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Superimposition of Contractional Structures in Models and Nature
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Superimposition of contractional structures is widely observed in different scales in the world. Superimposed structures form due to different processes: change in strain accommodation from one type of structure to another during a single progressive shortening; successive coaxial shortening phases separated by an unconformity; superimposition of different non-coaxial shortening phases. Using results of a series of systematic analogue models and detailed field structural mapping, this thesis focuses on the geometry and kinematics of such superimposed structures that are formed by these three processes. During a single progressive folding, thrusts develop within a fold to accommodate stain variations in different regime of the fold. Limited displacement along these thrusts does not significantly modify the geometry of the fold. However, during multiple shortening phases (coaxial or non-coaxial), early formed structures are modified by the later phase ones. The later thrusts can cut and displace the pre-existing structures. The early folds are tightened or interfered by the later folding phase. Pre-existing thrusts may be reactivated either in dip direction and/or along strike during the later shortening. The pre-existing structures in turn influence development of the later structures, which results in change in structure spacing. An angular unconformity between two shortening phases clearly truncates the early phase structures and separates structures of different levels. Unlike in the post-erosional layers, in the layers below the unconformity, complicated superimposed structures are visible. This thesis shows that geometry and sequence of structures formed during one progressive shortening or multiple shortening phases strongly depend on the mode of the superimposition (coaxial, orthogonal or oblique) and the orientation of pre-existing structures.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. , 45 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1223
Keyword [en]
superimposition, progressive deformation, multiple phases, coaxial, non-coaxial, analogue modelling, fold-and-thrust belts
National Category
Geology
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Identifiers
URN: urn:nbn:se:uu:diva-243235ISBN: 978-91-554-9160-4 (print)OAI: oai:DiVA.org:uu-243235DiVA: diva2:786525
Public defence
2015-03-27, Hambergsalen, Geocentrum, Villavägen 16, Uppsala, 10:00 (English)
Opponent
Supervisors
Available from: 2015-03-04 Created: 2015-02-05 Last updated: 2015-03-12Bibliographically approved
List of papers
1. Identifying the characteristic signatures of fold-accommodation faults
Open this publication in new window or tab >>Identifying the characteristic signatures of fold-accommodation faults
2013 (English)In: Journal of Structural Geology, ISSN 0191-8141, E-ISSN 1873-1201, Vol. 56, 1-19 p.Article in journal (Refereed) Published
Abstract [en]

Hand-specimen and outcrop scale examples of folds are analyzed here to identify the characteristic signatures of fold-accommodation faults. We describe and analyze the geometric and kinematic relationships between folds and their associated faults in detail including the structural position and spatial distribution of faults within a fold, the displacement distribution along the faults by applying separation-distance plots for the outcrop scale examples, and the change of cut-off angle when the fault cut across folded layers. A comparison between fold-accommodation faults and fault related folds based on their separation-distribution plots and the problem of time sequence between faulting and folding are discussed in order to distinguish fold-accommodation faults from the reverse faults geometrically and kinematically similar to them. The analysis results show that fold-accommodation faults originate and terminate within a fold and usually do not modify the geometry of the fold because of their limited displacement. The out-of-syncline thrust has a diagnostically negative slope (separation value decreasing away from the upper fault tip) in the separation-distance graph. The change of cut-off angle and the spatial distribution of faults display a close relationship with the axial surface of the fold. Our analyses show that fold-accommodation faults are kinematically consistent with the flexural slip of the fold. The interbedded strata with competence contrast facilitate formation of fold-accommodation faults. These characteristic signatures are concluded as a set of primary identification criteria for fold-accommodation faults.

Keyword
Fold-accommodation faults; Hand-specimen; Outcrop scale; Separation–distance plot; Characteristic signatures
National Category
Geology
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Identifiers
urn:nbn:se:uu:diva-190089 (URN)10.1016/j.jsg.2013.08.006 (DOI)000327922200001 ()
Available from: 2013-01-07 Created: 2013-01-07 Last updated: 2017-12-06Bibliographically approved
2. Modeling two sequential coaxial phases of shortening in a foreland thrust belt
Open this publication in new window or tab >>Modeling two sequential coaxial phases of shortening in a foreland thrust belt
2014 (English)In: Journal of Structural Geology, ISSN 0191-8141, E-ISSN 1873-1201, Vol. 66, 400-415 p.Article in journal (Refereed) Published
Abstract [en]

Analog sandbox models are used to simulate two sequential coaxial phases of shortening in a foreland thrust belt. A sufficient hiatus is considered so that erosion and sedimentation after the first phase create an angular unconformity that is subsequently deformed. The effect of variation in thickness of post-erosional sediment package and presence of a weak layer at the unconformity level are analyzed. During the second phase, some first phase thrusts are reactivated and new thrusts are also initiated. Thrust reactivation results in a structure spacing that is smaller than the expected spacing for a thicker sediment package. Reactivation of pre-existing structures prevents the weak layer from acting as an intermediate decollement. An increase in thickness ratio tends to weaken reactivation of pre-existing thrusts. Model results also show that total displacement along individual reactivated thrusts generally increases downwards across the unconformity, which could be used to distinguish thrust reactivation in the field. Two regional examples from the northern Eastern Cordillera in Colombia and from the Variscan frontal zone in Western Europe, respectively, where multiphase coaxial shortening occurred, are compared with model results. Both natural cases show features, such as partially eroded first-generation folds and truncated first-generation thrusts that are indicators for two sequential phases of deformation as observed in the models.

