Faulting mechanisms in high-porosity sandstones; New Red Sandstone, Arran, Scotland

Geological Society Special Publication - Tập 29 Số 1 - Trang 91-105 - 1987
John R. Underhill1,2, Nigel Woodcock3
1Department of Geology, University College PO Box 78, Cardiff CF1 1XL, Wales
2Present address: Shell Internationale Petroleum Maatschappij B.V. c/o Shell (UK) Exploration and Production, Shell-Mex House (Annexe), Little Adelphi Building, 10 John Adam Street, Charing Cross, London WC2R 0DX
3Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ

Tóm tắt

Summary Faults in the ‘New Red’ aeolian sandstones of Arran are unusual, firstly for occurring as closely-spaced (less than 1 m) often conjugate sets affecting large volumes of rock, and secondly for forming upstanding fault zones with numerous anastomosing strands of granulated rock, each preserving a small increment of slip. Anisotropy, such as bedding and cross-bed sets, has no discernible effect on fault behaviour. In contrast to the underlying Carboniferous rocks, large displacements are rarely concentrated on a single fault plane within the high-porosity sandstones. The proposed cause is slip-hardening of each fault after a very small displacement (less than 10 mm) causing the next slip increment to be taken up through undeformed rock rather than on the original plane. The common factor in recent records of similar faults elsewhere is their occurrence in high-porosity sandstones. Because of the low grain-contact strength, these rocks are partly analogous to unconsolidated sediment. The high porosity promotes high grain-contact stresses which induce rapid cataclasis during initial slip. Grain fracture and spalling of iron oxide coatings and quartz overgrowths produce a seam with reduced grain size, poorer sorting, higher angularity and lower porosity than the unfaulted rock. These factors collectively strengthen the seam because the coefficient of friction is increased, even though cohesion is reduced. This results in a Mohr failure envelope that lies outside the envelope of the undeformed rock for most stress states. A transient pore pressure increase in the fault seam may be important during slip. Rocks deformed by this slip-hardened faulting preserve a record of each increment of strain. If the displacement on each individual fault seam is the same, the geometry of the total fault systems is directly related to the bulk strain. Quadrimodal systems observed by us in Arran, and by others elsewhere, are probably a response to triaxial strain and show that bimodal ‘Andersonian’ fault systems are only special plane strain cases. If the bulk strain is irrotational, both the orientation and relative magnitude of the principal strains might be estimated.

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Tài liệu tham khảo

Anderson E. M. The Dynamics of Faulting 1951 Edinburgh Oliver & Boyd

10.1144/sjg19010047

10.1007/BF00876546

10.1007/BF00876547

10.1016/0191-8141(83)90004-4

10.1130/0091-7613(1982)10<107:NAOOFS>2.0.CO;2

10.1016/S0016-7878(85)80005-5

10.1111/j.1365-3091.1983.tb00676.x

10.1016/0040-1951(75)90060-8

10.1144/transglas.15.2.174

10.1016/0191-8141(85)90048-3

Jamison W. R., Stearns D. W. Tectonic deformation of Wingate Sandstone, Colorado National Monument Amer. Assoc. Pet. Geol. Bull. 1982 66 2584 2608

Lambe T. W., Whitman R. V. Soil Mechanics 1969 Wiley

10.1144/sjg07020162

McLean A. C. , Bowes D. R., Leake B. E. Evolution of fault-controlled ensialic basins in northwestern Britain Crustal Evolution in Northwestern Britain and Adjacent Regions 1978 Liverpool Seel House Press 325 346

McLean A. C., Deegan C. E. The solid geology of the Clyde sheet (55 N/6 W) Rep. Inst. Geol. Sci. 1978 No. 78/9

10.1144/sjg06030295

Pittman E. D. Effect of fault-related granulation on porosity and permeability of quartz sandstones, Simpson Group (Ordovician), Oklahoma Amer. Assoc. Pet. Geol. Bull. 1981 65 2381 2387

10.1016/0040-1951(78)90154-3

Taylor G. I. Plastic strain in metals Jour. Inst. Metal. 1938 62 307 324

Underhill J. R. Triassic evaporites and Plio-Quaternary diapirism in western Greece J. Geol. Soc. in press

Woodcock N. H. Underhill J. R. Emplacement-related fault patterns around the Northern Granite Arran Scotland Bull. geol. Soc. Am. in press

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Geological Society Special Publication

Tập 29 Số 1

91-105

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