Lecture 15 Fold-Thrust Belts, and the NJ Ridge and Valley Thrust System Earth Structure (2 nd Edition), 2004 W.W. Norton & Co, New York Slide show by Ben van der Pluijm WW Norton; unless noted otherwise
Mt. Kidd, Alberta Canadian Rocky Mountain Front Ranges Fig. 18.1 The folds affecting the Paleozoic strata exposed on these cliffs developed in association with transport on the Lewis and Rundle Thrusts. View is to the north. EarthStructure (2nd ed) 2
Continent-continent Collision Fig. 18.3 Regional cross sections depicting stages of foldthrust belt development first during convergent-margin tectonism and then during continent-continent collision. Thick-skinned tectonics involves slip on basementpenetrating reverse faults that uplifts basement and causes monoclinal forcedfolds ( drape folds ) to develop in the overlying cover. Time Thin-skinned tectonics involves folding and faulting above a mid-crustal detachment. EarthStructure (2 nd ed) 3
Continent-continent Collision Fig. 18.3 Passive margin strata are deposited on thinned continental crust In this sketch, basins on opposite sides of the margin do not have the same shape, because the basement beneath underwent different amounts of stretching. The so-called lower-plate margin underwent more stretching, whereas the so-called upper-plate margin underwent less stretching. EarthStructure (2 nd ed) 4
EarthStructure (2 nd ed) 5
Continent-continent Collision Fig. 18.3 Onset of convergence An accretionary prism develops that verges towards the trench, and a backarc fold-thrust belt forms cratonward of the volcanic arc and verges towards the upper-plate craton. EarthStructure (2 nd ed) 6
Continent-continent Collision A fold-thrust belt is geologic terrane in which upper-crustal shortening is accommodated by development of a system of thrust faults and related folds that form in the foreland on both sides of an orogen. Slivers of obducted ocean crust may separate lower-plate rocks from the metamorphic hinterland of the orogen and define the suture between the two plates. EarthStructure (2 nd ed) 7
Thrust System Mechanics and Geometry Work in the 1960s through 1980s led to a refinement of the geometric and kinematic rules governing the shape and evolution of folds formed as a result of the development of and slip on thrusts faults, and provided geologists with insight into the fundamental issue of how, why, where, and when fold-thrust belts developed.
How do thrust systems work? Gravity sliding models became very popular as a cause for thrusting, and in the 1960s, most structural geologists envisioned that development of fold-thrust belts occurred as thrust sheets glided down an incline created by uplift of the hinterland during orogeny But seismic-reflection data from petroleum exploration of many fold-thrust belts shortly thereafter provided showed basal detachments beneath almost all classic fold-thrust belts dip toward the hinterland, not the foreland! To account for this, some structural geologists suggested gravity spreading, as an alternative model when the thickened crust of an orogen collapses and spreads laterally under its own weight, much like a continental ice sheet spreads away from the region where snow accumulates. EarthStructure (2 nd ed) 9
How do thrust systems work? ii The next step in formulating an understanding of how fold-thrust belts develop came in the 1970s, when researchers began to study laboratory models that simulated the development of the belts. Models involving the formation of a sand wedge building in front of a plow proved to be particularly informative because sand is a Coulomb material, meaning an aggregate composed of grains that can frictionally slide past one another, and at the scale of a mountain range, rock of the upper crust behaves EarthStructure (2 nd ed) 10
Thrust Paradox: Fluid Pressure Pushed from rear, a thrust sheet on dry rock is crushed before overcoming frictional resistance But, high fluid pressure at the basal detachment lowers the effective stress and allows the thrust sheet to move under very small applied load. σ n is stress resulting from horizontal loading, σ f is frictional resistance, σ l is boundary load at end of thrust sheet, and P H2O is pore pressure. EarthStructure (2 nd ed) 11
How do thrust systems work? iii Two sources of stress drive the development of a Coulomb wedge. 1) The horizontal boundary load (the horizontal push directed toward the foreland) 2) The gravitational potential energy of the foreland topographic slope As the backstop moves toward the foreland, the wedge contracts and thickness internally with folds, faults, and grain-scale distortion and, as a consequence, its surface slope increases. When the wedge reaches a certain critical taper angle (φ c ) (defined by the surface slope angle (α 1 ) and the detachment dip (β)), the wedge as a whole slides toward the foreland along the weak detachment. Slip occurs on the detachment because the coefficient of sliding friction on the detachment is less than the coefficient of internal friction in the wedge. EarthStructure (2 nd ed) 12
