Elsevier

Journal of Structural Geology

Volume 21, Issues 8–9, August–September 1999, Pages 1131-1142
Journal of Structural Geology

Is there a close spatial relationship between faults and plutons?

https://doi.org/10.1016/S0191-8141(99)00024-3Get rights and content

Abstract

Using spatial relationships between individual plutons and faults to support genetic correlations between faulting and magmatism is meaningless since even random magmatic or tectonic processes will result in some plutons adjacent to faults. Our initial analyses of populations of faults and Carboniferous plutons in the Armorican Massif, France and faults and Alleghanian plutons in the southern Appalachians, USA indicate that plutons have broad distributions with respect to faults but with a tendency for plutons to occur away from faults. Maxima of integrated pluton areas occur at 1/4 (Appalachians) and 1/2 (Armorican) of the distance between the average fault spacing in these orogens. Although there is a great need for statistical evaluations of relationships between populations of igneous bodies and structures in a wide variety of settings and crustal depths, our initial studies suggest that faults do not preferentially channel magma during ascent or emplacement and that these are relatively unfocused processes within orogenic belts.

Introduction

Plutons and faults of similar age invariably occur together in a variety of tectonic settings, an observation which has led to a growing perception that faults and plutons generally have a close spatial relationship Hutton, 1988, Glazner, 1991, D'Lemos et al., 1992, Hutton and Reavy, 1992, Tikoff and Teyssier, 1992. Similar correlations have been made at surface levels between linear arrangements of volcanic centers and faults or fractures Draper et al., 1994, Lutz and Gutmann, 1995 and at deeper crustal levels between igneous bodies and a variety of co-eval structures including faults Hutton and Reavy, 1992, Brown et al., 1995, Collins and Sawyer, 1996. These relationships, typically based on qualitative observations of geological maps, are repeatedly cited as evidence that the ascent, emplacement, and sometimes the generation of magmas are controlled by faults Strong and Hanmer, 1981, Hutton, 1988, D'Lemos et al., 1992. If these hypotheses are correct, then regional deformation, particularly faulting, plays a fundamental role in magmatic processes at all crustal levels. This is such an important conclusion that its validity needs careful evaluation.

If indeed a causative relationship exists between faults and plutons, then the following should be demonstrable: (1) a strong spatial correlation; (2) a close geometric relationship; (3) a close temporal relationship; (4) compatible rates of faulting and plutonic processes; and (5) specific thermal–mechanical mechanism(s) by which magma is generated, ascended, and/or emplaced along faults. We suggest that even the first step of establishing and evaluating spatial relationships between faults and plutons is difficult at best. This is true for several reasons. First, devising useful methods for comparing spatial relationships between curviplanar faults and irregularly shaped plutons is not straightforward. Second, even in regions where previous authors have argued for a close spatial relationship between faults and plutons, there are invariably examples of plutons not in contact with, or adjacent to, faults. Thus, there is rarely, if ever, a simple one-to-one relationship between faults and plutons, emphasizing the need to quantitatively compare populations of plutons and faults rather than simply noting that one or more plutons lie along one or more faults. This step is necessary because even if faulting and plutonism are two independently operating processes, some plutons will invariably occur adjacent to faults. Finally, even if a close spatial relationship between populations of faults and plutons can be established, it may not indicate a causative link between faulting and magma ascent and emplacement. It is necessary to determine whether or not the established spatial relationship has a high probability of resulting from two independently operating processes, or requires a causative link.

Our goals in this paper are the following: (1) to explore issues that need consideration when evaluating spatial relationships between populations of faults and plutons; (2) to present an initial evaluation of two natural examples, Alleghanian plutons in the southern Appalachians, USA (Fig. 1), and Carboniferous plutons in the Armorican Massif, France (Fig. 2); and (3) to discuss the difficulties faced when attempting to determine if our results imply a genetic relationship between faulting and magma ascent and emplacement.

Section snippets

Establishing spatial relationships

Much of the literature pertaining to analysis of spatial relationships concerns establishing whether a distribution of a single type of object, typically represented by a pattern of points on a map, fits one of the following hypotheses (e.g. Boots and Getis, 1988): (1) complete spatial randomness in which all locations in the area studied have an equal chance of containing a point, and the location of one point in no way influences the location of another point; (2) a regular pattern in which

Armorican Massif, France

One orogenic belt in which it has long been argued that faults and plutons are intimately related is the Armorican Massif, Brittany, France Gapais and Le Corre, 1980, Strong and Hanmer, 1981, Guineberteau et al., 1987, Roman-Berdiel et al., 1997. Rocks of the Paleozoic Variscan orogen are cut by numerous strike-slip faults (Fig. 2), the most famous of which is the South Armorican shear zone. This region is of historical interest because it contains the shear zone in which SC structures were

Discussion

We emphasize that the above measurements help to characterize the spatial relationship between pluton and fault populations, but do not by themselves establish the significance of these relationships. To do so we need to evaluate whether it is statistically likely that two independently operating processes (in our case faulting or the existence of non-active faults and pluton emplacement) develop datasets such as those determined earlier, or whether it is likely that these datasets require some

Conclusions

When evaluating relationships between magmatism and deformation in orogenic belts, we suggest that it is important to consider ways of testing a variety of contrasting hypotheses such as whether faulting and magmatism are closely linked or whether there is no relationship between faulting and magmatism except that both occur in orogenic belts with restricted areal extent. More specifically we can ask whether: (1) the production of magma is triggered by faulting or magma-induced fracturing

Acknowledgements

We gratefully acknowledge support from NSF grants EAR-9627986 and EAR-9614682 awarded to Paterson while completing this research. We thank Scott Johnson for reading the manuscript and David Bowman, Scott Johnson, and Steve Lund for many discussions about fault and pluton relationships. Paterson thanks Michel Faure for organizing a field trip to the Armorican Massif and for his hospitality while staying in France. We also wish to take this opportunity to thank Sue Treagus for her role in shaping

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