Structural development of the Håsteinen Devonian Massif, its Caledonian substrate and the subjacent Nordfjord–Sogn Detachment Zone - a contribution to the understanding of Caledonian contraction and Devonian extension in West Norway
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In western Norway, the contractional history of the Caledonian Orogeny ended with the Scandian continentcontinent collision between Baltica and Laurentia at around 420-400 Ma, resulting in formation of eclogites in the subducted Baltic margin (Western Gneiss Region, WGR) and the top-E emplacement of outboard terreranes (for example the Høydalsfjorden Complex, HC) and nappes of Baltic affinity. The collapse of the orogenic belt started at the end of the Early Devonian period, with reverse top-W movement of the entire Scandian nappe pile on the shallowly W-dipping basal decollement zone. This movement, termed Mode-I, was dated to 402–394 Ma (Fossen & Dunlap 1998), and interpreted to mark the start of divergent movements between the Laurentian and Baltic plates. During the Mode-I movements, the eclogitic WGR experienced exhumation together with the above-lying Mode-I zone. At the end of the Mode-I movements, the Mode-I zone had lost its westward dip, and the resulting locking of the zone led to formation of the Mode-II Nordfjord- Sogn Detachment zone (NSDZ), cutting through the Scandian nappe pile. The NSDZ zone was a shallowly Wdipping mega-shear zone that divided the orogenic belt into an eastern "Lower Plate" and a western "Upper Plate". The Upper Plate moved down- and W-wards on the NSDZ, causing formation of basins that were filled with continental Devonian clastic sediments. In the study area, located 15 km ESE of the town of Florø in the municipality Flora, Sogn og Fjordane, three tectonostratigraphic units are present. These are from base to top, the Eikefjord Group (EG), the Høydalsfjorden Complex (HC) and the Håsteinen Devonian Massif (HDM). Each of the units displays different tectonometamorphic histories. The EG is penetratively mylonitised and interpreted to be part of the NSDZ. The HC is part of the above-lying Upper Plate, and the sediments of the HDM were deposited on top of the HC. Precambrian/early Caledonian geological history The earliest geological history in the study area can be found in the Eikefjord Group, which contains Precambrian orthogneisses dated to 1511 +/- 64 Ma, interpreted as a minimum intrusive age (Abdel-Monem & Bryhni 1978). The rocks consist of meta-anorthosite, grey monzonitic biotite-epidote gneiss, amphibolites, metagabbro and minor micaschist, and have been assigned to the "anorthosite-Jotun Kindred" which is correlated with the Jotun and Dalsfjord Nappes. Remnants of an amphibolite facies mineralogy that define the highest metamorphic grade in the rocks is thought to be Caledonian or post-Caledonian. Caledonian (Scandian) contractional geological history In the Eikefjord Group (EG), the only possible relict Scandian feature is the amphibolite facies mineralogy that may have formed during the subduction of the WGR. The mineralogy has been strongly retrograded during the post-Scandian NSDZ. The Høydalsfjorden Complex (HC) displays the Scandian (top-E) development of the rocks in the study area. The complex essentially consists of gabbro-intruded metasediments with well preserved bedding (S0). The rocks are either the cover sequence to an ophiolite and/or a melange and may be related to the 443 +/- 4 Ma Solund–Stavfjorden Ophiolite Complex (Dunning & Pedersen 1988). Obduction onto the Baltoscandian platform occurred in the time interval 425–400 Ma, yielding a bedding-parallel S1-foliation with a prograde lower greenschist facies (upper chlorite-grade) M1-metamorphism. The S1-fabric was folded into WE to WNW-ESE trending F2-folds. S2-fabrics are represented by kink bands and locally by a crenulation cleavage that displays only vague recrystallisation, with metamorphism still corresponding to lower greenschist facies (upper chlorite grade). The F2-folds are cut by the sub-Devonian unconformity, and the D1- and D2- deformations are thus pre-dating deposition of Devonian sediments. Post-Caledonian, Devonian, extensional geological history General: The subdivision of the Devonian extensional phase into Mode-I movements and the subsequent NSDZ/Mode-II structures is reviewed above. The extensional tectonics produced two tectonic styles in the field area: first the onset of top-W extensional movements, then the onset of folding. Mismatch: The present thesis reveals that a discrepancy appears to exist between the 40Ar/39Ar muscovite ages of the NSDZ in the Eikefjord–Gloppen area and the status of the NSDZ as a Mode-II zone. The 40Ar/39Ar muscovite ages in the Eikefjord–Gloppen area fall in the range 404–398 Ma (Chauvet & Dallmeyer 1992; Andersen 1998), which is contemporaneous with the movements at 402–394 Ma on the Mode-I zone (Fossen & Dunlap 1998). In addition, the NSDZ shows extensive movements at