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Simple-shear deformation of the Skagerrak lithosphere during the formation of the Oslo Rift
Te~~onopbysi~s 232 0994) 133-141
Simple-shear deformation of the Skagerrak lithosphere during the fixation of the Oslo Rift J.E. Lie a~*,ES. Husebye b a Narsk f&h as, P.O. &Lx ZOO,N-1320 StabekrG,NWwy ’ Institute ofSolid Earth Physi~s~A&g&n 41, .%I#7Bergen, Norwuy (Received February 12,1993; revised version accepted July 27, lY93)
Abstract Simple-shear deformation of the entire lithosphere has been postulated by Wenicke (198.5) and others, but up to now unequivocaf seismic evidence in support of this hypotheses has been lacking. Here we describe we&defined seismic reflectors below the Skagerrak Sea, one of which is interpreted as a low-angle fauit underiying the Skagerrak Graben segment of the Permian Oslo Rift. This reflective ~neament can be traced from the mid-crust through the lower crust, offsetting Moho and continuing downwards to ca. 50 km depth (16 s>. A separate mantle reflection beneath the graben may be associated with an earlier period of thrusting. The 1730 km of deep seismic reflection data in Skagerrak indicate that the crust and mantle inherited a pronounced structural fabric from the Proterozoic ~re~~~lian-Sv~cono~egian orogeny. During formation of the Oslo Rift, reactivation of these implied weak zones as localized detachment planes would explain the extensional deformation style of the non-magmatic Skagerrak
Graben.
The Permian Ode Rift shown in Fig. fb {Skagerrak Graben and associated features) is centraIIy located in the Svecono~egian province of the Baltic Shield (Ramberg and Spjeldnaq 1978; Gaal and Gorbatchev, 1987). The domin~t tectonic fabric in the crust is N-S representing the effects of alternating E-W compressional and extensional events of which the GrenvillianSvecono~egian orogeny (X2-0.9 Ga.) is the most prominent. During this orogeny, the Skagerrak
* Corresponding
au&or. Fux: 47-22-739756.
Block appears to have been upthrusted and ductile-sheared against the Telemark craton to the west along the Bamble fault (FaIkum, 1983 Starmer, 1985, 1991). The deveIopment of the Permian Oslo Rift remains a puzzIe despite extensive geological mapping (Ramberg and Spjeldnaq 1978; Bj~rIy~e et al., 1990). The Iandward “volcanic” rift segment was apparently only moderately stretched C/3= 1.0, while the offshore segment (the Skagerrak Graben) was subjected to larger stretching Q3 = 1.4-1.6) and appears to be non-volcanic (Wessel and Husebye, 1987; Pedersen et al., 1991; Lie et al., 1993). In general, the volume of melt generated should increase rapidfy with the amount of
~4~-~951/94/$0~.~ 0 1994 Eisevier Science B.V. Al1 rights reserved SSLM Oa40-~951(93~EO23~-R
J.E. Lie, E.S. Husebye / Tectorzophysics 232 (1994) 133-141
134
stretching and the potential temperature of the mantle. In a relatively small region, such as that occupied by the Oslo rift, the latter parameter should be fairly constant and hence magmatism primarily a function of the amount of extension. One possible solution to this enigma is that the rifting in the Skagerrak Graben was governed to a large extent by pre-existing fabric resulting in lithospheric deformation dominated by simple shear. Extension by simple shear produces significantly less magma than extension by pure shear (McKenzie, 1978; Latin and White, 1990) We address this problem with observations from deep seismic reflection data stemming from the M/V Mobile Search cruise in Skagerrak in 1987 (Fig. 1). 2. Observational deep seismic
data, tectonic profiling results
setting
6”
8”
12”
and new
The seismic reflection data recorded on the grid shown in Fig. 1 has been reprocessed recently to 16 s using the computer facilities at the Bullard Laboratories, Cambridge, UK. The reprocessing has greatly improved the resolution of the seismic sections. The seismic lines having the most direct bearing on the northern Skagerrak Graben area are displayed in Fig. 2. Below the Skagerrak Graben there is an outstanding reflective lineament denoted the Skagerrak Fault (SF in Fig. 2a). It cuts through the lower crust, apparently offsetting Moho by 2-4 km and continuing into the upper mantle. We are unable to resolve the SF in the uppermost crust, but its landward projection coincides with the Bamble Sector as shown by the dotted lines in Fig. 2b. The SF is most clearly seen on profile OG-8 (for a length of 130 km). It is also recorded in the mantle on the crossing and parallel profiles OG-12 and OG-7. In the mantle its reflective character is similar to that of the Flannan reflectors off northwest Scotland, appearing to be only 2-3 cycles wide (Flack and Warner, 1990). On all northern Skagerrak profiles, including OG-8, a prominent reflective feature is observed originating in the lower crust and dipping into the mantle. It is referred to as the Telemark Craton
Fig. 1. (a) Profiling grid for the M/V Mobil Search cruise in Skagerrak, 1987. Further details are given in Husebye et al. (1988). Bold lines indicate locations of seismic sections in Figs. 2 and 3. (b) Tectonic map of the southwestern part of the Baltic Shield based on Gaal and Gorbatschev (1987) and Kinck et al., (1991). AC = Akershus Graben, BF = Bamble Fault, BS = Bamble Sector, CDF = Caledonian Deformation Front, FBZ = Fennoscandian Border Zone, FFZ = Fjerritslev Fault Zone, KS = Kongsberg Sector, MZ = Mylonite Zone, SC = Skagerrak Graben, TC = Telemark Craton, VG = Vestfold Graben. The area referred to as the Skagerrak Block in the text is taken to comprise the Skagerrak Sea and the Bamble Sector (ES).
Tongue (7’CT) and is shown in Fig. 2. Its upper “surface” in the mantle is mapped out in Fig. 3a. On the iandward side, the high-grade metamorphic terrain, the Bambie Sector (BS), is separated from the Telemark Craton by a sharp
bounda~ of highly mylonized rocks here denoted the Bamble Fault (BF in Fig. lb). During the ~renvi~I~~u-Svecono~e~~an (GS) orogeny the Bamble Sector was duct~ie-defo~ed and sheared against the craton. After the orogeny, movement
OG-8
NW *_--__.