Keyword
Coaxial multiple shortening, Deformed unconformity, Sandbox modeling, Thrust reactivation
National Category
Geology
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Identifiers
urn:nbn:se:uu:diva-232585 (URN)10.1016/j.jsg.2014.06.006 (DOI)000340978000026 ()
Available from: 2014-09-24 Created: 2014-09-22 Last updated: 2017-12-05Bibliographically approved
3. Mega arrowhead interference pattern in the Central part of the Yanshan Orogenic Belt, North China
Open this publication in new window or tab >>Mega arrowhead interference pattern in the Central part of the Yanshan Orogenic Belt, North China
2014 (English)In: Journal of Structural Geology, ISSN 0191-8141, E-ISSN 1873-1201, Vol. 80, 25-37 p.Article in journal (Refereed) Published
Abstract [en]

The Chengde-Pingquan region is located in the central part of the Yanshan Orogenic Belt (YOB). At Daheishan and Pingquan in the central YOB, thrusts and folds of variable trends are displayed in 2 km-scale fold interference patterns. Detailed field mapping was conducted to decipher the geometry of these two superimposed structures. Map-view geometry and stereonet plots for outcrop-scale folds indicate that the superimposed structures form arrowhead interference pattern where NW-SE-trending F1 folds are refolded by later ENE-WSW F2 folding. After remove the effects of later faulting, restored map-views of the superimposed structures show that when the F1 folds have inclined axial surfaces but with no an overturned limb, an arrowhead interference pattern (here called modified type-2 pattern) can form. Our field data and reinterpretation of the findings of previous studies suggest that five major shortening phases have occurred in the Chengde-Pingquan region. The first two phases, which formed the superimposed folds, occurred earlier than the Late Triassic (D1) and during the Late Triassic to Early Jurassic (D2). These two phases were followed by three deformation phases that are mainly characterized by thrusting and strike-slip faulting, which strongly modified the large-scale fold interference patterns.

Keyword
Fold interference patterns; Arrowhead; The Yanshan Orogenic Belt; Kilometer scale
National Category
Geology Geosciences, Multidisciplinary
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Identifiers
urn:nbn:se:uu:diva-240390 (URN)10.1016/j.jsg.2015.06.004 (DOI)000364245100004 ()
Funder
Swedish Research Council
Available from: 2015-01-07 Created: 2015-01-07 Last updated: 2017-12-05Bibliographically approved
4. Superimposed folding and thrusting by two phases of mutually orthogonal or oblique shortening in analogue models
Open this publication in new window or tab >>Superimposed folding and thrusting by two phases of mutually orthogonal or oblique shortening in analogue models
2016 (English)In: Journal of Structural Geology, ISSN 0191-8141, E-ISSN 1873-1201, Vol. 83, 28-45 p.Article in journal (Refereed) Published
Abstract [en]

Orogens may suffer more than one phase shortening resulting in superposition of structures of different generations. Superimposition of orthogonal or oblique shortening is studied using sandbox and centrifuge modelling. Results of sand models show that in orthogonal superimposition, the two resulting structural trends are approximately orthogonal to each other. In oblique superimposition, structures trend obliquely to each other in the relatively thin areas of the model (foreland), and mutually orthogonal in areas where the model is thickened during the first phase of shortening (i.e. the hinterland). Thrusts formed during the first shortening phase may be reactivated during the later shortening phase. Spacing of the later phase structures is not as wide as expected, considering they across the pre-existing thickened wedge. Superposition of structures results in formation of type 1 fold interference pattern. Bedding is curved outwards both in the dome and basin structures. Folded layers are dipping and plunging outwards in a dome, while they are dipping and plunging inwards in a basin. In the areas between two adjacent domes or basins (i.e. where an anticline is superimposed by a syncline or a syncline is superimposed by an anticline), bedding is curved inwards, and the anticlines plunge inwards and the synclines outwards. The latter feature could be helpful to determine the age relationship for type 2 fold interference pattern. In tectonic regions where multiple phases of shortening have occurred, the orogenic-scale dome-and-basin and arrowhead-shaped interference patterns are commonly formed, as in the models. However, in some areas, the fold interference pattern might be modified by a later phase of thrusting. Similar to models results, superimposition of two and/or even more deformation phases may not be recorded by structures all over the tectonic area.

Keyword
multiple orthogonal/oblique shortening; superimposed deformation; structure spacing; fold interference patterns
National Category
Geology
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Identifiers
urn:nbn:se:uu:diva-242960 (URN)10.1016/j.jsg.2015.08.005 (DOI)000371842600003 ()
Available from: 2015-02-03 Created: 2015-02-03 Last updated: 2017-12-05Bibliographically approved

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