amphibolite facies conditions, indicating that the movements on the NSDZ started even earlier than the time interval 404–398 Ma. Recently, Johnston et al. (2007b) performed P/T analyses and radiometric dating in the Eikefjord area, and interpreted the NSDZ to have moved in the time interval 410–400 Ma. Viewed together, these ages may seem to indicate that the NSDZ initiated before the Mode-I zone, i.e. opposite to the conventional order of appearance. However, as the order of the “Mode-I first” and “Mode-II next” in this area is based on independent field work (Milnes et al. 1997), the conventional order of appearance has been maintained. The cause(s) of the discrepancy is not clear. Effects of Devonian extensional movements on the Høydalsfjorden Complex (top-W movements predating deposition of the HDM). The post-Scandian extensional movements led to formation of top-W S3-mylonites in the Høydalsfjorden Complex (HC). A very limited number of folds with NE-SW trending fold axes has been recorded, i.e. oriented with a high angle to the abundant WNW-ESE trending HC F2-folds, the latter cut by the unconformity below the Håsteinen deposits. The NE-SW trending folds may be F3-folds related to the D3-top-W mylonites that are cut by the unconformity below the Devonian Håsteinen sediments. This suggests that the S3- mylonites formed at an early stage of the Devonian extension, prior to the Devonian basin formation. It appears that the HC S3-mylonites are unrelated to the Eikefjord Group mylonites of the Mode-II/NSDZ. This is consistent with the lower metamorphic grade in the HC S3-mylonites, where brittle quartz suggests temperatures below greenschist facies. Instead, the HC S3-mylonites may be related to the foregoing Mode-I extension. Effecs of Devonian extensional movement on the Eikefjord Group. The top-W D2-movements of the NSDZ produced the strong and penetrative mylonitic S2-fabric that generally obliterates all older structures and is retrogressive from amphibolite facies (mentioned above) down to middle or lower greenschist facies. The amphibolite facies might originally have formed as a result of partly “down-transport” of the EG rocks during subduction of the WGR. The metamorphic retrogression of the EG reflects the uplift of the rocks during the movements on the NSDZ. The related L2-stretching lineations have a WNW-ESE trend and shallow plunges. Formation of the Håsteinen Devonian basin. The sediments were accumulated unconformably on top of the HC Upper Plate D1- and D2-structures (top-E) as well the D3-mylonites (top-W). The sediments consist of continental conglomerates deposited by mass flow processes in a proximal alluvial fan environment, and minor sandstones along the westernmost margin, deposited by fluvial processes in a more central or distal part of the fan environment. Along an E-W section through the middle of the massif, the beds are constantly standing with an eastward 53° dip against the subjacent, subhorisontal unconformable surface of the Høydalsfjorden rocks. This means that the Devonian beds – after having been deposited sub-horizontally – were rotated during the extension to achieve a steep easterly dip. The basin floor, defined by the Upper Plate (HC), must have rotated accordingly. The present-day 53° angle between the steeply dipping bedding and the overall subhorisontal contact towards the substrate represents an extraordinary geometry. This angle must have been high also at the time of sedimentation, but then with a sub-horizontal Devonian bedding deposited against a W-dipping palaeo-slope. This geometry cannot be explained by hitherto published models, which suggest that the sediments were deposited directly against the fault plane of a constantly active (listric?) growth fault, implying that the bedding/substrate contact being a tectonic fault rather than a primary unconformity as in the HDM. Hence a new model – termed the ramp basin model – is presented in the present work. The unique feature of this model is that the deposition is believed to have occurred in a depression that formed on top of the Upper Plate as a result of a westward-dipping frontal ramp in the subjacent detachment zone. In contrast, previous models suggest that the deposition occurred in a half-graben type basin. In the ramp basin model setting, a traditional half-graben basin would form at a considerable distance east of the “ramp basin”, at a position above the listric detachment zone present near the cut-off line. The Hornelen Devonian deposits appear to have formed in a half-graben basin of the latter type. The ramp basin model has been tested by numerical forward modelling. The modelling has been carried out by means of the structural modelling program 2D-Move. In the model, horizontal beds are deposited in the ramp depression, with an unconformable contact against the W-dipping surface of the HC. As the W-dipping Upper Plate moves W-wards down the ramp, the Devonian beds progressively rotate to obtain