.__.~
--- ..-----
~_
SE
_-
a
I
c
f_.
-*
5rlKH
c
Fig. 2. (a) Migrated seismic section of profile OG-8 (see Fig. la for location). Notice the narrow reflective event, the Skagerrak Fault (SF), dipping through the crust and mantle from NW (6 s) to SE (15 s) on the section. On the unmigrated sections of profiles OG-8 and OG-12 this event can be traced clearly to the bottom of the sections at 16 s, equivalent to a depth of about 55 km. (b) Interpretation of profile GG-8 (Fig. 2a) and its relationship to the Iandward geology of southeast Norway (Fig. 1). The E-dipping mantle feature is interpreted to be the remains of the underthrusted crustal segment, referred to as the Tefemark Craton Tongue Q%YI’).SF marks the interpreted low-angle Skagerrak Fault. Cc>Interpreted migrated seismic section of intersecting profile OG-2 (Fig. I), Note the 4-5 km offset in Moho directly below the Fjerritslev Fault Zone (FIX) indicating that vertical faults are cutting the entire crust. Base Triassic Un~nfo~i~ WI’Uf, Skagerrak Graben and Moho are marked. In the northeast, the Telemark Craton Tongue (TCTI is also marked.
J.E. Lie, E.S. Husebya / Tectonophysics 232 (1994) 133-141
136
continued with brittle deformation along the Bamble fault and within the Bamble sector (Starmer, 1985, 1991). Normal faulting occurred
during the Permian with SE downthrow throughout the Bamble Sector (Selmer-Olsen, 1956), although these faults cannot clearly be connected
a
C Fig. 3. (a) Contours of the upper surface of the underthrusted segment of the Telemark Craton Tongue in the mantle (see also Fig. 2). (bf Thickness of the crystalline crust in southern Scandinavia taken from Kinck et al., (10411. Contour interval is 2 km. cc) interpreted migrated section of line OG-7. The post-rift sediments are here shifted eastward rehrtive to the Skagerrak Graben axis. The Base Triassic Unconformity (BTU) of regional extent is marked. The E-dipping mantle event is interpreted as a relic of the Skgerrak Fault (SF).
J.E. Lie, E.S. Husebye / Tectonophysics 232 (1994) 133-141
to the SF (Fig. 2b). The total amount of movement along these faults is not well-determined due to erosion since Permian times; current estimates are in the range of l-3 km (Prof. N. Spjeldmes, pers. commun., 1992). Considering the offset geometry of the Moho, the Skagerrak Fault (Fig. 2b) is taken to be of extensional origin, postdating the GS orogeny. This in turn implies that contemporary movements should have been accommodated along faults in the upper crust within the Bamble sector and offshore west of the Skagerrak Graben. Small sedimentary basins of thicknesses less than 1 km are observed on the western flank of the Skagerrak Graben. Delineated on the basis of seismic velocities, these small basins apparently have the same Cambrian-Silurian sedimentary fill as elsewhere in the graben (Lie et al., 1993) and their formation is taken to be contemporary with the rifting. On the OG-8 profile (Fig. 21, the Skagerrak Graben is represented by half grabens bounded by faults antithetic to the SF, which we take as an indication of coupling between the SF and the rifting process. An observational problem in the Skagerrak is that all syn-rift sediments are regionally eroded, marked by the Base Triassic Unconfo~i~ (BTU on Figs. 2c and 345 In short, there are strong indications that the Skagerrak Fault was the controlling fault in the formation of the northern segment of the Skagerrak Graben. We interpret the “seismic” TCT as a remnant of the Telemark Craton underthrusted during the GS orogeny along which a minimum of 50 km crustal shortening has taken place. This style of crustal shortening is similar to that observed along the coeval Grenvillian front in eastern Canada although features like the TCT have not been reported (Green et al., 1988; Rivers et al., 1989; Gower, 1990). On the other hand, crust-mantle imbrications as seen in Skagerrak are commonly seen below young erogenic zones like the Pyrenees, Alps and Himalayas (Matthews and Hirn, 1984; Bois, 1991). On the profiles roughly perpendicular to the Bamble Sector we observe numerous reflections in the lower crust dipping consistently southeastward (Fig. 2b), coincident with the dominant tectonic trend onshore. This reflective fabric is inter-
137
preted to constitute part of the tectonic imprint from the Grenvillian-Svecono~egian deformation of the Skagerrak Block. Another likely imprint of this orogen is the 6-8 km Moho offset beneath the Skagerrak- Kattegatt transition (Fig. 2b; also seen on OG-12). Although there are no constraints on the original shape of the Proterozoic Telemark Craton Tongue its survival below a segment of the Skagerrak Graben during rifting is not easily explainable in terms of uniform stretching (pure shear) models (McKenzie, 1978); the implied temperature perturbations and magma generation would be expected to erase the seismic signature of any mid-lithospheric structural features such as the TCT. Pronounced crustal thinning coincides with the Oslo Rift axis except in the south where it is shifted slightly westward relative to the Skagerrak Graben (Figs. lb and 3b; Kinck et al., 1991). Paleozoic sediments have been reported along the entire Oslo Rift, presumably deposited in an older large pre-rift basin covering a major part of southern Scandinavia and with a depositional center coinciding with the rift (Ramberg and Spjeldn=s, 1978; Bjorlykke, 1983). Part of the crustal thinning in this area must have taken place during the earlier basin formation (Pedersen et al., 1991).
