Description:

_Journal of Structural Geology 32 (2010) 1933-1942_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Paleomagnetic dating of fracturing using breccia veins in Durness group carbonates, NW Scotland R. Douglas Elmore a,*, Rika Burr a, Michael Engel a, John Parnell b a Geology and Geophysics, University of Oklahoma, Norman, OK, USA b Department of Geology and Petroleum Geology, University of Aberdeen, Aberdeen, United Kingdom Article history: Received 27 September 2009; Received in revised form 8 May 2010; Accepted 24 May 2010; Available online 23 June 2010 Keywords: Paleomagnetic dating Breccias Diagenesis Chemical remagnetization Abstract A paleomagnetic study of red fault-related breccia veins in the Cambro-Ordovician Durness Group in NW Scotland was conducted to determine the time of brecciation, the origin of the veins, and the nature and timing of associated fluid-related diagenetic alteration. The veins contain brecciated fragments of the host Durness Group and strike either east-west or north-south. Clasts of breccia cemented by calcite suggest multiple brecciation events. The host Durness Limestone is a gray dolomite and contains a Devonian chemical remanent magnetization (CRM) that resides in magnetite. The veins contain magnetizations that reside in hematite and are interpreted as CRMs. The breccias in north-south veins contain a Triassic CRM whereas the veins with east-west strikes contain a Jurassic CRM. Authigenic hematite is common in the breccias along growth planes in the calcite cements. The two CRMs within the veins are interpreted as dating two separate brecciation and fluid flow events that precipitated authigenic hematite. The brecciation and fluid flow events are interpreted to be related to extension in the Mesozoic which is consistent with the extensional history of the northern Atlantic margins. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction Clastic veins or breccias are becoming recognized in numerous geologic terranes (e.g., Wright et al., 2009; Wilson et al., 2010) and some contain sediment infills (e.g., Beacom et al., 1999). Most of these breccias are interpreted to be related to faulting (e.g., Woodcock et al., 2006) and they can also be sites for precipitation of hydrothermal minerals (e.g., Sibson, 1986). Determining the timing of movements on such faults is frequently problematic. Paleomagnetic analysis is one approach that can provide constraints on the timing of faulting events (e.g., Dulin et al., 2005). Fault-related breccias occur in the Cambro-Ordovician Durness Group carbonates in northwest Scotland, adjacent to the Moine Thrust Zone (MTZ) (Wilson et al., 2010). These breccias cut through the carbonates of the Durness Group and display evidence that they were conduits for the migration of fluids that caused diagenetic alteration (e.g., hematite authigenesis, calcite cementation). Understanding the origin and timing of these breccias is important because it may provide clues to the timing of faulting and the structural history of the Durness Group rocks west of the MTZ. In addition, determining the nature and timing of the diagenetic * Corresponding author. ConocoPhillips School of Geology and Geophysics, University of Oklahoma, 100 E. Boyd St, Norman, OK, 73019, USA. E-mail address: delmore@ou.edu (R. Douglas Elmore). alteration in the veins should provide information on fluid migration pathways and possible connections between the fluids and orogenic or other geological events. The major objective of this paper is to date the brecciation by determining the timing of fluid-related diagenetic alteration in the fault-related breccias in the Cambro-Ordovician Durness Group near Durness, Scotland (Fig. 1). Another objective is to test if the faults and breccias were conduits for fluids as has been reported for the MTZ (Parnell et al., 2003; Blumstein et al., 2005) and other faults in Scotland (Elmore et al., 2002, 2006). Paleomagnetic data is used to date the migration events. Petrographic and geochemical studies are used to determine the nature and origin of alteration produced by the fluids. 2. Geologic history The MTZ in the Highlands of Scotland extends from the Isle of Skye to the Durness area in northwest Scotland (Fig. 1). Cambro-Ordovician clastic and carbonate rocks in northwest Scotland unconformably overlie Archean Lewisian gneisses and Proterozoic Torridonian sandstones. The Cambrian Eriboll Group is at the base and consists primarily of quartz-cemented marine quartzarenites. These deposits are overlain by the Durness Group, a thick Lower Cambrian to Lower Ordovician carbonate sequence that was deposited on the shallow passive Laurentian continental margin of the Iapetus Ocean. 1934 R. Douglas Elmore et al. Journal of Structural Geology 32 (2010) 1933-1942 Fig. 1. Geologic map of study area in northwest Scotland showing the major geologic units and faults to the west of the Moine Thrust Zone. The Moine thrust is located to the east of the map area. Inset map shows the location of the study area in Scotland. Stars are sampling locations (1: Balnakeil; 2: A838 Road; 3: Durness beach). Modified from British Geological Survey (2002) map. In the mid to late Ordovician an island arc collided with northern Scotland which deformed the Durness Group during this early Caledonian collision (Dalziel and Soper, 2001). During the Silurian and early Devonian Baltica collided with Avalonia and they subsequently collided with Laurentia (Torsvik and Rehnstrom, 2003) producing the MTZ where Proterozoic metasedimentary rocks of the Caledonian belt were carried over the foreland sequence of the Lewisian complex, Torridonian sandstones, and the Cambro-Ordovician shelf carbonates (Coward, 1988). The MTZ (Peach et al., 1907) consists of centimeter to kilometer scale thrusts and folds with the deformation occurring in the late Silurian-early Devonian (Elliott and Johnson, 