The kinematics of soft-linked thrust systems
Complex structural styles can be developed within orogenic belts, with crustal shortening achieved through a combination of distributed strain and discreet thrusting.
Soft-linked faults can be defined as structures with discontinuous fault surfaces which operate together with folds and other forms of distributed strain as a single kinematic and mechanical entity.
The Haut Giffre region of the French Subalpine Chains at the south-western extent of the Morcles nappe, with its well-defined stratigraphy provides an excellent opportunity to study the development of such a system of spatially discontinuous yet kinematically linked thrust faults, and their interaction with the surrounding rock in three dimensions. A novel computer-assisted photogrammetric method is presented which, in conjunction with new structural mapping aids the construction of accurate geologic profiles and maps of the Haut Giffre fold-thrust complex over a wide scale range.
The diversity of strain localisation behaviour observed in the district, from large-scale folds to discrete thrusts is largely a result of mechanical heterogeneities provided by stratigraphic rheological contrasts within the marl/limestone multi-layer and inherited weaknesses in the form of extensional faults.
A rolling-hinge model for fold growth supported by outcrop-scale field observation of vein arrays is proposed. A geometric analysis of predicted flexural-slip dynamics for folding shows that rolling hinge folding may be energetically preferable to fixed hinge folding.
Soft-linkage of thrust faults may be a common feature of thrust system development if a model of fault growth by segment linkage is accepted. During the soft-linked phase of thrust system development, bulk strength of the multi-layer, and therefore strain-rate, is likely to be controlled by transfer-zone processes rather than fault-propagation and fault slip accumulation processes.
The full thesis is available in Adobe Portable Document Format (PDF) (15.3 MB) suitable for on-screen display. A higher resolution version more suited to printing is available on request (37 MB). Printed copies of the thesis may be loaned from the Edward Boyle Library at the University of Leeds.
A geological map of the Haut Giffre study area (ISO A0 true size) is available in Xara format on request (47.9 MB) or as Portable Network Graphics (23.3 MB) or PDF (30.0 MB)
Conference presentation abstracts
Smallshire, R.J. (1996) The Kinematics of a Soft-linked Thrust System; an example from the Haut Giffre, French Alps. 28th Annual Meeting of the Tectonic Studies Group of the Geological Society of London.
Smallshire, R.J. & M. Casey, (1997) Finite strain compatibility models applied to soft-linked thrust systems. Abstract Supplement No. 1, Terra Nova, 9, 174.
Smallshire, R.J. (1997) Rolling hinges as a mechanism for buckle fold growth: Evidence from the Haut Giffre, French Alps. 29th Annual Meeting of the Tectonic Studies Group of the Geological Society of London.
Smallshire, R.J. (2000) Soft-linkage in thrust systems – geometry and kinematics. 30th Research in Process Meeting of the Tectonic Studies Group of the Geological Society of London.