3. A new hypothesis for the Oslo Rift evolution Current models for the evolution of the Oslo Rift stem mainly from the work of Ramberg and Spjeldmes (1978) who suggested an extensional regime with lithospheric thinning and subsequent influx and surface manifestation of magmatic material. This model has hardly changed during the last two decades due to lack of high-quality geophysical information bearing on the lower crust and the lithospheric mantle. In view of the deep seismic results presented here, we reconsider the above rift hypothesis with emphasis on the Skagerrak Graben segment. Dunbar and Sawyer (1990) have modelled the continental lithosphere as a composite unit comprising alternating high- and low-strength layers. in this model a weak lower crust can act as a
138
J.E. Lie, ES. Hurebye/Teclonopkysics
distributed detachment zone separating a strong upper crust from a strong upper-mantle lithosphere. For a region subjected to extensional forces, initial rifting would be located in areas having pre-existing weaknesses. The entire Oslo Rift is in such an area, where the dominant directional trend of its axis closely follows the older Precambrian structures (Ramberg et al., 1977; Lie and Husebye, 1993). Landward and adjacent to the rift is the narrow Kongsberg Sector (krs in Fig. lb), which is geologically similar to the Bamble Sector (Starmer, 1991). However, their tectonic trend directions are somewhat different (N-S and NE-SW, respectively), and in contrast to the KS, the BS has a clear gravity signature, The geometry of the TCT (Fig. 3a) is taken to be indicative of the seaward extension of the Bamble Sector. The above gravity and tectonic “disruptions” coincide with the transition from the onshore magmatic rift segment to the seaward non-magmatic Skagerrak Graben. Finally, the above mentioned lower-crustal reflective fabric, most prominent on the northern Skagerrak profiles OG-7, OG-8 (Fig. 21, OG-9 and OG-12 are also taken to be indicative of pre-existing weaknesses. The amount of stretching across the Oslo Rift axis increases progressively southward (p-values range from 1.1 to 1.6) with a hypothesized rotation pole several hundred kilometers to the north. Landward, the p-values were obtained from geological observations (Ramberg, 1976) and from differential crustal thickness estimates (Wessel and Husebye, 1987). Seaward, in the Skagerrak, a back projection technique was used in combination with sedimentary thickness observations C/3 = 1.3-1.6; Pedersen et al., 1991). Although different p-estimation schemes seldom produce identical results (McKenzie and Bickle, 1988) and uncertainties are attached to each estimate, it is noteworthy that the quoted P-values are consistent in progressively increasing southward. In the extreme, if the p-factors are grossly underestimated, then the total lithosphere would have been significantly thinned, but there is no seismic evidence in support of this view (Aki et al., 1977). During early Permian extension, the present landward and seaward parts of the Oslo Rift
232 fl904/
133-341
apparently responded very differently. In the northern onshore segment vertical faulting dominated, cracking a weakened crust. The subsequent influx of basaltic magmas was not preceded by pre-rift dooming (Ramberg and Spjeldnaes, 1978). Stretching appears to have been very modest (/3 N 1.1) and cannot explain the present-day thin crust in the greater Oslo Rift area (Fig. 3b). In the northern Skagerrak, simple-shear deformation appears to have taken place preferentially along the Skagerrak Fault (Fig. Zb), with the lithosphere acting as a rheologically uniform block. Extensional reactivation of old Sveconorwegian compressional fabrics would explain the favoured motion along a localized fault plane through the lower crust and into the mantle. The lower crust is otherwise, on the basis of rheological and temperature considerations, presumed to deform by distributed plastic stretching and thus unable to accommodate simple-shear deformation (Kusznir and Egan, 1989; Kusznir and Ziegler, 1992). An indication of the complexity of these processes is the lack of continuity of SF at the Moho, which could be explained by later overprinting by pure-shear plastic deformation of the lower crust in response to a slower extensional rate and subsidence. The lack of magmatism in the northern Skagerrak is in accord with the simple-shear numerical predictions of Latin and White (1990). They calculated that in the presence of an initial planer detachment fault in the lithosphere it is extremely difficult to generate melt from the asthenosphere. Further south in the Skagerrak, where the largest stretching is inferred, the SF cannot be observed, except possibly in the mantle on line OG-7. Here, later stage deformation appears to have taken place dominantly by pure shear in the lower crust and lower lithosphere (McKenzie and Jackson, 1987). For deep detachment zones in the lowermost crust, the corresponding crustal thinning would be small and shifted relative to the graben axis as is actually observed (Fig. 3b). The axis of the post-rift sedimentary basin also appears to be shifted eastward (Fig. 3~). Simple-shear deformation (Wernicke, 1985) would give rise to rotation, such as the observed sinistral movements along the faults in the Bam-
139
J.E. Lie. E.S. Husebye / Tectonophysics 232 (1994) 133-141
Northern rift segment, iandward Eas
I
Moho
I
I
i
Central rift segment, N. Skagerrak East Vest
Southern rift segment, S. Skagerrak west
East
(
Fig 4. Schematic view of the Oslo Rift evolution for its northern, central and southern graben segments. Not to scale. The upper figures represent the pre-rift and the lower the post-rift stages. Northern segment: (A) Pre-rift sedimentaty basin, thinned crust and a weakened lithosphere. (B) Rifting with graben formation and extensive magmatism. Pre-rift sediments only preserved within the graben. Central segment: (C) Pre-rift crustal fabric stemming from the Grenvillian-Sveconorwegian orogeny. CD) Simple shear-deformation of the entire lithosphere. No magmatism and less dominant graben formation. Southern segment: (E) Weakened Precambrian crust. (F) Later stage defo~ation by pure shear in the lower crust with the detachment slightly offset to the east.
ble Sector (Selmer-Olsen, 1956). The Skagerrak Block is detached along the Fjerittslev Fault Zone (FEZ; Figs. 1 and 26) where Permian magmatic activity has been reported &I, 1973; Klemperer and Hiirich, 1990). The mentioned enigma of basaltic magmatism in a rift apparently subject to little stretching runs counter to current models that require a p-factor of around 2 or higher (McKenzie and BickIe, 1988; Latin and White, 1990). However, the estimated rates of extension across the Oslo Rift are uncertain due to erosion of post-rift sediments and because the amount of pre-rift lithospheric thinning is not well established.
