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Fold-Thrust Belts, and the NJ Ridge and Valley Thrust System

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Fold-Thrust Belts, and the NJ Ridge and Valley Thrust System 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. 11/10/2014 © EarthStructure (2 nd ed) 2 Continent-continent Collision Fig. 18.3 • Regional cross sections depicting stages of fold- thrust belt development first during convergent-margin tectonism and then during continent-continent collision. • Thick-skinned tectonics involves slip on basement- Time penetrating reverse faults that uplifts basement and causes monoclinal forced- folds (“drape folds”) to develop in the overlying cover. • Thin-skinned tectonics involves folding and faulting above a mid-crustal detachment. 11/10/2014 © 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. 11/10/2014 © EarthStructure (2 nd ed) 4 11/10/2014 © 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. 11/10/2014 © 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. 11/10/2014 © 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. 11/10/2014 © 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 11/10/2014 © 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 σn is stress resulting from horizontal loading, σf is frictional load. σ resistance, lll is boundary load at end of thrust sheet, and PH2O is pore pressure. 11/10/2014 © 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 (φc) (defined by the surface slope angle ( α1) 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. 11/10/2014 © EarthStructure (2 nd ed) 12 Critical Taper Theory Critical taper ( φc) is sum of surface slope angle ( α1) and detachment slope angle ( β). (a) Stress acting on a wedge partly horizontal boundary load caused by backstop ( σbs ) and partly caused by gravity (σg). (b) If backstop moves, wedge thickens, so surface slope increases, and taper ( φ) eventually exceeds φc. (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. 11/10/2014 © EarthStructure (2 nd ed) 13 1983 11/10/2014 © EarthStructure (2 nd ed) 14 1983 11/10/2014 © EarthStructure (2 nd ed) 15 2012 11/10/2014 © 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 11/10/2014 © 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. 11/10/2014 © 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. 11/10/2014 © 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 11/10/2014 © 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, Consequently, normal faults reactivate as thrust faults, like a rift or passive margin basin, and the sedimentary fill of the rift or passive-margin transforms into a site of tectonic basin may be shoved up and over the margins of the shortening. 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 11/10/2014 © 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 • A footwall flat that lies parallel to bedding in Cross-sectional trace of the fault after slip. the footwall. 11/10/2014 © EarthStructure (2 nd ed) 25 Thrust ramps curve and change strike along their length Fig. 18.9 3D diagram with • A frontal ramp s trikes about hanging wall perpendicular to the direction removed in which the thrust sheet moves • 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, • Note that frontal ramps have strike-slip fault striking sub-parallel to the dip-slip, oblique ramps show regional transport direction that accommodates oblique slip, and lateral ramps differential displacement of one part of a thrust show strike-slip.
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