13. Contractional Tectonics - Supplements
Contents
Terminology of Convergence and Collision
Terminology of Fold-Thrust Belts
SUPPORTING INFORMATION
Terminology of Convergence and Collision
Accretionary wedge A wedge-shaped sequence of deformed sediment and some basalt, that forms along the edge of the overriding slab; the material of the accretionary prism consists of pelagic sediment and oceanic basalt scraped off the downgoing plate, as well as sediment that has collected in the trench. Also called accretionary prism.
Arc-trench gap The horizontal distance between the axis of the volcanic arc and the axis of the trench.
Backarc basin A narrow ocean basin located between an island arc and a continental margin.
Backarc region A general term for the region that is on the opposite side of the volcanic arc from the trench.
Continental arc A volcanic arc that has been built on continental crust.
Convergent plate boundary The surface between two plates where one plate subducts beneath another; as a consequence of subduction, oceanic lithosphere is consumed.
Coupled subduction A type of subduction in which the overriding plate is pushing tightly against the downgoing plate.
Décollement Synonym for detachment; the term is French.
Detachment A basal fault zone of a fault system; in accretionary prisms it marks the top of the downgoing plate.
Downgoing slab (plate) Oceanic lithosphere that descends into the mantle beneath the overriding plate.
Forearc basin A sediment-filled depression that forms between the accretionary prism and the volcanic arc. The strata of a forearc basin buries the top of the accretionary wedge and/or trapped oceanic crust, and in general, are flat-lying.
Forearc region A general term for region on the trench side of a volcanic arc.
Island arc A volcanic arc built on oceanic lithosphere; the arc consists of a chain of active volcanic islands.
Marginal sea Synonym for a backarc basin that is underlain by ocean lithosphere.
Mélange A rock composed of clasts of variable origin distributed in a muddy matrix.
Oceanic plateau A broad region where seafloor rises to a shallower depth. It is underlain by anomalously thick oceanic crust, probably formed above a large mantle plume.
Offscraping The process of scraping sediment and rock off the downgoing slab at the toe of the accretionary prism.
Outer swell A broad arch that develops outboard of the trench, in response to flexural bending of the lithosphere at the trench.
Overriding plate (slab) The plate beneath which another plate is being subducted at a convergent plate boundary.
Peripheral bulge A broad arch that develops outboard of the trench, in response to flexural bending of the lithosphere at the trench. Synonym for outer swell.
Retroarc basin Synonym for backarc basin.
Rollback The seaward migration of the bend in the downgoing plate as subduction progresses.
Subduction The process by which one plate sinks into the mantle beneath another.
Subduction complex Used as a synonym for accretionary wedge or the full convergent margin system.
Underplating In the context of subduction, this is the process of scraping of material from the downgoing slab beneath the accretionary prism, so that the material attaches to the base of the prism.
Trench A deep marine trough that forms at the boundary between the downgoing and overriding slabs; the trench may be partially or completely filled with sediment eroded from the volcanic arc or continental margin.
Trench slope break Topographic ridge marking a sudden change in slope at the top of the accretionary wedge.
Uncoupled subduction A process of subduction in which the downgoing plate is not pushing hard against the overriding plate, so the subduction system is effectively under horizontal tension.
Volcanic arc A chain of subduction-related volcanoes.
Accreted terrane A piece of exotic crust that has been attached to the margin of a larger continent. (Note the spelling of terrane, which differs from that of the geographic term for a tract of land, spelled terrain.)
Basin inversion The process whereby a region that had undergone crustal extension during basin formation subsequently undergoes crustal shortening during collisional tectonics; in the process, faults that began as normal faults are reactivated as reverse faults, and the strata of the basin thrusts up and over the former basin margin.
Collision An event during which two pieces of buoyant lithosphere move toward each other and squash together, after the intervening oceanic lithosphere has been subducted.
Delamination In the context of collisional tectonics, this refers to the separation of the basal portion of the thickened lithospheric mantle beneath a collisional orogen; this delaminated lithospheric mantle then sinks downward through the asthenosphere.
Exotic terrane An independent block of buoyant crust that has been brought into a convergent margin during subduction, where it collides and docks against the continent (also called accreted terrane). The adjective “exotic” in this context is used to emphasize that the block in question did not originate as part of the continent to which it is now attached, but rather came from somewhere else.
