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Fault Rocks and Fault Mechanisms

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Fault Rocks and Fault Mechanisms Fault rocks and fault mechanisms R. H. SIBSON SUMMARY Physical factors likely to affect the genesis of the with the production of mylonite series rocks various fault rocks--frictional properties, tem- possessing strong tectonite fabrics. In some cases, perature, effective stress normal to the fault and fault rocks developed by transient seismic fault- differential stress--are examined in relation to ing can be distinguished from those generated the energy budget of fault zones, the main by slow aseismic shear. Random-fabric fault velocity modes of faulting and the type of fault- rocks may form as a result of seismic faulting ing, whether thrust, wrench, or normal. In a within the ductile shear zones from time to conceptual model of a major fault zone cutting time, but tend to be obliterated by continued crystalline quartzo-feldspathic crust, a zone of shearing. Resistance to shear within the fault elastico-frictional (EF) behaviour generating zone reaches a peak value (greatest for thrusts random-fabric fault rocks (gouge--breccia-- and least for normal faults) around the EF/OP cataclasite series--pseudotachylyte) overlies a transition level, which for normal geothermal region where quasi-plastic (QP) processes of gradients and an adequate supply of water, rock deformation operate in ductile shear zones occurs at depths of lO-15 km. SINCE LAPWORTH'$ (I885) description of the type mylonite from the Moine Thrust in NW Scotland, there have been many petrographic descriptions and classifications of the texturally distinctive rocks found associated with fault zones (e.g. Waters & Campbell 1935, Hsu 1955, Christie 196o , 1963, Reed 1964, Spry I969, Higgins 1971 ). These rocks provide a tangible source of information on the processes which operate in major fault zones, but little has been done to correlate the various types of fault rock with different deformation environments and modes of faulting. In this paper, I discuss the physical factors which may affect the genesis of fault rocks, and make some tentative correlations between their textures and deforma- tion environments. For the most part, the effects of faulting in crystalline quartzo- feldspathic crust are considered. I. Textures and occurrence of fault rocks In this paper fault rocks is used as a collective term for the distinctive rock types found in zones of shear dislocation at both high and low crustal levels, whose textures are thought to arise at least in part from the shearing process. It is not denied that similar textures may develop in association with other geological structures (e.g. protoclastic textures arising from igneous intrusion). The term cataclastic rocks, introduced by Waters & Campbell (1935) as a collective name for all rocks of the gouge--breccia--cataclasite--mylonite kindred, is not used because of the misleading implication that such rocks have developed solely by cataclasis. In the strict sense, cataclasis involves the brittle fragmentation of mineral grains with rotation of grain fragments accompanied by frictional grain boundary sliding dTl geol. Soc. Lond. vol. I33 , 1977, pp. 191-213, 8 figs., 3 plates, 3 tables. Printed in Great Britain. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/133/3/191/4885339/gsjgs.133.3.0191.pdf by guest on 01 October 2021 I92 R. H. Sibson and dilatancy, and it is now clear that these are not the dominant processes leading to the formation of mylonite series textures. In quartzo-feldspathic mylonites, quartz grains have been reduced in grain size by the dynamic recovery and recrystallization of highly strained grains which have undergone intense intra- crystalline plastic deformation (Bell & Etheridge I973, White I973). It is also apparent, as originally noted for the type mylonite (Lapworth I885, Teall I918), that extensive recrystallization and neomineralization occurs in the groundmass of most cohesive 'cataclastic' rocks. (A) TEXTURAL CLASSIFICATION Because changes in textural type tend to be gradational (P1. 3 a) and some fault rocks retain mixed textures resulting from polyphase deformation under different T A B L E I : Textural classification of fault rocks RANDOM - FABRIC FOLIATED FAULT BRECCIA (visible fragments >30% of rock mass) FAULT GOUGE (vlslble fragments 430% of rock mess) L w PSEUDOTACHYLYTE CR~SH BRECCIA (frasments • 0.5 cm) I FINE CRUSH BRECCIA (O.lcm • frasso < 0.Scm) CRUSH NICROBRECCIA (fragments < 0. I cm) O ! PROTOCATACLASITE PROTOHYLOKITE ..... N ¢.o '-t- O O ! CATACLASITE I~fLONITE ~O Z ULTRACATACLASITE ULT~ONITE BLASTONYLONITE Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/133/3/191/4885339/gsjgs.133.3.0191.pdf by guest on 01 October 2021 Loch Eport SCOTLAND C~ Loch 0bisary % // I / ~EAVAL'. ",',', "," .''," • 347m ......... '.'o'.''