExpe

    rimen

    talM

    odels

    Fault

    Type

    FaultD

    ip()

    Trish

    earAx

    ialS

    urface

    Dips

    ()

    Second

    aryFaults

    Trish

    earGe

    ometry

    Reverse

    30An

    ticlin

    al=76

    ;syn

    clina

    l=34

    .Bothdipin

    samedirectionas

    major

    fault.Synclin

    alaxisiscurved

    .Re

    versefaults

    with

    decreasin

    gdipne

    arthetop

    ofthecla

    y.Ap

    icala

    ngle

    =70

    (w

    ide)

    Reverse

    45An

    ticlin

    al=78

    ;syn

    clina

    l=30

    .Bothdipin

    samedirectionas

    major

    fault.

    Reversefaults

    with

    decreasin

    gdipne

    arthetop

    ofthecla

    y.Ap

    icala

    ngle

    =72

    (w

    ide)

    Reverse

    60An

    ticlin

    al=71

    ;syn

    clina

    l=43

    .Bothdipin

    samedirectionas

    major

    fault.

    Reversefaults

    with

    decreasin

    gdipne

    arthetop

    ofthecla

    y.Ap

    icala

    ngle

    =66

    (m

    edium)

    Vertical

    90An

    ticlin

    al=67

    ;syn

    clina

    l=71

    .Anticlinal

    andSynclin

    alaxes

    dipin

    oppo

    sitedirections.

    Verticaltoreversefaults

    near

    anticlin

    alaxis;

    reversefaults

    near

    synclin

    alaxis.

    Faults

    decrease

    indipne

    arthetopof

    thecla

    y.

    Apica

    lang

    le=42

    (narrow)

    Norm

    al75

    Anticlin

    al=66

    ;syn

    clina

    l=80

    .Anticlinal

    axisdips

    insame

    directionas

    major

    fault;synclin

    alaxisdips

    inop

    posite

    direction.

    Norm

    alfaults

    near

    anticlin

    alaxis;

    verticalfau

    ltsne

    arsynclin

    alaxis.

    Norm

    alfaults

    increase

    indipne

    arthetopof

    thecla

    y.

    Apica

    lang

    le=34

    (narrow)

    Norm

    al60

    Anticlin

    al=55

    ;syn

    clina

    l=67

    .Anticlinal

    axisdips

    insame

    directionas

    major

    fault;synclin

    alaxisdips

    inop

    positedirection.

    Norm

    alfaults

    near

    anticlin

    alan

    dsynclin

    alaxes.

    Faults

    increase

    indipne

    arthetopof

    thecla

    y.Ap

    icala

    ngle

    =58

    (m

    edium)that in natural rocks (Oertel, 1965; Reches, 1988),because the strain fields associated with faulting inclay are similar to those predicted by dislocationcalculations (Hilderbrand-Mittelfeldt, 1979), andbecause the faults are developed as discrete surfaceswhose geometry and evolution can be easily stud-ied (Cloos, 1968).

    The experimentswere conducted using a 5-cm(1.9-in.) layer of wet clay to represent the sedimen-tary units and aluminum blocks to represent thebasement. The clay had an average density of 1.40 g/cc and a shear strength value of 0.011 kg/cm2, asmeasured using a Torvane CL-600A meter distrib-uted by Soiltest. The clay did not contain additionallayering, and therefore, all deformational observa-tions relate to that of a homogeneous medium.Rigid basement blocks separated by faults with dipsof 30, 45, and 60 were used for the compressionalstructures, and dips of 60 and 75 were used fordifferent basement structures, the similarities anddifferences between the structures formed in com-pressional, extensional, and vertical uplift settingscan be easily analyzed (Table 1).

    This article provides detailed information onthe orientations, densities, and evolution of sec-ondary faults within the trishear zone. Some gen-eral conclusions about the similarities and differ-ences among the different end-member scenariosare discussed. In addition, the evolution of the tri-shear zones, including their expansion with pro-gressive deformation, and the concentration of de-formation in the central parts of these zones arestudied. Finally, the models are compared withsome natural structures to show how the character-istic features summarized from the models can beused to interpret surface and subsurface basement-involved structures.

    EXPERIMENTAL SETUP

    Scaled experimentalmodels (Hubbert, 1937) havebeen widely used to document and understand theevolution of structures and related faults. Wet clayhas been documented to be an appropriate model-Miller and Mitra 677

  • 678 Deformation and Secondary Faultingextensional structures. Vertical uplift was studiedusing blocks separated by 90 planes.