The factors that constitute the justification for our Oslo Rift evolution model are sketched in Fig. 4, with emphasis on the postulated simpleshear deformation in northern Skagerrak. This Iatter hypothesis, generally rejected by the tectonophysics community, would be further strengthened if a close connection can be established between the SF and the bounding faults of the Skagerrak Graben. 4. Summary Our new interpretation accounts for the localized detachment plane cutting the lower crust
and upper mantle. The existence of the Telemark Cratou Tongue (TCT) and the Skagerrak Fault (33 indicates that the lower crust and lower lithosphere are exceptionally strong in certain shield and platform areas. Fault zones extending through the lower crust, as shown in Fig. 2, imply a combination of a more mafic lawer crust and lower temperature gradients (Hall, 1989; Dunbar and Sawyer, 19901. Subjecting such areas to moderate extension may result in whole-lithospheric deformation by simple shear. If the rifting process progresses further, the temperature field would be perturbed (McKenzie and Jackson, 1987) with an accompanying transformation in dominant lower-crustal deformational processes from simple to pure shear. Finally, we consider simple-shear deformation to be a viable mechanism for crustal extension in shield areas,
Acknowledgements
Once more we express our gratitude to Mobil Exploration Inc. ~~o~ay~ for making the M/V ~~~~1 Search available for surveying the Skagerrak. J.E.L. acknowledges a visiting fellowship to BIRPS, Cambridge, UK. We would like to thank the BIRPS Core Group members for helpful discussions and especially R. Hobbs for his advice during the data processing. Discussions with A. Andersen, A. Bjorlykke, T. Falkum, C. Hurich, T. Pedersen, B. Sturt and N. Spjeldmes are much appreciated. Instructive referee comments from R. England were much appreciated. This work was supported by the Norwegian Research Councii for Science and the Humanities (JJ3.L.I and Defense Advanced Research Projects Agencies under AFOSR grant 89-0259 (E.S.H.)
References Aki,K., Christoferssen, A. and Husebye, ES., 1977. Deformation of the three dimensional structure of the Lithosphere. J. Geophys. Res., 82: 277-296. h, K., 1973. Geophysical indications of Permian and Tertiary igneous activity in the Skagerrak. Nor. Geol. Unders., 287: l-25.
Babel Working Group, 1990. Evidence for early Proterozoic plate tectonics from seismic resection profiies in the Baltic Shield. Nature, 35: 34-38. Bjorlykke, K., 1983. Subsidence and tectonics in late Precambrian and Paleozoic sedimentary basins. Nor. Geol. Wnders., 380: 159-172. Bjorlykke, A., Jhlen, P.M. and Olsrud, S., 1990. Metallogeny and lead isotope data from the Oslo Paleorift. Tectonophysics, 178: 109-123. Bois, C., 1991. Geological significance of seismic reflections in collision belts, Geophys. J. Int., 105: 55-69. Dunbar, J.A. and Sawyer, D.S., 1990. How preexisting weaknesses control the style of continental breakup. J. Geophys. Res., 94: 7278-7292. Falkum, T.. 1985. Genetic evolution of Southern Scandinavia in light of a Late-Proterozoic plate-collision. In: A.C. Tobi and J.L.R. Touret fEditorsi, The Deep Proterozoic Crust in the North Atlantic Provinces. NATO Adv. Sci.lnst. Ser. C, 158: 309-322. Flack, C. and Warner, M., 1990. Three-dimensional mapping of seismic reflections from the crust and upper mantle, northwest of Scotland. Tectonophysics, 173: 469-481. Gaal, G. and Gorbatchev, R., 1987. An outline of the Precambrian evolution of the Baltic Shield. Precambrian Res., 35: 15-52. Gower, C.F., 1990. Mid-Proterozoic evolution of the Grenville Province, Canada. Geol. For. Stockholm Forh., 112: 127139. Green, A.G., Milkereit. 3.. Davidson, A., Spencer, C.. Hutchinson, D.R., Cannon, W.F., Lee, M.W., Agena, W.F., Behrendt, J.C. and Hinze, W.J., 1988. Crustal structure of the Grenville front and adjacent terranes. Geology, 16: 788-792. Hall. I., 1989. Base of the crust: seismological expression, geological evolution, and basin controls.In: A.J. Tankard and H.R. Balkwill (Editors), Extensional Tectonics and Stratigraphy of the North Atlantic Margins. Am. Assoc. Pet. Geol. Mem., 46: 41-50. Husebye, E.S., Ro, HE., Kinck, J.J. and Larsson. F.R., 1988. Tectonic studies in the Skagerrak Province.: The Mobil Search cruise. Nor. Geol. Unders. Spec. Publ., 3: 14-20. Kinck, J.J.. Husebye, E.S. and Lund, C.E., 1991. The south Scandinavian crust: Structurat complexities from seismic reflection and refraction profiling. Tectonophysics, 189: 117-133. Klemperer, S.L. and Hiirich, C.A., 1990. Lithosphere structure of the North Sea from deep seismic reflection pmfiling. In: D.J. Blundell and A. Gibbs (Editors), Tectonic Evolution af tltc North Sea Rifts. Oxford Univ. Press, Oxford, pp. 37-M. Kusznir, N.J. and Egan, S.S., 1989. Simple-shear and pureshear models of extensional sedimentary basin formation: application to the Jeanne d’Arc Basin, Grand Banks of Newfoundland. In: A.J. Tankard and H.R. Ealkwill (Editors), Extensional Tectonics and Stratigraphy of the North Atlantic Margins. Am. Assoc. Pet. Geol. Mem., 46: 305 322.
