EMSC 3002
Contractional regimes
- Louis Moresi (convenor)
- Chengxin Jiang (lecturer)
- Romain Beucher (former lecturer)
- Stephen Cox (curriculum advisor)
Australian National University
NB: the course materials provided by the authors are open source under a creative commons licence. We acknowledge the contribution of the community in providing other materials and we endeavour to provide the correct attribution and citation. Please contact louis.moresi@anu.edu.au for updates and corrections.
Resources
- Fossen, H, 2011. Structural Geology. Cambridge University Press, 2nd Edition.
- McClay, K.R. 1991. The Mapping of Geological Structures. John Wiley & Sons.
- Park, R.G., 1995. Foundations of Structural Geology. Blackie & Sons Ltd.
- Davis, G.H. and Reynolds, S.J., 1996. Structural Geology of Rocks and Regions. 2nd Edition, John Wiley & Sons.
Intended learning outcomes
Contractional Structures
Contractional structures occur in any tectonic regime, but they are more common along convergent boundaries.
Understanding contractional structures is important for understanding formation of mountain belts in general, but also for improved oil and gas exploration methods, because many world's oil ressources are located in fold and thrust belts.
Contractional Structures
Contractional deformation structures when rocks are shortened by tectonic or gravitational forces
We find contractional faults and folds in:
- All parts of collisional zones.
- Accretionary prisms associated with subduction zones.
- In gravitationally instable structures such as deltas and continental-margin sediments resting on weak mud or salt.
Contractional Structures
Shortening can be accomodated in different ways
- Volume loss and formation of dissolution seams (pressure solution, e.g. stylolites), compaction etc.
- Pure shear deformation where horizontal shortening is compensated by vertical thickening and where layers maintain their orientations.
- Buckling of layers (folding).
- Contractional faults and related folds structures.
Contractional faults
Contractional faults are faults that accommodate contraction or shortening.
In most cases, contrational faults correspond to reverse or thrust faults which accommodate shortening in the horizontal direction.
Contractional faults
Note that contractional faults may include normal and oblique faults.
In the example below the normal fault accommodate layer-parrallel shortening.
Thrusts Faults and reverse Faults
Contraction in the horizontal plane, or tectonic contraction, is associated with formation of reverse and thrust faults.
Thrust faults are reverse faults with shallow dip. (less than 30 degrees)
Thrust Faults
A thrust is a low angle fault or shear-zone where the hanging wall has been transported over the footwall
Thrusts faults bring older rocks on top of youger rocks, and rocks of higher metamorphic grade on top of rocks of lower metamorphic grade.
Stratigraphy and Metamorphic grade can both be used to map thrusts... Stratigraphic control is particularly important
Photo: Chief Mountain, Montana. pre-Cambrian limestones over Cretaceous shales.
Thrust Faults and Tectonic Units
Nappe Terminology
Thrust nappes consist of one or several subordinate Thrust sheets that have a common diplacement history.
The smallest tectonic units originating within the pile is called a horse
Nappes, sheets and horses are bounded at their base by a sole thrust or floor thrust and at the top by a roof thrust
Nappes are thin compared to their lateral extent. They commonly show a wedge geometry in cross-section.
Thrust Faults and Tectonic Units
Nappe Terminology 2
Thrust Faults and Tectonic Units
Nappe Terminology 3
Thrust Faults and Tectonic Units
Autochthons and Allochthons
Allochthon refers to nappes or tectonic units that have been transported. Autochthon designate in-situ rocks or tectonic units that have not been transported, i.e. basement rocks.
Locally transported rocks are referred to as parautochthonous.
The low-angle fault or shear-zone that separates allochthon from autochthon is called a detachment or décollement.
Thrust Faults and Tectonic Units
The directions and orientations of structures within nappes often refer to the hinterland or foreland.
The hinterland designates the central mountainous region of the orogen, wheres the foreland occupies the margins
Thrust Faults and Tectonic Units
Thrust nappes are characteristics of orogen such as the Alps and the Caledonian-Appalachian Orogen.
Cross-section through the Alps, Schmid and Kissling, 2000
Thrust Faults and Tectonic Units
Hinterland characteristics
- Thick-skinned deformation, meaning both basement and cover (wedge) are involved.
- Penetrative deformation
- Formation of large metamorphic nappes.
- Extensive internal folding of the nappes is common.
Thrust Faults and Tectonic Units
Foreland characteristics
- Thin-skinned contractional tectonics.
- Very localized deformation.
- Formation of nappe systems.
- Local basement not involved.
- Deposition of sediment following erosion of the hinterland.