    The structures with basement fault dips from30 to 75 were modeled using a combination ofthree aluminum blocks, two of which acted asfootwall ramps and the third as the rising or subsidinghanging-wall fault block, depending on whethercompressional or extensional structures were beingmodeled (Figure 2A). To create displacements, thethree blocks were placed between an immobile walland a mobile wall driven by threaded rods at a con-stant velocity of approximately 3.5 * 103 cm/s.Vertical fault motion was modeled by placing ahydraulic piston under the mobile block that wasraised or lowered at a constant velocity (Figure 2B).The velocity was kept at a constant, 2.0 * 103 cm/s,with the aid of an electric motor connected to amechanical speed control. Clay models conductedat this scale and velocity satisfy the conditions for

    approximate scaling of large-scale natural structures(Withjack and Schlische, 2006).

    Boundary conditions for the compressional ex-periments involved a regional stress system (s), inwhich 1 is horizontal and s3 is vertical, where s1> s2 > s3. For the extensional and vertical upliftexperiments, the s1 and s3 stress components werereversed. The top surface and the two sides of theclay cake were free surfaces and therefore had noshearing stress occurring on them.

    The Bighorn Basin and other foreland basementcompressional structures typically formed in sedi-mentary packages of approximately 3000 to 4000m(984313,123 ft) in thickness overlying crystallinebasement. Similarly, extensional basement rifts suchas in the North Sea and Gulf of Suez involved pack-ages of approximately 2000 to 4000 m (656213,123 ft) above the basement. Based on studies ofscale models by Hubbert (1937) and subsequent

    Figure 2. (A) Experimental setup for structures involving reverse and normal faults. q is the fault dip and ranges between 30 and 75.The clay cake, which is 5 cm (1.9 in.) thick, is deformed above sliding rigid aluminum blocks driven by a motor. (B) Experimental setupfor vertical fault experiments. The clay cake is deformed above rigid aluminum blocks bounded by vertical faults. Vertical motion issimulated using a hydraulic piston located below the central block.

  • ive strfaceexperimental studies involving clay, this set of con-ditions corresponds to an appropriate downwardscaling of the dimensions of the structure and rhe-

    Figure 3. Progressive evolution of basement-involved compressshow the locations of the anticlinal (A) and synclinal (S) axial suology, and upward scaling of the strain rate for theWith progressive shortening, these faults increasein length and are rotated downward within thedeformation zone and are replaced by new faults

    ructure above a 45 basement fault. Dashed lines for each panels in the final deformed stage.formed directly above the tip of the basement block

    structures to correctly model the structures in ques-tion. Because rigid blocks are used to model base-ment, the structures producedmost closely resemblenatural structures involving a strong competencycontrast between the crystalline basement and thesedimentary cover.

    REVERSE BASEMENT FAULTS

    Reverse fault experiments with faults dipping 30,45, and 60 were conducted. The structural evo-lution is well represented by the 45 experiments(Figure 3). Differences resulting from a change infault dip are described for the 30 and 60 models(Figure 4).

    Initial deformation results in the formation of amonoclinal structure terminating at the tip of thebasement fault (Figure 3A). A significant amountof deformation occurs by the formation of a fewsecondary reverse faults within the lowest units.

    in the hanging wall (Figure 3B, C).The entire deformation occurs within an ini-

    tially upwardwidening triangular deformation zonebounded by two axial surfaces: an anticlinal axialsurface (A) tied to the tip of the basement hangingwall and a synclinal axial surface (S), which ter-minates at the tip of the basement footwall. Theaxial surfaces separate the unfolded units from thefolded units in the monoclinal structures. This de-formation zone widens with progressive deforma-tion, thereby incorporating more material, whichmigrates through the anticlinal and synclinal axialsurfaces. The positions of the anticlinal and syncli-nal surfaces shown at each stage are those in thefinal stage of deformation. Most of the faults arelocated within the central part of the deformationzone. In the late stages of deformation, as the throwon the upthrown block approaches or exceedsabout half the thickness of the overlying sediments,one or more of the faults break through to the sur-face (Figure 3D).Miller and Mitra 679