J.E. Lie, E.S. Husebye / Tectonophysics
Kusznir, N.J. and Ziegler, P.A., 1992. The mechanics of continental extension and sedimentary basin formation: A simple-shear/pure-shear flexural cantilever model. Tectonophysics, 215: 177-131. Latin, D. and White, N., 1990. Generating melt during lithospheric extension: Pure-shear vs. simple-shear. Geology, 18: 327-331. Lie, J.E. and Husebye, ES., 1993. Seismic imaging of upper crustal basement faults in the Skagerrak sea. Tectonophysics, 219: 119-128. Lie, J.E., Husebye, ES., Kink, J.J. and Larsson, F.R., 1993. Geophysical evidence of Cambrian-Silurian sedimentary rocks in the northern Skagerrak. Geol. Foren. Stockholm Forh., 115: 181-188. Matthews, D. and Hirn, A., 1984. Crustal thickening in Himalayas and Caledonides. Nature, 388: 497-498. McKenzie, D.P., 1978. Some remarks on the development of sedimentary basins. Earth Planet. Sci. Lett., 40: 25-32. McKenzie, D.P. and Bickle, M.J., 1988. The volume and composition of melt generation by extension of the Iithosphere. J. Petrol., 29: 625-679. McKenzie, D.P. and Jackson, J., 1987. The relationship between strain rates, crustal thickening, palaeo-magnetism, finite strain and fault movements within a deforming zone. Earth Planet. Sci. Lett., 65: 182-202. Pedersen, T., Pettersson, S.E. and Huebye, E.S., 1991. Skagerrak evolution derived from tectonic subsidence. Tectonophysics, 189: 149-163. Ramberg, LB., 1976. Gravity interpretation of the Oslo
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Graben and associated igneous rocks. Nor. Geol. Unders., 325: 1-194. Ramberg, LB. and Spjeldnres, N., 1978. The tectonic history of the Oslo Region. In: LB. Ramberg and E.R. Neumann (Editors), Tectonics and Geophysics in Continental Rifts. Nato Adv. Sci. Inst. Ser. C, 37: 167-194. Ramberg, LB., Gabrielsen, R.H., Larsen, T.B. and Solli, A., 1977. Analysis of fracture patterns in southern Norway. Geol. Mijbouw, 56: 295-310. Rivers, T., Martignole, J., Gower, C.F. and Davidson, A., 1989. New tectonic divisions of the Grenville province, southeast Canadian Shield. Tectonics, 8: 63-84. Selmer-Olsen, R., 1956. Om forkastninger og oppbrytningssoner i Bambleformasjonen. Nor. Geol. Tidskr., 28: 171-191. Starmer, I.C., 1985. The evolution of the South Norwegian Proterozoic as revealed by the major and mega-tectonics of the Kongsberg and Bamble sector. In: A.C. Tobi and J.L.R. Touret (Editors), The Deep Proterozoic Crust in the North Atlantic Provinces. NATO Adv. SciInst., Ser. C, 158: 309-32. Starmer, I.C., 1991. The Proterozoic evolution of the Bamble Sector shear belt, Southern Norway: Correlations across southern Scandinavia and the Grenvillian Controversy. Precambrian Res., 49: 107-139. Wernicke, B., 1985. Uniform sense simple shear of the continental lithosphere. Can. J. Earth Sci., 22: 108-125. Wessel, P. and Husebye, E.S., 1987: The Oslo Graben gravity high and taphrogenesis. Tectonophysics 142: 15-26.
Simple-shear deformation of the Skagerrak lithosphere during the fixation of the Oslo Rift J.E. Lie a~*,ES. Husebye b a Narsk f&h as, P.O. &Lx ZOO,N-1320 StabekrG,NWwy ’ Institute ofSolid Earth Physi~s~A&g&n 41, .%I#7Bergen, Norwuy (Received February 12,1993; revised version accepted July 27, lY93)
Abstract Simple-shear deformation of the entire lithosphere has been postulated by Wenicke (198.5) and others, but up to now unequivocaf seismic evidence in support of this hypotheses has been lacking. Here we describe we&defined seismic reflectors below the Skagerrak Sea, one of which is interpreted as a low-angle fauit underiying the Skagerrak Graben segment of the Permian Oslo Rift. This reflective ~neament can be traced from the mid-crust through the lower crust, offsetting Moho and continuing downwards to ca. 50 km depth (16 s>. A separate mantle reflection beneath the graben may be associated with an earlier period of thrusting. The 1730 km of deep seismic reflection data in Skagerrak indicate that the crust and mantle inherited a pronounced structural fabric from the Proterozoic ~re~~~lian-Sv~cono~egian orogeny. During formation of the Oslo Rift, reactivation of these implied weak zones as localized detachment planes would explain the extensional deformation style of the non-magmatic Skagerrak
Graben.
The Permian Ode Rift shown in Fig. fb {Skagerrak Graben and associated features) is centraIIy located in the Svecono~egian province of the Baltic Shield (Ramberg and Spjeldnaq 1978; Gaal and Gorbatchev, 1987). The domin~t tectonic fabric in the crust is N-S representing the effects of alternating E-W compressional and extensional events of which the GrenvillianSvecono~egian orogeny (X2-0.9 Ga.) is the most prominent. During this orogeny, the Skagerrak
* Corresponding
au&or. Fux: 47-22-739756.
