Joint development normal to regional compression during flexural-flow folding: the Lilstock buttress anticline, Somerset, England

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Abstract

Alpine inversion in the Bristol Channel Basin includes reverse-reactivated normal faults with hanging wall buttress anticlines. At Lilstock Beach, joint sets in Lower Jurassic limestone beds cluster about the trend of the hinge of the Lilstock buttress anticline. In horizontal and gently north-dipping beds, J3 joints ( 295–285° strike) are rare, while other joint sets indicate an anticlockwise sequence of development. In the steeper south-dipping beds, J3 joints are the most frequent in the vicinity of the reverse-reactivated normal fault responsible for the anticline. The J3 joints strike parallel to the fold hinge, and their poles tilt to the south when bedding is restored to horizontal. This southward tilt aims at the direction of σ1 for Alpine inversion.

Finite-element analysis is used to explain the southward tilt of J3 joints that propagate under a local σ3 in the direction of σ1 for Alpine inversion. Tilted principal stresses are characteristic of limestone–shale sequences that are sheared during parallel (flexural-flow) folding. Shear tractions on the dipping beds generate a tensile stress in the stiffer limestone beds even when remote principal stresses are compressive. This situation favors the paradoxical opening of joints in the direction of the regional maximum horizontal stress. We conclude that J3 joints propagated during the Alpine compression caused the growth of the Lilstock buttress anticline.

Introduction

An old paradigm is that outer-arc stretching leads to fold-related jointing (Van Hise, 1896). Outer-arc stretching is associated with tangential longitudinal strain folds that develop when a homogeneous, isotropic layer is buckled (Price and Cosgrove, 1990). The tangential longitudinal strain fold is divided by a neutral surface beyond which the inner arc of the fold is compressed. Joints that initiate in the outer arc of anticlines propagate down to but do not cross the neutral surface. When fold-related jointing penetrates the entire thickness of a bed, the tangential longitudinal strain model is less useful as a mechanism to explain the strain that may have caused jointing. This is also true when jointing is well developed in the limbs of folds or at inflection points where strain goes to zero in a tangential longitudinal strain fold. To understand the development of jointing in the limbs of folds, we must look to another model. Flexural flow folds have maximum strains at the inflection points and zero strain at the hinges (Price and Cosgrove, 1990).

We consider the possibility that one or more sets of unfilled joints of the Bristol Channel were contemporaneous with flexural-flow folding during Alpine shortening. Here, we describe field data gathered to test this hypothesis. We presume that the joint geometries interpreted to date from the Alpine inversion reflect the superposition of local and remote (i.e. regional) stress conditions. We also use the orientation of systematic joints in carbonate beds to constrain a mechanical model for stress distribution during flexural-flow folding in interlayered limestone–shale beds. With the help of this mechanical model, we propose a new mechanism for the joint driving stress. This mechanism does not rely on outer-arc extension commonly cited as the conventional fold-related jointing mechanism. Under this new mechanism, fold-related joints are driven completely through beds without arrest at a neutral surface.

Section snippets

The regional picture

The Bristol Channel Basin is an E–W striking Mesozoic rift basin (Fig. 1). Normal faults dip either north or south to divide the basin into fault blocks (Dart et al., 1995). The basin was inverted in the Eocene and Oligocene during a period of N–S compression associated with Alpine tectonics (Holloway and Chadwick, 1986; Arthur, 1989; Dart et al., 1995; Nemcok et al., 1995). Alpine deformation was accommodated by reverse-reactivation of normal faults, new thrust faults, new strike-slip faults,

Data collection

Our field approach at Lilstock Beach was to sample systematic joints in a volume of the Blue Lias measuring 40 m thick by 200 m long (E–W). The repetition of the section by high angle faults permitted sampling up to 50 m in the third dimension (i.e. N–S) of some beds. In sampling the volume, we were particularly interested in the affect of Alpine inversion on joint development. Our premise is that if joints propagated during Alpine inversion, they are likely to be found in the south-dipping

Identification of joint sets

A joint set is a collection of systematic parallel joints (Hodgson, 1961). Poles to joints of a systematic set fall, however, in a cluster several degrees wide because individual joints are never perfectly planar and there are measurement errors (Fig. 5). Because of these variations, joint sets separated by <10° in orientation may have overlapping clusters of poles in a stereographic plot. In this case, placing an individual joint into one set or the other is impossible during post-fieldwork

The association between J3 joints and the Lilstock buttress anticline

Six observations lead us to conclude that J3 joints in the south-dipping beds at Lilstock Beach propagated during the development of the Lilstock buttress anticline reflecting Alpine inversion:

  • 1.

    J3 joints are rare in the subhorizontal beds of Lilstock Beach but are the most common set in the steep limb of the Lilstock buttress anticline. The mechanism driving J3 jointing must involve higher strain on the limbs of folds and little strain in the hinge area.

  • 2.

    J3 joints are most common in the vicinity

Other examples of fracture development during flexural-flow folding

Flexural-flow appears to be a common process in folds (e.g. Ramsay, 1962, Ramsay, 1967). Our model predicts fanning joint patterns that are similar to the strongly-convergent dip isogons shown by class 1A folds of Ramsay (1967; fig. 724). Similar patterns of joints within flexural-flow folds are shown by Hills (1963; fig. VIII-14B) and by Whitten (1966; fig. 140). Balk (1937) describes similar fanning patterns of joints developed in country rocks above plutons.

The literature also contains

Conclusions

At Lilstock Beach, a number of joint sets in the limestones beds of the Jurassic Blue Lias cluster about the strike of the axes of the Lilstock buttress anticline, including J1 (330°–310°), J2 (310°–295°), J3 (295°–285°), J4 (285°–275°), J5 (275°–265°), and J6 (< 265°). In subhorizontal beds, including those near the hinge surface of the Lilstock buttress anticline, J3 joints are rare, whereas J2, J4, J5, and J6 abut to indicate an anticlockwise sequence of development. In the south-dipping

Acknowledgements

Paul Hancock first introduced TE to the wonderful Lilstock outcrop more than 15 years ago. This trip was the beginning of a fruitful collaboration that lead to our work on neotectonic joints (Hancock and Engelder, 1989). Our collaboration reached its zenith when Paul and his wife, Ann, hosted Jan Engelder and TE in their lovely Georgian row house in Bristol after our field campaign at Lilstock during the summer of 1997. We only wish that Paul could have seen the fruits of our work at Lilstock.

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