  • Figure 4. Variation in de-formation zone geometry forcompressive structures for (A)30, (C) 45, and (E) 60 base-ment faults. Note the greaterwidth of the deformation zonefor lower basement fault dips.The synclinal axial surface iscurved in the case of panel A.Panels B, D, and F show rosediagrams of the relative fre-quency of secondary faultlengths of different orientationsnormalized against the totallengths of all faults. A = anti-clinal axial surface; S = syncli-nal axial surface.680 Deformation and Secondary FaultingThe overall pattern of deformation is similarfor the 30 and 60 dipping faults (Figure 4). Theprimary differences are the dips of the secondaryfaults and width of the deformation zone as mea-sured by the apical angle between the anticlinal andsynclinal axial surfaces. The apical angle is higherfor the 30 and 45 models than for the 60 model.Regardless of the dip of the main basement fault, afairly large distribution of dips of the secondaryfaults always exists.

    Particle paths traced with respect to a fixedfootwall were determined for all experiments. Thecenters of circles inscribed within the clay weretraced onphotographs taken during regularly spaced

    increments of deformation. The particle pathswerethen determined in Arc GIS by connecting thesepointswith reference to fixedpoints in the footwall.

    The particle paths show a similar pattern for allof the models (Figure 5). The particle paths are ap-proximately parallel to the fault and have the samelength as the fault slip in the hanging wall away fromthe fault. Within and in the vicinity of the trisheardeformation zone, the particle paths tend to begenerally convex upward, suggesting a transfer ofmaterial from the anticlinal to the synclinal area.The particle paths are progressively shorter towardthe synclinal axial surface within the deformationzone.

  • Figure 5. Particle paths tracedwith respect to a fixed footwallfor compressive structures withbasement fault dips of (A) 30,(B) 45, and (C) 60. Gray zonesin this and other particle pathfigures represent the final trishearzone. Large circles show thefinal position of each particle.Miller and Mitra 681

  • ward the downthrown block. Progressively lessmovement occurs close to the synclinal axial surface.NORMAL BASEMENT FAULTS

    Normal fault experiments with faults dipping 60and 75 were conducted. The structural evolutionis shown for the 60 experiment (Figure 7). Initialnormal separation on the master basement faultresults in the formation of secondary normal faultsimmediately above the master fault, which prop-agate upward, and are progressively rotated tosteeper dips and translated down the master fault,being replaced by new faults forming immediatelyabove the master fault. Some of the earlier formednormal faults are rotated sufficiently to exhibit re-verse separation in their final position. After thefault throw approaches one-third of the thicknessof the cover units, one or more of the faults imme-diately above the master fault breaks through theentire sedimentary section. Most of the secondaryfaults dip between 70 in the direction of themasterfault and vertical dips, although a few faults withdips opposite to the master fault and apparent re-verse separations are also found. A set of antitheticVERTICAL BASEMENT FAULTS

    Vertical uplift on basement faults results in a mono-clinal structure with a broad anticlinal arch and anarrower synclinal arch (Figure 6). The zone of de-formation is bounded by a steeply dipping synclinalaxial surface tied to the top of the basement in thestationary block, and an anticlinal axial surface thatdips in the opposite direction, and is tied to the topof the basement in the upthrown block. Most ofthe deformation is localized along a central zone offaults, which originates at the tip of the basementmaster fault. These faults are initially close to ver-tical, but as they propagate upward, they developa convex upward reverse geometry, and rotatetoward the downthrown block. A few of the faultsbreak through the cover sediments, when the throwon the basement fault exceeds a third of the throwon the basement fault. The final geometry of thedeformation zone and the secondary faults approx-imates the geometry depicted in the upthrustmodel of Prucha et al. (1965).682 Deformation and Secondary FaultingParticle paths for normal faults show smallamounts of movement along a convex upward ro-tational path (Figure 9B, C). In the vicinity of thesynclinal axial surface, the particle paths show sig-nificant movement at angles slightly higher thanthe main fault.

    IMPLICATIONS FOR INTERPRETATION

    Many surface and subsurface examples of forelandbasementinvolved structures are characterized bypoor data, and details of the secondary faults andthe geometry of the main basement fault are com-monly amatter of controversy. Although some earlymodels used normal fault geometries to explainforeland basement structures, these structures arenowbelieved to bemostly related to thrust or high-angle reverse faults; however, it is still necessary todetermine the dips of the underlyingmaster faults.Because the experiments of normal, reverse, andvertical faulting were all conducted using similarmaterials, they enable a comparison of the natureof deformation in these different tectonic settings.Comparison of the characteristic features observedin the experiments and the natural structures canfaults dipping at a relatively low angle in an oppo-site direction to the master fault are formed in thecentral part of the cover units.