Block appears to have been upthrusted and ductile-sheared against the Telemark craton to the west along the Bamble fault (FaIkum, 1983 Starmer, 1985, 1991). The deveIopment of the Permian Oslo Rift remains a puzzIe despite extensive geological mapping (Ramberg and Spjeldnaq 1978; Bj~rIy~e et al., 1990). The Iandward “volcanic” rift segment was apparently only moderately stretched C/3= 1.0, while the offshore segment (the Skagerrak Graben) was subjected to larger stretching Q3 = 1.4-1.6) and appears to be non-volcanic (Wessel and Husebye, 1987; Pedersen et al., 1991; Lie et al., 1993). In general, the volume of melt generated should increase rapidfy with the amount of
~4~-~951/94/$0~.~ 0 1994 Eisevier Science B.V. Al1 rights reserved SSLM Oa40-~951(93~EO23~-R
J.E. Lie, E.S. Husebye / Tectorzophysics 232 (1994) 133-141
134
stretching and the potential temperature of the mantle. In a relatively small region, such as that occupied by the Oslo rift, the latter parameter should be fairly constant and hence magmatism primarily a function of the amount of extension. One possible solution to this enigma is that the rifting in the Skagerrak Graben was governed to a large extent by pre-existing fabric resulting in lithospheric deformation dominated by simple shear. Extension by simple shear produces significantly less magma than extension by pure shear (McKenzie, 1978; Latin and White, 1990) We address this problem with observations from deep seismic reflection data stemming from the M/V Mobile Search cruise in Skagerrak in 1987 (Fig. 1). 2. Observational deep seismic
data, tectonic profiling results
setting
6”
8”
12”
and new
The seismic reflection data recorded on the grid shown in Fig. 1 has been reprocessed recently to 16 s using the computer facilities at the Bullard Laboratories, Cambridge, UK. The reprocessing has greatly improved the resolution of the seismic sections. The seismic lines having the most direct bearing on the northern Skagerrak Graben area are displayed in Fig. 2. Below the Skagerrak Graben there is an outstanding reflective lineament denoted the Skagerrak Fault (SF in Fig. 2a). It cuts through the lower crust, apparently offsetting Moho by 2-4 km and continuing into the upper mantle. We are unable to resolve the SF in the uppermost crust, but its landward projection coincides with the Bamble Sector as shown by the dotted lines in Fig. 2b. The SF is most clearly seen on profile OG-8 (for a length of 130 km). It is also recorded in the mantle on the crossing and parallel profiles OG-12 and OG-7. In the mantle its reflective character is similar to that of the Flannan reflectors off northwest Scotland, appearing to be only 2-3 cycles wide (Flack and Warner, 1990). On all northern Skagerrak profiles, including OG-8, a prominent reflective feature is observed originating in the lower crust and dipping into the mantle. It is referred to as the Telemark Craton
Fig. 1. (a) Profiling grid for the M/V Mobil Search cruise in Skagerrak, 1987. Further details are given in Husebye et al. (1988). Bold lines indicate locations of seismic sections in Figs. 2 and 3. (b) Tectonic map of the southwestern part of the Baltic Shield based on Gaal and Gorbatschev (1987) and Kinck et al., (1991). AC = Akershus Graben, BF = Bamble Fault, BS = Bamble Sector, CDF = Caledonian Deformation Front, FBZ = Fennoscandian Border Zone, FFZ = Fjerritslev Fault Zone, KS = Kongsberg Sector, MZ = Mylonite Zone, SC = Skagerrak Graben, TC = Telemark Craton, VG = Vestfold Graben. The area referred to as the Skagerrak Block in the text is taken to comprise the Skagerrak Sea and the Bamble Sector (ES).
Tongue (7’CT) and is shown in Fig. 2. Its upper “surface” in the mantle is mapped out in Fig. 3a. On the iandward side, the high-grade metamorphic terrain, the Bambie Sector (BS), is separated from the Telemark Craton by a sharp
bounda~ of highly mylonized rocks here denoted the Bamble Fault (BF in Fig. lb). During the ~renvi~I~~u-Svecono~e~~an (GS) orogeny the Bamble Sector was duct~ie-defo~ed and sheared against the craton. After the orogeny, movement
OG-8
NW *_--__.
.__.~
--- ..-----
~_
SE
_-
a
I
c
f_.
-*
5rlKH
c
Fig. 2. (a) Migrated seismic section of profile OG-8 (see Fig. la for location). Notice the narrow reflective event, the Skagerrak Fault (SF), dipping through the crust and mantle from NW (6 s) to SE (15 s) on the section. On the unmigrated sections of profiles OG-8 and OG-12 this event can be traced clearly to the bottom of the sections at 16 s, equivalent to a depth of about 55 km. (b) Interpretation of profile GG-8 (Fig. 2a) and its relationship to the Iandward geology of southeast Norway (Fig. 1). The E-dipping mantle feature is interpreted to be the remains of the underthrusted crustal segment, referred to as the Tefemark Craton Tongue Q%YI’).SF marks the interpreted low-angle Skagerrak Fault. Cc>Interpreted migrated seismic section of intersecting profile OG-2 (Fig. I), Note the 4-5 km offset in Moho directly below the Fjerritslev Fault Zone (FIX) indicating that vertical faults are cutting the entire crust. Base Triassic Un~nfo~i~ WI’Uf, Skagerrak Graben and Moho are marked. In the northeast, the Telemark Craton Tongue (TCTI is also marked.
J.E. Lie, E.S. Husebya / Tectonophysics 232 (1994) 133-141
136
continued with brittle deformation along the Bamble fault and within the Bamble sector (Starmer, 1985, 1991). Normal faulting occurred
during the Permian with SE downthrow throughout the Bamble Sector (Selmer-Olsen, 1956), although these faults cannot clearly be connected
a
C Fig. 3. (a) Contours of the upper surface of the underthrusted segment of the Telemark Craton Tongue in the mantle (see also Fig. 2). (bf Thickness of the crystalline crust in southern Scandinavia taken from Kinck et al., (10411. Contour interval is 2 km. cc) interpreted migrated section of line OG-7. The post-rift sediments are here shifted eastward rehrtive to the Skagerrak Graben axis. The Base Triassic Unconformity (BTU) of regional extent is marked. The E-dipping mantle event is interpreted as a relic of the Skgerrak Fault (SF).