Lateral escape The process, accompanying collision, during which crustal blocks of the overriding plate slide along strike-slip faults in a direction roughly perpendicular to the regional convergence direction; effectively, the “escape” from the collision zone resembles the movement of a watermelon seed that you squeeze between your fingers. Also extrusion tectonics.
Orogenic collapse The process that occurs when thickened crust in a collisional orogen weakens and starts to sink under its own weight. Effectively, gravitational loads cause horizontal extensional strain to develop. In some cases, extension is coeval with thrusting (syn-orogenic collapse), in other cases it occurs after thrusting has ceased (post-orogenic collapse). Also called extensional collapse.
Suspect terrane A crustal block in an orogen whose tectonic origin is unclear. The block does not appear to correlate with adjacent crust in the orogen where it now resides and thus may be exotic. A block remains “suspect” only until its origin (i.e., whether it’s exotic or not) has been determined.
Suture The zone within an orogen that marks the boundary between once-separate continents; commonly, slivers of ophiolites occur along a suture and the tectonic fabric reflect shear displacement.
Tectonic collage A region of crust that consists of numerous exotic terranes that have been sutured together; in other words, a tectonic collage consists of accreted terranes that docked during a protracted period of convergent-margin tectonism.
Terminology of Fold-Thrust Belts
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 still at the site where they originally formed and have not been displaced by movement on a thrust fault or detachment.
Backarc The region that lies behind the volcanic arc along a convergent plate boundary; the backarc and the trench are on opposite sides of the volcanic arc.
Backstop A representation of the boundary load in the hinterland of a fold-thrust belt. The backstop generates horizontal compressional stress, which contributes to driving fold-thrust belt development. The backstop represents rock of the hinterland that is moving toward the foreland. As such, a backstop is like a snowplow pushing snow toward the foreland.
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.
Branch line The line of intersection between two fault surfaces, e.g., where a ramp branches (splays) off of a detachment, or where one ramp splays off another.
Break-forward A sequence of thrusting during which younger thrusts initiate to the foreland of older thrusts (the sequence is called a foreland-breaking sequence).
Break-thrust fold A fold that initiates prior to thrusting, but later ruptures so that a thrust cuts through its forelimb.
Cutoff (cutoff line) The line of intersection between a fault and a bedding plane.
Décollement A major subhorizontal fault (also called a detachment)
Detachment A major subhorizontal fault (also called a décollement)
Detachment fold A fold that forms in response to slip above a subhorizontal fault, much like fold in a rug that wrinkles above a slick floor.
Duplex A type of thrust system where a series of thrusts branch from a lower detachment to an upper detachment.
Fault-bend fold A fold that forms in response to movement over bends in a fault surface.
Fault-propagation fold A fold that forms immediately in advance of a propagating fault tip (also called a tip fold).
Floor thrust The lower detachment of a duplex; it forms the base of the duplex.
Fold nappe A thrust sheet that contains a regional-scale recumbent fold.
Fold-thrust belt A geologic terrane in which upper-crustal shortening is accommodated by development of a system of thrust faults and related folds.
Footwall block The body of rock beneath the fault.
Footwall cutoff The intersection between bedding planes of footwall strata and a fault surface.
Footwall flat The portion of the footwall where bedding surfaces parallel the fault.
Footwall ramp The portion of the footwall where bedding surfaces truncate against the fault (i.e., the portion of the footwall along which there are footwall cutoffs).
Forearc The region to the trench side of the volcanic arc of a convergent plate boundary. The forearc is not the same as the foreland. The forearc lies on the ocean side of a continental volcanic arc.
Foreland The part of the undeformed craton adjacent to an orogenic belt; some authors have used the term in a more general sense to include the portion of an orogenic belt closer to the undeformed continental interior.
Foreland basin A sedimentary basin formed on the continent side of a fold-thrust belt that forms because the weight of the stack of thrust sheets in the belt depresses the lithosphere.
Forethrust A thrust on which the transport direction is the same as the regional transport direction for the whole fold-thrust belt.
Frontal ramp A ramp that strikes perpendicular to transport direction.