eastward dips. When the Upper Plate and attached beds have reached the flatter part of the NSDZ west of the ramp, the Devonian beds have achieved a prominent E-ward dip, dipping against the subhorisontal Upper Plate, just as in the Håsteinen case. Hence, the bedding-normal, cumulative stratigraphic thickness of 5.8 km for the HDM, as calculated along the E-W trending axial trace of the Osstrupen syncline, is reproduced. As the purpose of the modelling has been to test whether the geometry of Håsteinen can be reproduced, a full testing of various parameters has not been necessary to carry out. Devonian folding in general: Folding of Devonian age has affected all three units in the study area. In all the units, the fold axes are trending WNW-ESE, indicating roughly NNE-SSW directed contraction. In the literature, folding of the Devonian deposits and subjacent units in Western Norway have been interpreted as a result of a regional contractional event, affecting the entire area between Bergen and Trøndelag. In the present thesis, however, it is suggested that the folding of the Håsteinen basin and its substrate has mainly been a result of intrabasinal processes related to the extension, indicating that regional N-S contraction may not be necessary to explain the folding. Devonian folding of the Håsteinen Devonian Massif (HDM). In the HDM, the Devonian folding led to formation of the Osstrupen F1-syncline; an upright, plane fold with an axial trace oriented WNW-ESE (295°–115°), and with a average fold axis with a trend/plunge of 115/53. The limbs are straight, with a mean strike/dip orientation of 070/62 SE for the northern limb and 161/62 NE for the southern limb, and an interlimb angle of 103°. In the Gravaneset sandstone unit in the far west, a set of parasitic F1-folds with related axial planar S1-cleavage have been developed, showing that the major part of the deformation occurred during semiductile/ductile (plastic) conditions. In the thesis, three models have been presented to explain the folding in the HDM: (i) folding due to transpression along converging lateral ramps; (ii) folding due to ridge-shaped frontal ramps (“Ridge-ramp” model). (iii) folding due to narrowing basin at depth (“Narrowing basin depth” model). Of these, model (iii) appears to offer the most likely explanation. Devonian folding in the Høydalsfjorden Complex (HC). In the Høydalsfjorden Complex, the effects of Devonian N-S folding are less obvious. Both the HC F2-fold structures that trend roughly parallel to the F1-fold axis of the HDM Osstrupen syncline, and the top-to-the-west S3-mylonites, are cut by the sub-Devonian unconformity and thus predate the folding of the HDM. A small number of fold axes that are trending NE-SW, i.e. at a high angle to the F2-folds, are possibly F3-folds, although their relationship to the S3-mylonites and the unconformity have not been observed. S3-mylonite fabrics may possibly be folded by WNW-ESE trending F4-fold axes that may have formed during the N-S contraction that also folded the Håsteinen deposits. Generally, it is difficult to reveal the possible effects that the N-S contraction of the HDM had on the HC rocks. Possibly, HC-F2-folds became tightened during the Håsteinen contraction, or the HC may actually contains Devonian folds that have been erroneously recorded as HC-F2-folds due parallelism with the latter folds. The only place where Devonian folding can be seen to clearly affect HC rocks is at Gravaneset in the far west, where the unconformity below the Gravaneset Sandstone Unit is folded along with the sandstone beds. Devonian folding in the Eikefjord Group (EG). In the Eikefjord Group, the Devonian contraction folded the S2/L2-mylonite fabric into F3-folds with fold axes having a WNW-ESE oriented trend and shallow plunge, making the L2-stretching lineations and the F3-fold axes parallel. An S3-axial planar fabric was not developed. Furthermore, the trend of the F3-folds in the Eikefjord Group is parallel to the trend of the Osstrupen F1-syncline in the HDM. The Devonian N-S contraction produced the F3-folding of the S2-mylonites of the Eikefjord Group, since the F3-folds are folding the extensional S2-mylonites. The parallel orientation of L2-stretching lineation and F3-fold axes in the Eikefjord Group might indicate a genetic relationship, although this is still somewhat uncertain. Since the F3-folding in the Eikefjord Group appears to be more intense than the F1-folding giving the Osstrupen syncline, the F3-folding could be a result of shear-related contraction within the detachment zone. Post-Devonian geological history A large number of faults and joints are developed in the HDM. The amount of displacement has been tested for the most prominent faults crossing the massif, and the movements are found to be negligible. A minor fault at the Devonian-substrate contact beneath the Gravaneset sandstone unit has been dated by palaeo-magnetism to a Triassic/Early Jurassic age (Torsvik et al. 1987).