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_Journal of Structural Geology 32 (2010) 641-655_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com/locate/jsg_ _Measurement of diagenetic compaction strain from quantitative analysis of fault plane dip_ _Raluca Cristina Neagu a,*, Joe Cartwright a, Richard Davies b 3D Laboratory, School of Earth and Ocean Sciences, Cardiff University, Main Building, Park Place, Cardiff CF10 3YE, UK b Department of Earth Sciences, Durham University, Science Labs, Durham DH13LE, UK_ _article info_ _Article history: Received 3 May 2009; Received in revised form 12 March 2010; Accepted 29 March 2010; Available online 2 April 2010_ _Keywords: Biogenic silica diagenesis Polygonal faults Fault dip Compaction strain_ _abstract_ _We developed a new technique for quantifying the amount of porosity loss during diagenetic transformation of opal-A to opal-CT. The technique is based on 3D seismic data from offshore Norway, where the widely developed biosiliceous sediments of the Brygge Formation are deformed by a polygonal fault system. Evidence is shown from two study areas that the dip of the polygonal fault planes reduces abruptly across the opal-A to opal-CT diagenetic boundary, suggesting that the fault planes were passively rotated into shallower dips due to diagenesis. The reduction of fault plane dip was used to quantify the vertical compaction strain due to diagenesis. Using an independent assessment of the magnitude of the porosity loss, we are able to validate the method based on fault plane dips, and also to evaluate whether the porosity loss results in an exclusively vertical strain. Additionally, the impact of silica diagenesis on the shear strength of the sediments and fault growth is investigated. We present evidence suggesting that deviations from classical Mohr-Coulomb behaviour may be expected during the combined processes of diagenesis and porosity collapse, and may indeed be sufficient to promote continued fault growth._ _? 2010 Elsevier Ltd. All rights reserved._ _1. Introduction_ _It is well established that listric curvature of fault planes can result from compactional flattening of the fault plane as a passive marker during burial, either during or after the main phase of fault activity (Jones and Addis, 1984; Davison, 1987). This compactional effect may explain at least some of the widely observed listric fault geometries exhibited by normal faults in shallow parts of sedimentary basins (e.g. Shelton, 1984). Compactional flattening of fault planes is likely to be most pronounced where a fault transects a dominantly fine-grained sedimentary succession, because the amount of compaction due to consolidation is much greater for finer than for coarser lithofacies (Rieke and Chilingarian, 1974; Velde, 1996). The quantification of compaction strain is an important element in basin analysis, because it accrues as a result of the advective loss of pore fluid from the originally deposited sediment, and this flux is a critical input into analyses of pore fluid overpressure, fluid flow, and thermal evolution of sedimentary basins (Audet and Fowler, 1992; Bayer and Wetzel, 1989; Bethke and Corbet, 1988)._ _* Corresponding author. Tel.: ?44 (0)29 208 76294; fax: ?44 (0)29 208 74326. E-mail addresses: neagurc@cf.ac.uk, ralukaneagu@yahoo.com (R.C. Neagu)._ _0191-8141 $ e see front matter ? 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2010.03.010_ _Based on normal faults from an outcrop in the Rec?ncavo Basin (NE Brazil), Davison (1987) recognised that the early normal faults in shale act like passive markers subjected to compaction. He suggested that their dip shallows due to compaction and discussed the decompaction of the fault plane dips. The present study takes further his observations, applying them to a much larger scale. In general, small scale methods have been employed to estimate vertical flattening strains, e.g. from the deformation of bedding around nodules (Craig, 1987), or from ptygmatic folding of sandstone dykes (Parize and Beaudoin, 1988). On a slightly larger scale, differential compaction of isolated channel sand bodies encased in a muddier host lithology has also been used to infer vertical flattening strains (Hillier and Cosgrove, 2002). More recently, forward modelling of differential compaction folding due to irregular advancement of a silica diagenetic front has been undertaken to quantify the effects observed during diagenetic transformation of opal-A to opal-CT (Davies, 2005; Meadows and Davies, 2008). This diagenetic transformation involves a relatively sudden loss of a considerable fraction of the porosity, due to mineral dissolution and precipitation over a narrow depth interval of typically a few tens of meters (Isaacs, 1981; Chaika and Dvorkin, 1997; Nobes et al., 1992; Davies et al., 2008)._ _This paper focuses on the impact of this diagenetically-induced compaction on fault plane dips, where a pre-existing fault is embedded in a sedimentary succession that is subject to silica diagenesis. The main aims of this paper are to document the nature of the relationship between fault plane dip and porosity loss due to silica diagenesis, and to develop a method for quantifying the amount of porosity loss from measurement of fault plane dip. By attempting an independent assessment of the magnitude of the porosity loss, we are able to validate the method based on fault plane dips, and also to evaluate whether the porosity loss results in an exclusively vertical strain. A purely one-dimensional flattening would be the predicted result for classical models of soil or sediment consolidation (Terzaghi and Peck, 1948; Skempton, 1981), rather than an alternative of a more fully three-dimensional strain, which might be expected for chemical compaction where cementation is a major process for reduction of porosity (Cartwright and Lonergan, 1996)._ _Our method for estimating the compaction strain during diagenesis is based on observations of fault plane dip using 3D seismic data from the continental margin of Norway, in which widely developed biosiliceous sediments of the Brygge Formation (Brekke, 2000) are deformed by a polygonal fault system. Polygonal fault systems are laterally extensive arrays of extensional faults arranged in polygonal patterns in map view and confined to a specific stratigraphic interval (Cartwright and Dewhurst, 1998). Polygonal faults have been extensively described in recent years based mainly on reflection seismic data (e.g. Henriet et al., 1991; Cartwright, 1994, 1996; Cartwright and Dewhurst, 1998; Dewhurst et al., 1999; Watterson et al., 2000; Goulty, 2001, 2002; Stuevold et al., 2003; Goulty and Swarbrick, 2005), but there has been only limited investigation of the relationship between polygonal faulting and silica diagenesis (R. Davies et al., 1999; Davies, 2005; Davies et al., 2008, 2009), and this has not addressed the question of fault plane dip. We show evidence from two study areas that the dip of the polygonal fault planes reduces abruptly across the opal-A to opal-CT (from here abbreviated A CT) diagenetic boundary, and a central theme of this paper is that this reduction in dip is due to the diagenetic loss of porosity._ _A secondary theme of this paper is the extent of any effect of the silica diagenesis on the shear strength of the sediments and whether, if at all, this impacts fault growth e.g. through changing the physical properties of the deforming sediments. It is pertinent to ask, for example, whether slip accumulation on the faults is reduced or ceases altogether either as a result of the reduction in fault plane dip or due to the change in physical properties once opal-A has been converted into opal-CT. Investigations of the role of diagenesis on the evolution of sediment physical properties during burial and deformation are in their infancy (Shin et al., 2008), so our motivation is that the quantification here of the relationship between fault plane dip and physical property changes due to diagenesis may help promote further research on these complex, coupled phenomena._ _Biosiliceous sediment is mainly composed of diatom frustules and radiolarian tests (amorphous silica or opal-A). They are relatively resistant to mechanical compaction, showing a remarkably high retention of porosity during burial (Hamilton, 1976). Opal-A converts to opal-CT (cristobalite and tridymite) by a set of reactions involving dissolution of opal-A and precipitation of opal-CT (e.g. Murata and Larson, 1975; Isaacs, 1981; Williams and Crerar, 1985). The diagenetic transformation occurs at low temperature, generally in the range 20?50°C the precise value depending on lithology, pH, presence or absence of clays and carbonate, and sediment age (Kastner et al., 1977; Williams and Crerar, 1985). The diagenesis results in a dramatic reduction in porosity (e.g. Chaika and Dvorkin, 2000). Typical ODP (Ocean Drilling Program) boreholes calibrating this diagenetic transformation show ranges from 5 to 25% reduction in porosity values, with the lower values relating to lesser proportions of opal-A in the bulk sediment. In many ODP and industry borehole calibrations, the reduction in porosity occurs over a vertical interval of approximately 10?20 m (e.g. Nobes et al., 1992). It has been shown that this porosity reduction results from diagenesis and not from gradual mechanical compaction due to increasing sediment load (e.g. Isaacs, 1981)._ _The decrease in porosity during silica diagenesis is correlated to an increase in sediment bulk density and compressional wave velocity (Nobes et al., 1992). Importantly for this study, these changes combine to give a pronounced acoustic impedance contrast and this vertically abrupt contrast ensures a high-amplitude response marking the diagenetic boundary on seismic reflection data (e.g. Hein et al., 1978; Bohrmann et al., 1992; Brekke, 2000; Davies and Cartwright, 2002)._ Ключевые слова: sedimentary, silica, ridge, velocity, variation, silica diagenesis, density, transformation, parallel, combined process, monterey formation, shear, data, upward migration, biosiliceous mudstones, jones, gay, structural analysis, survey, porosity, structural element, kai formation, case study, interval velocity, cross, ormen, strain, segment, sea, fault segment, eds, measurement, upper, fault zone, isaacs, geological, soil, opal-ct, polygonal fault, angle, seismic data, uid, throw, act boundary, measured, eds proceedings, cartwright, journal geological, throw measurement, burial depth, norwegian, map, porosity loss, fault strike, diagenetic, neagu, dip reduction, study, lonergan, direct measurement, solid, total range, marine, nobes, opal-a opal-ct, normal fault, geological society, opal, distribution, ct, biosiliceous, geophysics, structure, soil mechanics, york, sediment compaction, polygonal, noaa amante, structural, ormen lange, bulk density, kai formations, survey area, marine geology, gjallar ridge, physical property, opal-ct transformation, tez plot, sedimentary layer, independent assessment, fault representing, geology, normal, tectonic evolution, compactional attening, america davies, compaction, dvorkin meadows, review, study area, davies, range, rich, fault plane, fault dip, margin, faul, structural geology, basin, skogseid, diagenetic boundary, pore space, situ stress, dip angle, diagenetic transformation, brown, odp scientic, seismic survey, area, biosiliceous sediment, stratal reections, interval, table, plane dip, histogram showing, shallower dip, located, differential compaction, depth, wa, shear strength, phase, change, burial, seismic proles, siliceous rocks, thickness, result, density difference, ha, conning stress, ct re?ection, porosity collapse, bulletin, formation, vertical, location, polygonal faulting, selected fault, fault plane dip, abrupt change, lange, passively rotated, correlation coefcients, act reection, berndt, re?ection, reduction, compaction strain, porosity change, paper, discussion, ?uid, biogenic silica, high, depth conversion, journal structural, permit slip, stratigraphic, growth, diagenesis, brygge, sea-oor spreading, odp, porosity reduction, kai, neagu journal, effectively ceased, late pliocene, average, sedimentary basin, dip, boundary, davies cartwright, maximum throw, eldholm, odp odp, marker, case, preferred orientation, mre basins, seismic, early, horizontal distance, petroleum, schematic representation, initial, walsh, ne-grained sediment, elsevier, faul faul, society, time, sediment, brekke, journal structural geology, plane, fault, journal, solid fraction, fault growth, error bar, dip map, density log, observed, goulty, grain scale, measurement precision, brygge formation, opal-a, gjallar