Confluence and intersection of interacting conjugate faults: A new concept based on analogue experiments

Abstract

Examination of the interaction area of conjugate faults yields evidence of a yet unknown structural pattern. Analogue experiments revealed the evolution of this pattern during the interaction process. A fault propagating towards the highly active section of another conjugate fault becomes increasingly deflected towards the obtuse angle side of the conjugate system while approaching until joining it. During this process, the tip of the curved propagating fault retains its shear sense until immediately before joining with the highly active fault—a process called “confluence” in this report. Under constant remote stress, the deflected fault now becomes dominant. A fault branch splits off at the point where deflection commences, propagating straight ahead of the original direction, and may dissect the former master fault. Alternating activity at both of the faults finally produces the known intersection patterns. Further studies of the proposed concept on natural examples are recommended due to the close geometric similarity of the evolutionary structural stages and natural fault patterns.

Introduction

Study of conjugate faults yields evidence of their interaction and the dynamic processes governing growth of the fault sets and displacement along them. The key question has been investigated in detail: what happens in the region of intersection? Watterson et al. (1998) distinguish between three methods of establishing the volume balance: (a) ductile thinning by inter-grain slip, (b) pressure solution, and (c) multiple cross-cutting faulting. Another mode of volume accommodation is mass transport toward the surface resulting in flower structures. In this study, we describe an additional kinematic process and its structural features of interacting conjugate faults within the intersection region.

Most of the intersections of natural faults present sharp-cut junctions with or without displaced sections or display a strongly fragmented damage zone. The literature on conjugate faults generally recognizes these types. Surprisingly often, however, the region of potential or existing intersection of conjugate faults reveals a curved link between the faults on the obtuse angle side (Fig. 1).

The lack of a detailed description of such curved links (excluding short notes by Anderson, 1951, p. 53 ff. and Reches, 1988, p. 147) or even of an explanation for such a geometric feature induced us to perform analogue experiments and to examine the generation and kinematic function of these curved links and related structures. Their frequent occurrence in nature at a wide range of scales makes them appear the specific result of an interaction process at conjugate faults producing a sequence of characteristic geometric patterns.

We differentiate between four main types (Fig. 2). A small fault 2 propagating towards a pre-existing fault 1 becomes increasingly deflected into the shear direction of fault 1 (Fig. 2a,b). This deflection process results in a curved linkage between the conjugate faults, comparable with the joining of a confluent river with the main river (Fig. 2b). We call this type and process “confluence”. The occurrence of an additional fault 2 branch propagating in a straight direction towards fault 1 marks a new structural development (Fig. 2c), which may be followed by the intersection and displacement of fault 1 (Fig. 2d).

Although the intersection process is well known (Watterson et al., 1998: cross-cutting faulting; Ferrill et al., 2000: crossing conjugate faulting), it has seldom been considered part of a larger evolutionary process (e.g. Davatzes and Aydin, 2003, Flodin and Aydin, 2004: sequential jointing and shearing). Therefore, it appears appropriate to focus on the development of the structure types of Fig. 2 and to integrate them into the entire interaction process. Under this aspect, we restrict the term “intersection” to the cross-cutting patterns of type d of Fig. 2.

The kinematic principle of interacting faults can best be studied using single fault elements. However, the ubiquity of the structures of interest and the problem of their fractal order in any material and on any scale suggest that shear zones should also be included in the investigation. In this report, we use the term “shear zone” for a zone of synthetic and antithetic fault sets organized into domains or as a chain of en echelon faults, including their damage zones. Shear zones and faults can be constituents of a conjugate system with equal kinematic functions.