Hanging-wall block The rock mass that has been transported above a fault surface.
Hanging-wall cutoff The intersection between bedding planes of hanging-wall strata and the fault surface.
Hanging-wall flat The portion of the hanging wall where bedding surfaces parallel the fault.
Hanging-wall ramp The portion of the hanging wall where bedding surfaces truncate against the fault (i.e., the portion of the hanging wall where there are hanging-wall cutoffs).
Hinterland The region closer to the high-grade core of an orogen; as a directional reference, it is the direction opposite to the foreland direction.
Horse A body of rock in a duplex that is completely enveloped by faults.
Imbricate fan A type of thrust system where a series of thrusts branch from a lower detachment without merging into an upper detachment horizon.
Inversion tectonics The process by which a site of extension (e.g., a rift or passive margin basin) transforms into a site of shortening. During inversion, faults that had initiated as normal faults reactivate as thrust faults, and the sedimentary fill of the rift or passive-margin basin is shoved up and over the margins of the basin.
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.
Lateral ramp A ramp that strikes parallel to transport direction.
Mechanical stratigraphyThe succession of rock types comprising the stratigraphy of a region, defined in terms of their relative strength.
Oblique ramp A ramp that strikes oblique to transport direction.
Out-of-sequence thrustA thrust that initiates to the hinterland of preexisting thrusts.
Out-of-plane strain The strain due to movement outside the plane of cross section.
Regional transport directionThe dominant direction in which thrust sheets of a thrust belt moved during faulting. Some use the term regional vergence direction as a synonym.
Roof thrust The upper detachment of a duplex.
Stair-step geometry The geometry of a thrust that cuts up section via a series of flats and ramps. The shape of the fault resembles a staircase in cross section. Typically, the ramps form in stronger units, and the flats in weaker units.
Tear fault A nearly vertically dipping fault in a thrust sheet that that is parallel or subparallel to the regional transport direction. Motion on a tear fault is dominantly strike-slip and may accommodate differential displacement of one part of a thrust sheet relative to another (i.e., a tear fault is a nearly vertically dipping oblique ramp or lateral ramp). Kinematically it is equivalent to a transfer fault.
Tectonic inversion The reactivation of preexisting faults by a reversal of slip direction on the faults.
Thick-skinned tectonics The process of deformation that involves slip on basement-penetrating reverse faults; this movement uplifts basement and causes monoclinal forced-folds (“drape folds”) to develop in the overlying cover.
Thin-skinned tectonics The process of deformation in which folding and faulting are restricted to rock above a detachment. Some restrict the term to situations in which the detachment lies at or above the basement-cover contact. Others use the term even when basement occurs in thrust sheets, to imply that the basement has been transported or detached.
Thrust fault (thrust) A shallowly to moderately dipping (< 30°) contractional fault with dip-slip reverse movement; in detail, thrusts may include several ramps and flats, and thus on a regional scale, do not necessarily have a uniform dip.
Thrust sheet The hanging-wall block, above a thrust surface, that has been transported as a consequence of slip on the thrust (also called a thrust slice)
Thrust system An array of related thrusts that connect at depth; a regional-scale thrust system may represent shortening above a regional detachment.
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.
Window An erosional hole through a thrust sheet that exposes the footwall (i.e., an exposure of the footwall completely surrounded by hanging wall rocks). Also called fenster.
MORE EXPLORATION
Balanced Cross-sections
In this chapter, we have presented several cross sections depicting the subsurface geometry of fold-thrust belts, right down to the basal detachment, even where these depths are not exposed. Perhaps you’ve asked yourself the fundamental question, “How do people draw such cross sections?” and “How reliable are they?” Well, to begin with, it is important to remember that a cross section is just an interpretation of the subsurface geology, and nothing more. We do not have access to outcrops several kilometers below the ground surface to let us see exactly where formation contacts, faults, and cutoffs are positioned. 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 cross section that passes these tests is said to be a balanced cross section. A balanced cross section has a reasonable chance of being correct, whereas an unbalanced cross section is more likely wrong (unless a good explanation can be provided for why the section does not balance).