    The distribution of secondary fault orienta-tions for a vertical fault shows that most of thefaults show vertical or steep dips (7090) towardthe upthrown block (Figure 8A). For a master nor-mal fault dipping 75 (Figure 8B), both the de-formation zone and the central zone of faulting arenarrower than in the case of the 60 master fault(Figure 8C).

    The particle paths for a vertical fault tracedwithrespect to a fixed footwall are approximately parallelto the fault in the hanging wall outside the defor-mation zone (Figure 9A). Within the deformationzone, particles between the anticlinal axial surfaceand the extension of the fault into the cover unitsshow a slightly convex upward pattern sloping to-

  • be used to constrain the details of the geometry insubsurface interpretation.

    The experimental results also provide someimportant insights regarding deformation withinthe forelimb deformation zones. These observa-

    tions can be used to supplement data in surface orRegardless of whether the faults are normal,reverse, or vertical, deformation of the sedimen-tary cover occurs within a triangular zone. Unitscloser to the rigid basement undergo deformationover a narrower zone than the units higher up.Figure 6. Progressive evolutionof a basement involved struc-ture because of uplift on a ver-tical basement fault. The bound-ing axial surfaces (A and S)dip in opposite directions.This is reflected in generally steeper bed dips in the

    subsurface interpretations of these structures. basal units and lower dips in the upper units. In

    Figure 7. Progressive evolutionof an extensional basementstructure above a normal faultdipping 60. The anticlinal andsynclinal axial surfaces dip inopposite directions.Miller and Mitra 683

  • Figure 8. Variation in de-formation zone geometry forvertical faults (A) and normalfaults with dips of (C) 75 and (E)60. Note that in all cases, theanticlinal axial surface dips in anopposite direction to the syncli-nal axial surface. Panels B, D,and F show rose diagrams of thenormalized fault lengths for dif-ferent orientations of secondaryfaults. A = anticlinal axial surface;S = synclinal axial surface.684 Deformation and Secondary Faultingaddition, the density of secondary faults (the num-ber of faults per unit length measured normal tothe average fault dip) is higher in the basal units.Although deformation at any stage occurs over afairly wide zone, most of the deformation is local-ized within a central fault zone.

    Geometrically, this configuration is similar tothe heterogeneous trishearmodel of Erslev (1991).Zehnder and Allmendinger (2000) proposed ashape factor s to define the pattern of deformationin the trishear zone, with s =1, representing ho-mogeneous shear, and s > 1, representing heterog-eneous shear with the concentration of deforma-

    tion in the center of the trishear zone. Accordingto this definition, the configuration resembles acase where s > 1. However, both of these modelssuggest heterogeneous deformation within a con-stant area trishear zone, whereas the experimentssuggest that at least part of this geometry is relatedto an expanding zone of trishear deformation(Mitra and Mount, 1998).

    In all cases, one ormore of the secondary faultsonly break through the entire sedimentary coverwhen the structural relief of the basement is betweenone-third and one-half the thickness of the sedi-mentary package above it, provided the competency

  • Figure 9. Particle paths tracedwith respect to a fixed footwallfor (A) vertical uplift (90 fault)and extensional structures re-

    lated to fault dips of (B) 75 andcontrast between the basement and the sedimen-tary cover is high and similar to that in the ex-perimental setup.

    The width of the deformation zone as mea-suredby the apical angle between the axial surfacesand the shapes of the axial surfaces is dependent on

    (C) 60.Miller and Mitra 685

  • faults that curve to reverse geometries close to the

    surface.Normal faults exhibit secondary faultswithnormal separation except those that are rotated toorientations so that they show apparent reverseseparations. Therefore, the nature of secondaryfaults varieswith structural position, possiblywithinthe same layer, and the orientations of individualsecondary faults cannot always be used to inferwhether the underlying faults will be normal, ver-tical, or reverse faults. However, the overall dis-tribution of secondary faults and the dips of thebounding axial surfaces provide an indication of thedip of the underlying faults.

    COMPARISON WITH NATURAL STRUCTURES

    This section describes four natural examples ofbasement-involved structures with different inter-preted dips of the basement faults (Figure 10). Al-though there are differences between the naturalexamples and the experimental models, a numberof the characteristic features in the models are rep-licated in these examples, whereas others can bepredicted using the models.