Critical Taper Theory Critical taper (φ c ) is sum of surface slope angle (α 1 ) and detachment slope angle (β). (a) Stress acting on a wedge partly horizontal boundary load caused by backstop (σ bs ) and partly caused by gravity (σ g ). (b) If backstop moves, wedge thickens, so surface slope increases, and taper (φ) eventually exceeds φ c. (c) Wedge slides toward foreland and new material is added to toe, and extension of wedge occurs so that surface slope decreases. (d, e) If surface slope becomes too small, thrusting at toe stops, and wedge thickens by penetrative strain or out-of-sequence thrusting. EarthStructure (2 nd ed) 13
1983 EarthStructure (2nd ed) 14
1983 EarthStructure (2 nd ed) 15
2012 EarthStructure (2 nd ed) 16
Thrust faults, thrust sheets, and thrust slices Continental mountain ranges formed by plate convergence involve thrust faults, dip-slip faults on which hanging-wall blocks slide up the fault surface from a few kilometers to hundreds of kilometers. Geologists refer to the bodies of rock that move during thrusting as thrust sheets or thrust slices. 10 km EarthStructure (2 nd ed) 17
Glarner Thrust (Swiss Alps) Geologic domains, such as the Canadian Rocky Mountains, and the Swiss Alps are produced by regional tectonic shortening and thickening of the upper-crust where distinctive suites of thrust faults, folds, and associated mesoscopic structures, are called fold-thrust belts or fold-and-thrust belts. EarthStructure (2 nd ed) 18
Some key terms: The foreland direction is toward the undeformed continental interior, whereas the hinterland direction is toward the orogen s more intensely deformed and metamorphosed internal zone. Mechanical stratigraphy the succession of strong and weak rock layers of the sequence being deformed. For example, a sequence consisting of massive layers of limestone behaves differently from one consisting of thin layers of sandstone inter-bedded with thick layers of shale. The former may break to form several large thrust slices or may flex to form large-amplitude folds, whereas the latter may buckle to form a train of short wavelength folds. Because different mechanical stratigraphy develops in different tectonic settings, therefore not all fold-thrust belts look the same. EarthStructure (2 nd ed) 19
Location of a fold-thrust belt in an orogen. The belt occurs between the foreland basin and the internal metamorphic region of the hinterland. Thrusts eventually cut across the strata of the foreland basin and incorporate the basin material into the fold-thrust belt. A stack of thrust slices acts like a heavy load, pushing the surface of the crust down to create a depression that fills with sediment (i.e., to create the foreland basin EarthStructure (2 nd ed) 20
Fold-thrust belt occurrences 1) Foreland of an ocean-continent convergent margin 2) Accretionary prism bordering a trench EarthStructure (2 nd ed) 21
Fold-thrust belt occurrences 3) Foreland sides of a collisional orogenic belt. 4) Inverted rift basins Inversion tectonics - A tectonic setting in which a site of extension, like a rift or passive margin basin, transforms into a site of tectonic shortening. Consequently, normal faults reactivate as thrust faults, and the sedimentary fill of the rift or passive-margin basin may be shoved up and over the margins of the basin. EarthStructure (2 nd ed) 22
Fold-thrust belt occurrences 5) Restraining bends along large continental strike-slip fault. 6) Seaward edge of passive-margin sedimentary basins. EarthStructure (2 nd ed) 23
Fold-thrust belt on the seaward edge of passive-margin sedimentary basins EarthStructure (2 nd ed) 24
Thrust Ramps Fig. 18.8 Fault-ramp anticlines develop over fault steps. Cross-sectional trace of the fault before slip Hanging-wall ramp cuts across beds of the hanging wall Footwall ramp cuts across beds of the footwall. A hanging-wall flat lies parallel to bedding in the hanging wall Cross-sectional trace of the fault after slip. A footwall flat that lies parallel to bedding in the footwall. EarthStructure (2 nd ed) 25
Thrust ramps curve and change strike along their length Fig. 18.9 A frontal ramp strikes about perpendicular to the direction in which the thrust sheet moves 3D diagram with hanging wall removed A lateral ramp cuts up-section laterally and strikes approximately parallel to the direction in which the thrust sheet moves An oblique ramp strikes at an acute angle to the transport direction Tear fault - A nearly vertical-dipping, strike-slip fault striking sub-parallel to the regional transport direction that accommodates differential displacement of one part of a thrust sheet relative to another. Note that frontal ramps have dip-slip, oblique ramps show oblique slip, and lateral ramps show strike-slip. EarthStructure (2 nd ed) 26