Thrust systems geometries
Thrust typically propagates in a stepwise manner that gives rise to ramp-flat geometries.
Flats form along soft, incompetent layers. Ramps develop where the thrust cuts across relatively stiff layers.
Ramps categories are based on their orientation relative to the direction of transport.
- Frontal ramps are perpendicular with reverse dip-slip.
- Lateral ramps are parallel to the direction of transport. They are associated with vertical transfer faults.
- Oblique ramps form oblique slip faults.
Thrust systems geometries
Contractional faults in the foreland of an orogenic zone typically form imbrication zones and duplex structures.
Imbrication zones results from the formation of ramps. They are composed of a series of horses thrusted up along more or less parallel ramp faults.
Imbrications zones form preferentially in the foreland.
- Repeated series of thrusts sharing a dip direction
- Faults flatten out into the basal thrust (décollement)
- Fault dip depends on erosion level
- Blind thrust: No surface expression
Thrust systems geometries
Contractional faults in the foreland of an orogenic zone typically form imbrication zones and duplex structures.
Duplexes are structures confined between an overlying roof thrust and an underlying floor thrust.
Duplexes comprise a series of juxtaposed ramps that formed in front of a propagating floor thrust. It results in stacked horses.
- Form when sub-horizontal thrust propagation “sticks”.
- Bound by floor and roof thrusts.
- May contain many individual horses.
Thrust systems geometries
Duplex structure in dolomitic sandstones, Svalbard, Photo Steffen Bergh
Thrust systems geometries
Example of Duplex in the south Norwegian Caledonides, Based on Morley. 1986, Haakon Fossen
Thrust systems geometries
Thrust systems geometries
Stages of thrust development in a sandbox experiment. Two thrusts and associated fault propagation folds, after about 31% horizontal shortening. These experiments were led by students, supervised by Assoc. Prof. Sandra McLaren, Melbourne University.
Thrust systems geometries
Stages of thrust development in a sandbox experiment. Greater complexity after 56% shortening. The beginning of a new thrust is also apparent at the frontal edge of the “thrust belt”. These experiments were led by students, supervised by Assoc. Prof. Sandra McLaren, Melbourne University.
Orogenic Wedges
Geometry
Orogenic Wedges are wedge-shaped fold and thrust belts formed in association with crustal contraction and orogenesis.
Orogenic wedges are thickest in the hinterland and become progressively thinner toward the foreland.
Orogenic Wedges
The Buldozer model
Snow piling up at the front of a buldozer is a common analogue used to describe the growth of orogenic wedges.
The translation and internal deformation of the wedge are driven by lateral compression and thrusting toward the foreland.
The whole wedge is translated along a basal detachment, internal nappes are thrusted along floor faults and shear zones.
Orogenic Wedges
Wedge shape: controlling factors
At shallow depth, where deformation is brittle, the shape of the orogenic wedge is controlled by:
- Friction along the basal detachment
- Shear strength of the orogenic wedge
- Surface erosion.
The combination of these controlling factors can result in a wide range of orogen shapes.
Orogenic Wedges
Controlling Factors: Friction along detachment
High Friction and asociated strain hardening along the detachment may cause deformation to relocate into the wedge.
Folding, formation of duplexes and imbrications result in contraction and increase in thickness.
Orogenic Wedges
Controlling Factors: Friction along detachment
Orogenic Wedges
Controlling Factors: Strength
The internal strength, i.e. the ability to support loading and resist internal deformation also affects the shape of the wedge.
Weak orogenic wedges deform and generate topography more easily than strong orogenic wedges. Weak orogenic wedges are more likely to collapse due to increased topographic loading.
Orogenic Wedges
Controlling Factors: Strength
Gravity driven deformation occurs when the topographic load exceeds the internal strength of the wedge.
Orogenic Wedges
Gravity Models
Alternative models uses gravity as the most important driving force for the displacement of nappes and orogenic wedges.
Three gravity models have been proposed.
- The Gliding Model
- The Extrusion Model
- The Spreading Model
Orogenic Wedges
Gravity Models: the Gliding Model
- The basal detachment dips toward the foreland
- Nappe displacement do not require any internal deformation.
Orogenic Wedges
Gravity Models: the Extrusion Model
Deformation is essentially planar. No deformation occurs in the direction perpendicular to the direction of displacement. Pure Shear Deformation
Orogenic Wedges
Gravity Models: the Spreading Model
The wedge spreads out radially and therefore deforms internally by non-planar deformation.
Orogenic Wedges
Models
Models that explains orogeny and large-scale thrusting must consider gravity driven deformation as well as pure dynamic shortening.