.'','-',. |.. J.~ KEY Thrust r" Crush zone Phylbniti¢ shear belt ~. • . ° Crush • . melange. ". ° . ltl ;~ ...- , . ..:.,,. , ~ .I I . - ~,~ 0 b Mylonitic { 30-60° ' I foliatm ( 60_90o ..4" J increasing palaeotemperature , > IX~eudofachylyte- uitraca~¢lasite crush zone Fzo. I Map and schematic cross-section of the Outer Hebrides Thrust in Eaval block, N Uist. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/133/3/191/4885339/gsjgs.133.3.0191.pdf by guest on 01 October 2021 194 R. H. Sibson T A B L E 2 "Fault rocks and style of faulting in the Outer Hebrides Thrust zone StTle of Faulting Fault Rocks Brittle shearing of intact rock~ Pseudotachylyte Sliding on existing planes J Cataclastic crush zones Cataclaslte-Ultracataclasite (& some pseudotachylyte) Crush Melange Crush breccias, microbreccias & protocataclasite Quasi-plastlc shear zones Phyllonltic mylonites & ultramylonites Crush Melange with crude Protomylonites mylonitic foliation crustal conditions (PI. 2), (Christie I96O) attempts to classify the textures of fault rocks in a pigeon-hole manner that can never be entirely satisfactory. The scheme outlined in Table I serves as a simple reference system, adequate for the following discussion. It is based largely on that put forward by Spry (I969, p. 229), modified to avoid genetic connotations, with some additional elements from Higgins' (i 97 I) classification. The whole has been rearranged to emphasize a division which I consider to be of great mechanical significance; the separation of those fault rocks with an essentially random shape (and crystallographic) fabric from those that are foliated, usually with a strongly inosculafing L-S shape fabric (fluxion structure). Another main division is made on the presence or absence ofprirnary cohesion, the cohesive fault rocks then being further subdivided on the nature of their matrix. Cohesive rocks in which tectonic reduction in grain size has dominated processes of grain growth form the bulk of the commonly recognized fault rocks formed at other than near surface conditions (P1. I). The term phillonite is retained as a useful descriptive name for hydrated, mica-rich mylordtes and ultramylonites which have the silky appearance of phyllites. (B) DISTRIBUTION WITHIN FAULT ZONE'S Many major ancient fault zones are now exposed at erosion levels which corres- pond to considerable depths when the faults were active. They often consist of a mesh of shear zones enclosing lozenge-shaped areas of comparatively undeformed rock, the whole being perhaps a few kilometres or possibly tens of kilometres in width. Variations in both the style and the rock products of faulting occur across such zones when they have a large component of finite dip-slip. As an example we consider a section across the Outer Hebrides Thrust, a major dislocation of prob- able late Caledonian age which borders the eastern coastline of the Outer Isles in NW Scotland (Francis & Sibson 1973, Sibson 1975). This structure is especially suitable for discussion, because it disrupts crystalline Lewisian basement gneisses of fairly uniform bulk composition, and because fault rocks are generally best Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/133/3/191/4885339/gsjgs.133.3.0191.pdf by guest on 01 October 2021 J1 geol. Soc. Lond. x33 , i977 SIBSON a • . ~ '~,q~ ~ e :: PLATE I Mylonite series (a--protomylonite, b--mylonite, c--ultramylonite) versus Catacla- site series (d--protocataclasite, e--cataclasite, f--ultracataclasite). All specimens from the Outer Hebrides Thrust zone (crossed polars except c & f, × 7)- 194 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/133/3/191/4885339/gsjgs.133.3.0191.pdf by guest on 01 October 2021 J1 geol. Soc. Lond. I33 , 1977 SIBSON (a) (b) PLATE 2 Polyphase texture from Seaforth Head, Lewis (NB 305 158). (a) Vein material forming breccia matrix and disrupting banded mylonite-ultramylonite, was probably random- fabric pseudotachylyte developed by seismic faulting within a ductile shear zone (plane polars, × 3.0). (b) Porphyroclasts within vein are now aligned in a strong shape fabric as a result of continued ductile deformation (plane polars, x 15 . 75)- Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/133/3/191/4885339/gsjgs.133.3.0191.pdf by guest on 01 October 2021 J1 geol. Soc. Lond. i33 , i977 SIBSON (a) (b) PLAIE 3" (a) Gradational change in texture. Amphibolite -+ protomlyonite -~- mylonite --~ ultramylonite (Ness, Lewis NB 521664) (plane polars, × 14.o ). (b). Phyllonitic shear belt cutting crush m61ange, east coast ofN Uist (NF 922597). Note marginal curve-in of schistosity and asymmetric chevron folds developed within the belt. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/133/3/191/4885339/gsjgs.133.3.0191.pdf
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