More key terms: Allochthon - A mass of rock, comprising a thrust sheet (i.e., a hanging-wall block), that has been displaced by movement on a thrust fault; commonly, use of the term implies that the mass has moved a considerable distance on a detachment from its point of origin. Allochthonous - Adjective describing out-of-place rocks that have moved a large distance from their point of origin. Autochthonous -Adjective describing rocks that are close to the location where they originally formed and have not been displaced by movement on a thrust fault or detachment. Backthrust - A thrust on which the transport direction is opposite to the regional transport direction. Basal detachment - The lowest detachment of a thrust system; the regional basal detachment in a fold-thrust belt separates shortened crust above from unshortened crust below. In the foreland part of a fold-thrust belt, it typically lies at or near the basement-cover contact (also called a basal décollement). Blind thrust - A thrust that, while it is active, terminates in the subsurface. Emergent thrust - A thrust that, while it is active, cuts land surface. Break-forward sequence - Thrusting during which younger thrusts initiate to the foreland of older thrusts (also sequence called a foreland-breaking sequence). EarthStructure (2 nd ed) 27
More key terms: Out-of-sequence - A thrust that initiates to the hinterland of preexisting thrusts. Out-of-plane strain - The strain due to movement outside the plane of cross section. Cutoff (cutoff line) - The line of intersection between a fault and a bedding plane. Detachment - A subhorizontal fault Décollement or Sole fault The lower-most, main, subhorizontal detachment fault underlying a fold-thrust belt Regional transport - The dominant direction in which thrust sheets of a thrust belt moved during faulting (or regional vergence). Tip line - The line along which displacement on the thrust becomes zero. Triangle zone - A region in which a wedge of rock is bounded below by a forethrust and is bounded above by a backthrust. EarthStructure (2 nd ed) 28
Detachment folds form in response to slip above a subhorizontal fault, much like fold in a rug that wrinkles above a slick floor. Fig. 18.22 EarthStructure (2 nd ed) 29
Imbricate Fan - A type of thrust system where a series of thrusts branch from a lower detachment without merging into an upper detachment horizon. Fig. 18.12 Usually develops by progressive break-forward thrusting Note that successively younger thrusts cut into the footwall, and older faults and folds become deformed by younger structures Relatively small displacements EarthStructure (2 nd ed) 30
Cross section illustrating the concept of trishear deformation Fig. 18.21 In recent years, geologists have examined deformation in the region above the tip line of the fault, and have found that not all regions obey the classical geometric image of a fault-propagation fold with kink-style hinges. Rather, a triangular (in profile) region of deformation develops beyond the fault tip known as a trishear zone where strain is distributed throughout a triangular zone in the region beyond the fault tip. The solid line is the fault trace and the dashed lines outline the region of trishear.
Thrust-related Folding: Fault-propagation Fold Fig. 18.20 Fault-propagation folds form immediately in advance of a propagating fault tip (also called a tip fold). Progressive development of a simple fault-propagation fold. Lost River Range, Idaho, EarthStructure (2 nd ed) 32
Thrust Duplex Fig. 18.13 Duplex - A thrust system with a series of thrusts branched from a lower detachment to an upper detachment. An idealized flat-roofed duplex that develops by progressively breaking forward A horse is a fault-bounded body of rock in a duplex Note that the roof thrust undergoes a sequence of folding and unfolding, and that formation of the duplex results in significant shortening A roof thrust is the upper detachment of a duplex EarthStructure (2 nd ed) 33
Back Thrusts Fig. 18.14 Antithetic reverse faults develop in the back limb of a ramp anticline, and along bedding of the forelimb. Cross section of a triangle zone. Out-of-the-syncline forethrusts and backthrusts form to accommodate crowding in the hinge zone of the syncline. EarthStructure (2 nd ed) 34
Thrust-related Folding: Fault-bend Fold Fault-bend folds form in response to movement over bends in a fault surface. Cross-sectional model showing the progressive stages during the development of a fault-bend fold. The dashed lines are the traces of axial surfaces. EarthStructure (2 nd ed) 35
EarthStructure (2 nd ed) 36
Nappe structures EarthStructure (2 nd ed) 37
Klippe and Window (or Fenster) Lewis Thrust (52Ma), Crowsnest Klippe, Alberta Klippe - An erosional outlier of a thrust sheet that is completely surrounded by footwall rocks; it is an isolated remnant of the hanging-wall block above a thrust. Window (fenster) - An erosional hole through a thrust sheet that exposes the footwall (i.e., an exposure of the footwall completely surrounded by hanging wall rocks). EarthStructure (2 nd ed) 38