J.E. Lie, E.S. Husebye / Tectonophysics 232 (1994) 133-141
to the SF (Fig. 2b). The total amount of movement along these faults is not well-determined due to erosion since Permian times; current estimates are in the range of l-3 km (Prof. N. Spjeldmes, pers. commun., 1992). Considering the offset geometry of the Moho, the Skagerrak Fault (Fig. 2b) is taken to be of extensional origin, postdating the GS orogeny. This in turn implies that contemporary movements should have been accommodated along faults in the upper crust within the Bamble sector and offshore west of the Skagerrak Graben. Small sedimentary basins of thicknesses less than 1 km are observed on the western flank of the Skagerrak Graben. Delineated on the basis of seismic velocities, these small basins apparently have the same Cambrian-Silurian sedimentary fill as elsewhere in the graben (Lie et al., 1993) and their formation is taken to be contemporary with the rifting. On the OG-8 profile (Fig. 21, the Skagerrak Graben is represented by half grabens bounded by faults antithetic to the SF, which we take as an indication of coupling between the SF and the rifting process. An observational problem in the Skagerrak is that all syn-rift sediments are regionally eroded, marked by the Base Triassic Unconfo~i~ (BTU on Figs. 2c and 345 In short, there are strong indications that the Skagerrak Fault was the controlling fault in the formation of the northern segment of the Skagerrak Graben. We interpret the “seismic” TCT as a remnant of the Telemark Craton underthrusted during the GS orogeny along which a minimum of 50 km crustal shortening has taken place. This style of crustal shortening is similar to that observed along the coeval Grenvillian front in eastern Canada although features like the TCT have not been reported (Green et al., 1988; Rivers et al., 1989; Gower, 1990). On the other hand, crust-mantle imbrications as seen in Skagerrak are commonly seen below young erogenic zones like the Pyrenees, Alps and Himalayas (Matthews and Hirn, 1984; Bois, 1991). On the profiles roughly perpendicular to the Bamble Sector we observe numerous reflections in the lower crust dipping consistently southeastward (Fig. 2b), coincident with the dominant tectonic trend onshore. This reflective fabric is inter-
137
preted to constitute part of the tectonic imprint from the Grenvillian-Svecono~egian deformation of the Skagerrak Block. Another likely imprint of this orogen is the 6-8 km Moho offset beneath the Skagerrak- Kattegatt transition (Fig. 2b; also seen on OG-12). Although there are no constraints on the original shape of the Proterozoic Telemark Craton Tongue its survival below a segment of the Skagerrak Graben during rifting is not easily explainable in terms of uniform stretching (pure shear) models (McKenzie, 1978); the implied temperature perturbations and magma generation would be expected to erase the seismic signature of any mid-lithospheric structural features such as the TCT. Pronounced crustal thinning coincides with the Oslo Rift axis except in the south where it is shifted slightly westward relative to the Skagerrak Graben (Figs. lb and 3b; Kinck et al., 1991). Paleozoic sediments have been reported along the entire Oslo Rift, presumably deposited in an older large pre-rift basin covering a major part of southern Scandinavia and with a depositional center coinciding with the rift (Ramberg and Spjeldn=s, 1978; Bjorlykke, 1983). Part of the crustal thinning in this area must have taken place during the earlier basin formation (Pedersen et al., 1991).
3. A new hypothesis for the Oslo Rift evolution Current models for the evolution of the Oslo Rift stem mainly from the work of Ramberg and Spjeldmes (1978) who suggested an extensional regime with lithospheric thinning and subsequent influx and surface manifestation of magmatic material. This model has hardly changed during the last two decades due to lack of high-quality geophysical information bearing on the lower crust and the lithospheric mantle. In view of the deep seismic results presented here, we reconsider the above rift hypothesis with emphasis on the Skagerrak Graben segment. Dunbar and Sawyer (1990) have modelled the continental lithosphere as a composite unit comprising alternating high- and low-strength layers. in this model a weak lower crust can act as a
138
J.E. Lie, ES. Hurebye/Teclonopkysics
distributed detachment zone separating a strong upper crust from a strong upper-mantle lithosphere. For a region subjected to extensional forces, initial rifting would be located in areas having pre-existing weaknesses. The entire Oslo Rift is in such an area, where the dominant directional trend of its axis closely follows the older Precambrian structures (Ramberg et al., 1977; Lie and Husebye, 1993). Landward and adjacent to the rift is the narrow Kongsberg Sector (krs in Fig. lb), which is geologically similar to the Bamble Sector (Starmer, 1991). However, their tectonic trend directions are somewhat different (N-S and NE-SW, respectively), and in contrast to the KS, the BS has a clear gravity signature, The geometry of the TCT (Fig. 3a) is taken to be indicative of the seaward extension of the Bamble Sector. The above gravity and tectonic “disruptions” coincide with the transition from the onshore magmatic rift segment to the seaward non-magmatic Skagerrak Graben. Finally, the above mentioned lower-crustal reflective fabric, most prominent on the northern Skagerrak profiles OG-7, OG-8 (Fig. 21, OG-9 and OG-12 are also taken to be indicative of pre-existing weaknesses. The amount of stretching across the Oslo Rift axis increases progressively southward (p-values range from 1.1 to 1.6) with a hypothesized rotation pole several hundred kilometers to the north. Landward, the p-values were obtained from geological observations (Ramberg, 1976) and from differential crustal thickness estimates (Wessel and Husebye, 1987). Seaward, in the Skagerrak, a back projection technique was used in combination with sedimentary thickness observations C/3 = 1.3-1.6; Pedersen et al., 1991). Although different p-estimation schemes seldom produce identical results (McKenzie and Bickle, 1988) and uncertainties are attached to each estimate, it is noteworthy that the quoted P-values are consistent in progressively increasing southward. In the extreme, if the p-factors are grossly underestimated, then the total lithosphere would have been significantly thinned, but there is no seismic evidence in support of this view (Aki et al., 1977). During early Permian extension, the present landward and seaward parts of the Oslo Rift
232 fl904/
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apparently responded very differently. In the northern onshore segment vertical faulting dominated, cracking a weakened crust. The subsequent influx of basaltic magmas was not preceded by pre-rift dooming (Ramberg and Spjeldnaes, 1978). Stretching appears to have been very modest (/3 N 1.1) and cannot explain the present-day thin crust in the greater Oslo Rift area (Fig. 3b). In the northern Skagerrak, simple-shear deformation appears to have taken place preferentially along the Skagerrak Fault (Fig. Zb), with the lithosphere acting as a rheologically uniform block. Extensional reactivation of old Sveconorwegian compressional fabrics would explain the favoured motion along a localized fault plane through the lower crust and into the mantle. The lower crust is otherwise, on the basis of rheological and temperature considerations, presumed to deform by distributed plastic stretching and thus unable to accommodate simple-shear deformation (Kusznir and Egan, 1989; Kusznir and Ziegler, 1992). An indication of the complexity of these processes is the lack of continuity of SF at the Moho, which could be explained by later overprinting by pure-shear plastic deformation of the lower crust in response to a slower extensional rate and subsidence. The lack of magmatism in the northern Skagerrak is in accord with the simple-shear numerical predictions of Latin and White (1990). They calculated that in the presence of an initial planer detachment fault in the lithosphere it is extremely difficult to generate melt from the asthenosphere. Further south in the Skagerrak, where the largest stretching is inferred, the SF cannot be observed, except possibly in the mantle on line OG-7. Here, later stage deformation appears to have taken place dominantly by pure shear in the lower crust and lower lithosphere (McKenzie and Jackson, 1987). For deep detachment zones in the lowermost crust, the corresponding crustal thinning would be small and shifted relative to the graben axis as is actually observed (Fig. 3b). The axis of the post-rift sedimentary basin also appears to be shifted eastward (Fig. 3~). Simple-shear deformation (Wernicke, 1985) would give rise to rotation, such as the observed sinistral movements along the faults in the Bam-
139
J.E. Lie. E.S. Husebye / Tectonophysics 232 (1994) 133-141
Northern rift segment, iandward Eas
I
Moho
I
I
i
Central rift segment, N. Skagerrak East Vest
Southern rift segment, S. Skagerrak west
East
(
Fig 4. Schematic view of the Oslo Rift evolution for its northern, central and southern graben segments. Not to scale. The upper figures represent the pre-rift and the lower the post-rift stages. Northern segment: (A) Pre-rift sedimentaty basin, thinned crust and a weakened lithosphere. (B) Rifting with graben formation and extensive magmatism. Pre-rift sediments only preserved within the graben. Central segment: (C) Pre-rift crustal fabric stemming from the Grenvillian-Sveconorwegian orogeny. CD) Simple shear-deformation of the entire lithosphere. No magmatism and less dominant graben formation. Southern segment: (E) Weakened Precambrian crust. (F) Later stage defo~ation by pure shear in the lower crust with the detachment slightly offset to the east.
