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

Lecture 15

Earth Structure (2 nd Edition), 2004 W.W. Norton & Co, New York Slide show by Ben van der Pluijm

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

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.

5) Restraining bends along large continental strike-slip fault.

6) 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. sheet relative to another.

11/10/2014 © 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).

11/10/2014 © 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.

11/10/2014 © 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

11/10/2014 © 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

11/10/2014 © 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 The solid line is the fault trace and the strain is distributed throughout a dashed lines outline the region of triangular zone in the region beyond the trishear. fault tip. 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,

11/10/2014 © 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.

•A horse is a fault-bounded body of rock in a duplex • An idealized flat-roofed duplex that develops by progressively breaking forward

• Note that the roof • A roof thrust is the upper thrust undergoes a detachment of a duplex sequence of folding and unfolding, and that formation of the duplex results in significant shortening

11/10/2014 © 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.

• Out-of-the-syncline forethrusts and • Cross section of a triangle zone. backthrusts form to accommodate crowding in the hinge zone of the syncline.

• 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.

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).

11/10/2014 © 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 ( σ1) 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

11/10/2014 © EarthStructure (2 nd ed) 39 Thrust Belts in Map View Schematic map The “bow and- showing the traces arrow rule,” as of thrust faults in applied to the • Fold-thrust belts the southern McConnell form in response Canadian Rockies. Thrust, Alberta to layer-parallel (Canada). compression of the upper crust, meaning the maximum principal compressive stress ( σ1) is σ1 horizontal and has a bearing roughly perpendicular to map trace of folds and faults within the fold-thrust belt. • For most foreland thrust belts, the ratio b/a is about 0.07– • Slip transfers from 0.12, or dip-slip is one fault to another about 7-12% of the at a relay zone or fault trace length transfer zone

11/10/2014 © 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

• 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.

11/10/2014 © 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 Map showing thrust fault traces of the thrust faults.

• Northeast and southwest ends of the Pine Mountain thrust sheet are bounded by tear faults.

Cross section

11/10/2014 © EarthStructure (2 nd ed) 43 Classic Rocky Mt. Thrust Geometry Mt. Sevier Thrust belt Battle Mountain, WY (Prospect thrust)

• 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.

• 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”.

11/10/2014 © 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 W = Waterton; shaded = lower Altyn; uA = upper Altyn; Ap = Appekunny; G = Grinnell; S = Siyeh; cross sections using the equation: S = L SL = sea level; MCT = McConnell thrust – L’ GCH MS Thesis

11/10/2014 © EarthStructure (2 nd ed) 50 Work in the NJ Ridge and Valley Thrust system involved balancing thrust faults cutting and displacing F1 folds