    The SpringCreek anticline in the BighornBasin(Peterson, 1983) is an example of a basement-involved structure related to amoderately dippingbasement thrust fault. The cross section shown inFigure 10A is modified from Peterson (1983) andbased on surface data, and data from five projectedthe dip of the fault. Reverse or thrust faults withgentler dips generally result in a wider deforma-tion zone (high apical angles) andmore curvilinearbounding axial surfaces. The apical angles decreasewith increasing dip of the master fault. The anti-clinal axial surface usually dips in the same directionas the synclinal axial surface for low and moderatedipping reverse faults. The anticlinal axial surfacecommonly dips in a direction opposite to the syn-clinal axial surface for very high angle reverse faults,vertical faults, and normal faults.

    The separation associatedwith secondary faultsalso varies with structural position and the masterfault dip. Low-angle reverse faults show reverseor thrust separation on secondary faults. Verticalfaults exhibit secondary reverse faults or vertical686 Deformation and Secondary Faultingwells, one of which drills through the entire de-formation zone. The main thrust fault makes anangle of 50 with the footwall and 40 with thehanging wall, for an average basement fault angleof 45 (compare with Figure 4C). The forelimbdeformation zone is made up of a series of threeminor thrust faults with progressively decreasingdips from the anticlinal to the synclinal axial sur-face. Two out-of-syncline faults propagate from thesynclinal hinge toward the structure to accommo-date the layer-parallel shortening that occurs in thevicinity of the synclinal hinge. The bounding anti-clinal and synclinal axial surfaces dip in the samedirection as the main fault, and the angle betweenthe two surfaces is approximately 35. Because thestructural relief of the basement is less than one-fourth of the thickness of the sedimentary cover,all of the secondary thrust faults die out within theCretaceous sequence and do not propagate to thesurface.

    The Grass Creek structure (Figure 10B) in theBighorn Basin (Mitra and Mount, 1998) is asso-ciated with a reverse fault, making an angle of 69with basement in the footwall, and 55 with thehanging wall for an average angle of 62 (comparewith Figure 4E). The shallow dip of the hanging-wall block may be related to a change in the dip ofthe master fault at depth or the regional dip in thisarea related to a larger first-order structure. Surfacedips and the interpreted subsurface geometry sug-gest a progressive decrease in dip from the faultzone to the surface within a triangular deformationzone. No data are available to document the num-ber of secondary faults in the deformation zones;however, based on the experimental models, it canbe postulated that a number of these faults areprobably present. The angle between the bound-ing anticlinal and synclinal axial planes is approxi-mately 30 and similar to what might be predictedfrom the experimental model (Figure 4E). Becausethe basement throw is approximately or less thanone-half the thickness of the sedimentary coverabove it, no major faults crop out at the surfacewithin the deformation zone.

    The Owl Creek structure in central Wyomingshows considerable variation in geometry alongtrend, and a number ofmodels have beenproposed

  • for this structure. In the BoysenDam section in theWind River Canyon (Wise, 1963), the structuremost closely approaches an upthrust structure(Prucha et al., 1965), related to a steeply dippingmaster fault, with dips exceeding 70 (Figure 10C).The interpretation byWise (1963) suggests that themain fault branches into a number of shallowing-upward secondary reverse faults. However, closeto the anticlinal axial surface, a number of normalfaults are also present, most of which dip towardthe deformation zone. Themost important of theseis the Boysen fault, which also marks the approxi-mate location of the anticlinal axial surface (com-pare with Figure 8A). The anticlinal and synclinalaxial surfaces dip in opposite directions, and the

    angle between them is approximately 80. Becausethe structural relief is very high compared with thethickness of the sedimentary basin, the deforma-tion zone extends into the basement, and both themajor and a large number of the secondary faultsare exposed at the surface. The Golden fault in theSoda Lake area of the Colorado frontal zone, doc-umented by Berg (1962), shows a very similar ge-ometry and characteristic features.