MESOSCOPIC- AND MICROSCOPIC-SCALE STRAIN IN THRUST SHEETS Fig. 18.23 Fold-thrust belts form in response to layer-parallel compression of the upper crust, meaning the maximum principal compressive stress (σ 1 ) is horizontal and has a bearing roughly perpendicular to map trace of folds and faults within the fold-thrust belt. Compression causes mesoscopic- and microscopic-scale structures to form in most fold thrust belts including 1) cleavage 2) small folds 3) wedge faults 4) grain-scale brittle and plastic strains 5) tension and shear-fractures EarthStructure (2 nd ed) 39
Thrust Belts in Map View Fold-thrust belts form in response to layer-parallel compression of the upper crust, meaning the maximum principal compressive stress (σ 1 ) is horizontal and has a bearing roughly perpendicular to map trace of folds and faults within the fold-thrust belt. Slip transfers from one fault to another at a relay zone or transfer zone σ 1 Schematic map showing the traces of thrust faults in the southern Canadian Rockies. The bow andarrow rule, as applied to the McConnell Thrust, Alberta (Canada). For most foreland thrust belts, the ratio b/a is about 0.07 0.12, or dip-slip is about 7-12% of the fault trace length EarthStructure (2 nd ed) 40
MESOSCOPIC- AND MICROSCOPIC-SCALE STRAIN IN THRUST SHEETS Fig. 18.23 Compression causes mesoscopic- and microscopic-scale structures to form in most fold thrust belts including 1) cleavage 2) small folds 3) wedge faults 4) grain-scale brittle and plastic strains 5) tension and shear-fractures EarthStructure (2 nd ed) 41
Curvature of fold-thrust belts Regional map traces of fold-thrust belts typically are sinuous and include bulges into the foreland called salients, and recesses bulging toward the hinterland. Displacement estimates made on these patterns can be misleading if one assumes only effects form horizontal tectonics (less transport in recesses) The map pattern may be misleading with respect to the amount of foreland transport if orogens are inverted, or subjected to epeirogenic (vertical) strains. EarthStructure (2 nd ed) 42
Classic Appalachian Thrust Geometry: Pine Mountain Thrust of Va. And Tenn. Fig. 18.7 Barbs on mapped faults point toward hanging wall of the thrust faults. Map showing thrust fault traces Northeast and southwest ends of the Pine Mountain thrust sheet are bounded by tear faults. Cross section EarthStructure (2 nd ed) 43
Classic Rocky Mt. Thrust Geometry Mt. Sevier Thrust belt Battle Mountain, WY (Prospect thrust) EarthStructure (2nd ed) 44
Balanced Cross-sections Perhaps you ve asked yourself the fundamental question, How do people draw such cross sections? and How reliable are they? It is important to remember that a cross section is just an interpretation of the subsurface geology, and nothing more. Cross-section interpretations are constrained by projecting surface geology into the subsurface, by interpreting seismic-reflection profiles, and by interpreting well data. Such data rarely provide a complete picture of subsurface geology, so we always must extrapolate when making cross sections. However, geologists have established a set of tests that permit us to evaluate cross sections to determine if the sections at least have a good chance of being correct. EarthStructure (2 nd ed) 45
Balanced Cross-sections A balanced cross section has a reasonable chance of being correct, though we cannot guarantee it, whereas an unbalanced cross section is probably wrong (unless a good explanation can be provided for why the section does not balance). 1) The deformed-state cross section must be admissible with structures resembling observed structures in outcrop or seismic profiles. 2) Restoration of the cross section must yield reasonable geometries 3) The cross section should area balance when taking into account penetrative strains 4) The cross section must be kinematically reasonable. EarthStructure (2 nd ed) 46
Classic balanced cross section of a duplex within the Lewis Thrust Sheet, Waterton (Canada) Fig. 18.28 Hanging-wall strata of the Precambrian Belt Supergroup overlie footwall Cretaceous siliciclastics (K) beneath the Lewis Thrust. Shortening (S) is determined by comparison of the deformed and restored cross sections using the equation: S = L L W = Waterton; shaded = lower Altyn; ua = upper Altyn; Ap = Appekunny; G = Grinnell; S = Siyeh; SL = sea level; MCT = McConnell thrust
GCH MS Thesis EarthStructure (2 nd ed) 48
Prior interpretation on 1980 State Geological Map EarthStructure (2 nd ed) 49
Revised, balanced interpretation on 1980 State Geological Map EarthStructure (2 nd ed) 50
Work in the NJ Ridge and Valley Thrust system involved balancing thrust faults cutting and displacing F1 folds EarthStructure (2 nd ed) 51