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FIGURE S13.1. (a) Deformed-state cross section of a duplex within the Lewis Thrust Sheet, Waterton (Canada). No vertical exaggeration. (b) Restored version of the cross section. Hangingwall strata are composed of the Precambrian Belt Supergroup (W = Waterton; shaded = lower Altyn; uA = upper Altyn; Ap = Appekunny; G = Grinnell; S = Siyeh; SL = sea level; MCT = McConnell thrust) that overlie footwall Cretaceous siliciclastics (K) across the Lewis Thrust. Shortening (S) is determined by comparison of the deformed and restored cross sections using the equation: S = L – L′. Note that this previously published cross section and restoration does not exactly balance in the foreland; to see why, try to match slices in the deformed and restored cross sections. Datum is taken as the top of the shaded reference horizon. [18.28] |
Let’s briefly look at fundamental elements that help determine whether a cross section is balanced. Note that these observations only apply when deformation does not result in movement in or out of the cross-section plane, meaning that they only apply to cross-sections that are drawn parallel to the transport direction on faults.
- The deformed-state cross section must be admissible. Structures in the deformed-state cross section (i.e., the cross section depicting the way structures look today, after deformation) must resemble real structures that geologists have observed in outcrop or seismic profiles. For example, ramps should cut up section, not down section, unless they are out-of-sequence faults. We call cross sections that pass this test admissible sections. Figure 18.28a provides an admissible deformed-state cross section of the Lewis thrust sheet in Canada.
- Restoration of the cross section must yield reasonable geometries. A restored cross section (Figure 18.28b) represents the pre-deformation configuration of strata and the pre-deformational location of faults in the region. “Restoration” of a cross section involves returning beds to horizontal by removing the effect of folding, by returning rocks to their original locations by removing the displacement on faults, and by undistorting thrust sheets by removing the effect of mesoscopic-scale and microscopic scale deformation. The restored section must depict realistic-looking structures. For example, if a restored fault trace zigs and zags up section, then there’s likely something wrong with the section.
- The cross section must be “area balanced.” The area of rock shown on the restored cross section must equal the area shown on the deformed-state cross section unless pressure solution causes volume-loss strain. Cross sections that meet this criteria are area balanced. If volume-loss strain developed during deformation, this must be taken into account when restoring the section.
- The cross section must be kinematically reasonable. It should be possible to create the deformed-state cross section from the restored cross section in a kinematically reasonable way. This means that you should be able to draw a series of cross sections depicting stages in the evolution of originally horizontal beds into the faulted and folded beds of the deformed-state cross section.
The first two criteria in the above list ensure that the section doesn’t depict impossible structural geometries. The third criterion ensures that the configuration of structures shown on the deformed-state section does not imply that undocumented volume change occurred during deformation. The fourth criterion emphasizes that you do not really understand the geometry of a complex structure until you can demonstrate how the structure formed from undeformed rock. Balancing a cross section involves the following steps. First, you carefully examine the deformed-state section for admissibility. Then, you construct a restored section and check it for admissibility and area balance. Finally, you think through a scenario that can explain the evolution of the deformed-state cross section from initially horizontal beds.
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FIGURE S13.2. Diagram illustrating the quick-look technique for checking a cross section for potential problems using section balancing. The key is to recognize ramps and flats in the deformed-state section and realize that hanging-wall flats and ramps must exactly match footwall ramps and flats in number and in stratigraphic composition. [18.29] |
It is beyond the scope of this chapter to provide detailed guidelines for balancing cross sections; most structural geology laboratory manuals offer step-by-step instructions and exercises. But we do point out that, in some cases, you can use a quick-look technique to scan a deformed-state cross-section and determine if it can be balanced. To apply these steps (Figure 18.29):
- Identify ramps and flats in each part of the cross section, and count them. Are there the same number of ramps and flats in the hanging wall as in the footwall? There should be, because in an admissible, restored cross section, the hanging wall fits over the footwall with no gaps or overlaps.
- Paying close attention to the ramps, check to see if the same beds are truncated in the hanging wall as in the footwall. They must be, because the hanging-wall beds were originally adjacent to the footwall beds.
These two simple tests will identify the majority of cross-section errors that lead to construction of cross sections that cannot be balanced and, so, are likely incorrect.
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