    A surface structure from the Gulf of Suez(Figure 10D, modified fromWithjack et al., 1990;Jin and Groshong, 2006) provides a good exampleof an extensional fault-propagation fold developedabove anormal fault. The fault dips at approximately63 andhas cut through the already developing fold,

    Figure 10. Examples of basement involved structures related to different master fault geometries. (A) Spring Creek anticline from theBighorn Basin related to a moderately dipping reverse fault (modified from Peterson, 1983). (B) Grass Creek structure from the BighornBasin related to a steep reverse fault (modified from Mitra and Mount, 1998). (C) Owl Creek uplift from the Boysen Dam area related to avery steep master fault with an upthrust geometry (modified from Wise, 1963). (D) Extensional fault-propagation fold from the Gulf ofSuez exposed at the surface (modified from Withjack et al., 1990). Parts of the structure have been interpreted by Jin and Groshong(2006). Axial surfaces are interpreted in this study. PC = Precambrian; PKn = Nubian; Kum = Upper Cretaceous limestone and shale;Kuc = Upper Cretaceous and Paleocene limestone and shale; Te = Eocene limestone.Miller and Mitra 687

  • Particle paths show a convex upward path,

    ogy, v. 19, p. 617620, doi:10.1130/0091-7613(1991)suggesting movement across the projection of thefault from the anticlinal to the synclinal area. Thepaths indicate larger movements close to the anti-clinal axial surfaces for reverse faults and closer tothe synclinal axial surface for normal faults.

    The separation associated with the secondaryfaults also varies with structural position and themaster fault dip. Low-angle reverse faults show re-verse or thrust separation on secondary faults nearthe synclinal axial surface. Vertical faults exhibitresulting in dipping panels adjacent to the fault inboth the hanging wall and footwall. The structure isbounded by anticlinal and synclinal axial surfacesthat separate it from the regionally dipping strata.The structure is reconstructed on the basis of sparsesurface data. The experimental models (Figure 8C)suggest that a number of secondary normal faultsare likely to be present in both the hanging walland footwall within the trishear deformation zone.

    CONCLUSIONS

    Deformation of a weak sedimentary cover above afaulted rigid unit results in the formation of trian-gular deformation zones. Within the deformationzone, units closer to the rigid basement undergodeformation over a narrower zone than the unitshigher up. This is reflected in generally steeper beddips and higher fault densities in the basal units,and lower bed dips and lower fault densities in theupper units. The deformation zone in all caseswidenswith time instead of being confinedbetweenfixed and locked hinges, so that new material isincorporated into the deformation zone with on-going deformation. Although deformation at anystage occurs over a fairly wide zone, most of thedeformation is localized within a central zone ofupward propagating faults. The width of the de-formation zone and the shapes of the axial surfacesare dependent on the dip of the fault. Faults withgentler dips generally result in wider deformationzones andmore curvilinear bounding axial surfaces.The anticlinal axial surface dips in an opposite di-rection to the synclinal axial surface for verticalfaults and normal faults.688 Deformation and Secondary Faulting0192.3.CO;2.Erslev, E. A., and J. L. Rodgers, 1993, Basement-cover ge-

    ometry of Laramide fault-propagation folds: GeologicalSociety of America Special Paper 280, p. 125146.

    Friedman, M. R., H. H. Hugman III, and J. Handin, 1980,Experimental folding of rocks under confining pressure,part VIIIForced folding of unconsolidated sand and lu-bricated layers of limestone and sandstone: GeologicalSociety of America Bulletin, v. 91, p. 307312, doi:10.1130/0016-7606(1980)912.0.CO;2.

    Hardy, S., and M. Ford, 1997, Numerical modeling of tri-shear fault propagation folding: Tectonics, v. 16, p. 841854, doi:10.1029/97TC01171.

    Hildebrand-Mittlefehldt, N., 1979, Deformation near a faulttermination, part I: A fault in a clay experiment: Tectono-physics, v. 57, p. 131150, doi:10.1016/0040-1951(79)90145-8.

    Hodgson, R. A., 1965, Genetic and geometric relations be-tween structures in basement and overlying sedimentaryvertical or reverse faults, whereas normal faultsexhibit normal separation of all faults, except thosethat are rotated to orientations so that they showapparent reverse separations. Therefore, the natureof secondary faults varies with structural position,possiblywithin the same layer, and the orientationsof individual secondary faults cannot always be usedto infer whether the underlying faults will be nor-mal, vertical, or reverse faults. However, the dis-tribution of secondary faults, the orientations of theaxial surfaces and the angle between them, and thewidth of the deformation provide an indication ofthe dip of the overall structural geometry and thedip of the underlying major faults.

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