1980). The rocks in the MTZ have been subjected to low-grade metamorphism (Johnson et al., 1985). The MTZ is located to the east of Durness and Loch Eriboll. In the Durness area (Fig. 1), faults have down-thrown rocks of the overlying Moine Nappe to form the Durness and Faraid outliers (Holdsworth et al., 2006, 2007). In the Devonian, extensional faulting along with magmatic and volcanic activity occurred in Scotland (Butler and Coward, 1984; Serranne, 1992). Early Permian rifting in the North Atlantic involved siliciclastic sedimentation in extensional basins and widespread extrusive and intrusive magmatic activity (Francis, 1991; Hitchen et al., 1995). Normal faults developed in the hanging wall of Caledonian thrusts (Coward et al., 1989), suggesting the thrust zones were reactivated during the Permian extension (Hitchen et al., 1995). The normal fault-bounded basins were part of the initial stages of the breakup of Pangea and the opening of the Atlantic (Hitchen et al., 1995). Wilson et al. (2010) hypothesized that faulting occurred in the Mesozoic and describe examples of ‘red breccias’ associated with the faults in the Durness area. The major rift associated with opening of the Atlantic eventually developed west of Scotland (present orientation) in the Tertiary. In the Tertiary, a large igneous complex and associated dike swarm was intruded (55 Ma; Dagley et al., 1990) on the Isle of Skye. Previous workers have attributed the regional SE dip (w10 x14) of the lower Paleozoic strata in NW Scotland to flexure because of thrust sheet loading (e.g., Soper and Barber, 1982) or to post-Caledonian tilting (Butler and Coward, 1984). Some studies point out that there is evidence for post-Mesozoic exhumation and tilting in NW Scotland that is probably related to Tertiary underplating associated with the proto-Iceland plume (e.g., Wilson et al., 2010). Three localities were sampled in northwest Scotland (Fig. 1). Most samples of breccias (matrix and clasts) and host rock were collected in a fault zone with an east-west trend at Balnakeil, west of Durness (Figs. 1 and 2A, B). Here the host Durness Group consists of dark gray fractured crystalline dolomite (Fig. 2A). The breccias extend for over 100 m (Fig. 2A) and range from several cm up to w25 cm in width (Fig. 2B). Some of the breccia clasts were large enough to be sampled. The breccias are red and contain clasts of the Durness Group carbonates that range from sand size up to w20 cm (Fig. 2B). The contacts between the red breccias and the host dolomites are relatively sharp although red staining occurs in some thin fractures which extend away from the main fractures. Clasts composed of breccia are found in some breccias, suggesting multiple brecciation events. Samples were also collected along the road (A838) south of Durness next to the Kyle of Durness (Figs. 1 and 2C, D). The host Durness is a gray dolomite that is not as fractured as at Balnakeil. The breccias here are not as defined as at Balnakeil but they have an orientation that is approximately north-south. The samples include clasts and matrix; individual clasts were too small to sample. Two sites were also collected in a dolomitized breccia from the Durness outlier (e.g., Holdsworth et al., 2006) along a fault in the intertidal zone on the beach in Durness (Fig. 1). 3. Methods Twenty paleomagnetic sites (w8 cores per site) were sampled at the three localities (Fig. 1). Samples were collected from the host Durness Group around the veins for contact tests at Balnakeil and the Durness road location to determine if a remagnetization is localized in the veins. The cores were collected with a portable gasoline drill and oriented with an inclinometer and Brunton compass. The natural remanent magnetization (NRM) of standard paleomagnetic samples (2.2 cm) was measured using Ключевые слова: brecciation, caused, group, elmore journal, authigenic hematite, diaz-tushman, set, locality, road, mtz, francis, ?uids, carbonate, doi, host durness group, moine thrust, breccia clast, component, treatment, douglas elmore, eds, strike, dolomite, fault zone, butler, geological, extensional history, reported, remagnetization, durness group, breccia vein, gray dolomite, uid, triassic, representative specimen, tectonics, host dolomite, thermal demagnetization, journal geological, geological magazine, durness, host durness, diagenetic, site, fracture, regional tilt, growth plane, geophysical, study, evidence, zijderveld, sharp, west, event, tertiary, age, modern vrm, host rock, geological society, paleomagnetic dating, paleomagnetism, rock, devonian, conduit, authigenic, parnell, thrust, structural, permian, dened, sturt, holdsworth, magazine, johnson, triassic crm, pole, douglas, northwest scotland, acquired, calcite cement, remanent, geology, woodcock, christiansen, specimen, demagnetization step, study area, range, torsvik, elmore, magnetization, basin, society london, durness figs, test, nw, thrust zone, breccia specimen, declination, area, balnakeil, demagnetization, resides, dickson, tarling, table, journal geological society, circle representing, vein, hematite, thermal, wa, contact test, dunlop, shallow inclination, geographic coordinate, calcite phase, result, dagley, moine, direction, weak, location, journal geophysical, ?uid ?ow, series, northwest, engel, durness area, zone, scotland, lohmann, sibson, faulting occurred, inclination, southerly declination, step, ?uid, mesozoic, sample, high, journal structural, london, interpreted, nrm, post tilting, jurassic, atlantic, wilson, fisher, serranne, timing, blumstein, analysis, temperature, coercivity, crm, breccia, paleomagnetic, coward, remanent magnetization, faulting, form, magnetite, fault-related breccia, structural geology, crms, cemented, ?ow, elsevier, breccia sample, society, northeasterly declination, history, fault, journal, host, kirschvink, kent, calcite, collected, extension, clast