Analogue models of folds above a wrench fault

folds first appear fault. Further wrench
at heterogeneities
and with axes at about 45’ to the trace of the wrench with the basement stabilize forming
fault displacement
fault, and a resulting weakens
of fold axes at larger fault displacement. an area which tends to quickly deformation is concentrated vergence, with an external down in width, since folding cover and subsequent Deeper upper in it. Axial surfaces of folds are initially on the basement that are greater between surface
zones between
the layers of the cover and at the basement-cover
33
Fig. 2. Triangulation base determine Each segment
of the crests of folds of a model. The CI, coordinates
is represented a wrench fault.
by two plates The cover.
of expanded on these
(120 X 40 X 5 cm). These two plates slide one against
each other along a resting wax (melting
(Received August 23, 1982; revised version accepted April 19, 1983)
ABSTRACT Odonne, F. and Vialon, P., 1983. Analogue models of folds above a wrench fault.
METHOD
Experimental The basement, plane polystyrene vertical
structures assumed of contact
are produced to he rigid, simulating
under
controபைடு நூலகம்led
laboratory
conditions.
NORTH
SO”TN
r”
a basement wrench fault under a sedimentary interface.
Fig. 1. Principle cover. Shaded
of experimental part: greased
apparatus
used to simulate
* Present address: Laboratoire de Radiogeologie, Universite de Bordeaux I, F-33405 Talence (France).
0 1983 Elsevier Science Publishers
B.V.
2. EXPERIMENTAL
Tectonoph_~sics, 99:
3 l-46.
This paper simulating folds appear bedding bulk rotation Folding describes the experimental deformation wrench of models fault. made with sheets of paraffin wax, in the causes a curvature the sheared upright, wrench then fault.
Tectonophysics, 99 (1983) 3 l-46
Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands
31
ANALOGUE
MODELS
OF FOLDS ABOVE A WRENCH FAULT
FRANCIS
forcing blocks, is represented
by sheets (80 x 80 x 0.21 cm) of paraffin
point 46”-48’C, manufactured by Merck). These sheets simulate relatively competent layers. Silicone grease coatings, simulating thinner weaker layers. are placed between the wax layers and permit slipage, i.e. “bedding plane slip”. The experiment consists of sliding one of the two basement plates. llorizo~~tall~ and parallel to the contact. under a cover made up of a pile of six. initially flat. paraffin wax layers. Under these conditions, when the displacement of the basement causes the cover to move, the anisotropy due to the simulated stratification causes folding to occur easily and to become visible (Ramberg. 1964). The basic mechanics of the cover deformation consists of a simple shear (Ramsay and Graham. 1970) which is centered on the plane of the basement the layers. “En echelon” folds appear of the wrench fault. fault (see Fig. 1). combined with an axial trend oblique with slip between to the orientation
involves fold reorientations line, there is no continuity folds.
than in the and deep by
structures.
of folds appearing
at later stages of wrenching
is controlled
tend to become inclined layer-deformation, layers. Therefore, The geometry
a fan centered
close to the basement, a given vertical extant and orientation of already
a bedded
cover resting
on a basement
During
the experiments,
“en echelon”
in the cover. As a result of early fault motion, of fold axes towards affects parallelism
ODONNE
* and PIERRE
VIALON
~~tit~t de ~eeher~he~ ~~te~dis~ipIj~ai~es de Gkoiogie et de ~~eani4~. Ikmaine Un~vers~taiTe de Grenoble, BP 68, F-38402 Saint Martin d’H&es <France)and ERA no. 603, C.N.R.S. (France)
the geometry
and orientation
1. INTRODUCT
ION
An “en echelon” array of folds associated with a strike-slip fault is a well-known geological structure (see Goguel, 1943; Campbell, 1958; Vialon et al., 1976). However laboratory models of these structures have not been made systematically, but only to simulate specific cases (e.g., Pavoni, 1961; Wilson, 1970; Lowell, 1972; Wilcox et al., 1973; Graham, 1978). This work does not aim to reproduce an individual special case. Its purpose is to investigate the general geometrical pattern of ‘<en echelon” folds that develops in a simplifying model. Thus “en echelon” folds are seen as structures formed in a stratified cover above a wrench fault in its basement.