ble Sector (Selmer-Olsen, 1956). The Skagerrak Block is detached along the Fjerittslev Fault Zone (FEZ; Figs. 1 and 26) where Permian magmatic activity has been reported &I, 1973; Klemperer and Hiirich, 1990). The mentioned enigma of basaltic magmatism in a rift apparently subject to little stretching runs counter to current models that require a p-factor of around 2 or higher (McKenzie and BickIe, 1988; Latin and White, 1990). However, the estimated rates of extension across the Oslo Rift are uncertain due to erosion of post-rift sediments and because the amount of pre-rift lithospheric thinning is not well established.
The factors that constitute the justification for our Oslo Rift evolution model are sketched in Fig. 4, with emphasis on the postulated simpleshear deformation in northern Skagerrak. This Iatter hypothesis, generally rejected by the tectonophysics community, would be further strengthened if a close connection can be established between the SF and the bounding faults of the Skagerrak Graben. 4. Summary Our new interpretation accounts for the localized detachment plane cutting the lower crust
and upper mantle. The existence of the Telemark Cratou Tongue (TCT) and the Skagerrak Fault (33 indicates that the lower crust and lower lithosphere are exceptionally strong in certain shield and platform areas. Fault zones extending through the lower crust, as shown in Fig. 2, imply a combination of a more mafic lawer crust and lower temperature gradients (Hall, 1989; Dunbar and Sawyer, 19901. Subjecting such areas to moderate extension may result in whole-lithospheric deformation by simple shear. If the rifting process progresses further, the temperature field would be perturbed (McKenzie and Jackson, 1987) with an accompanying transformation in dominant lower-crustal deformational processes from simple to pure shear. Finally, we consider simple-shear deformation to be a viable mechanism for crustal extension in shield areas,
Acknowledgements
Once more we express our gratitude to Mobil Exploration Inc. ~~o~ay~ for making the M/V ~~~~1 Search available for surveying the Skagerrak. J.E.L. acknowledges a visiting fellowship to BIRPS, Cambridge, UK. We would like to thank the BIRPS Core Group members for helpful discussions and especially R. Hobbs for his advice during the data processing. Discussions with A. Andersen, A. Bjorlykke, T. Falkum, C. Hurich, T. Pedersen, B. Sturt and N. Spjeldmes are much appreciated. Instructive referee comments from R. England were much appreciated. This work was supported by the Norwegian Research Councii for Science and the Humanities (JJ3.L.I and Defense Advanced Research Projects Agencies under AFOSR grant 89-0259 (E.S.H.)
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Graben and associated igneous rocks. Nor. Geol. Unders., 325: 1-194. Ramberg, LB. and Spjeldnres, N., 1978. The tectonic history of the Oslo Region. In: LB. Ramberg and E.R. Neumann (Editors), Tectonics and Geophysics in Continental Rifts. Nato Adv. Sci. Inst. Ser. C, 37: 167-194. Ramberg, LB., Gabrielsen, R.H., Larsen, T.B. and Solli, A., 1977. Analysis of fracture patterns in southern Norway. Geol. Mijbouw, 56: 295-310. Rivers, T., Martignole, J., Gower, C.F. and Davidson, A., 1989. New tectonic divisions of the Grenville province, southeast Canadian Shield. Tectonics, 8: 63-84. Selmer-Olsen, R., 1956. Om forkastninger og oppbrytningssoner i Bambleformasjonen. Nor. Geol. Tidskr., 28: 171-191. Starmer, I.C., 1985. The evolution of the South Norwegian Proterozoic as revealed by the major and mega-tectonics of the Kongsberg and Bamble sector. In: A.C. Tobi and J.L.R. Touret (Editors), The Deep Proterozoic Crust in the North Atlantic Provinces. NATO Adv. SciInst., Ser. C, 158: 309-32. Starmer, I.C., 1991. The Proterozoic evolution of the Bamble Sector shear belt, Southern Norway: Correlations across southern Scandinavia and the Grenvillian Controversy. Precambrian Res., 49: 107-139. Wernicke, B., 1985. Uniform sense simple shear of the continental lithosphere. Can. J. Earth Sci., 22: 108-125. Wessel, P. and Husebye, E.S., 1987: The Oslo Graben gravity high and taphrogenesis. Tectonophysics 142: 15-26.