TECTONICS, VOL. 27, TC6016, doi:10.1029/2007TC002242, 2008
Styles of extension offshore mid-Norway and implications for mechanisms of crustal thinning at passive margins P. T. Osmundsen1 and J. Ebbing1 Received 3 December 2007; revised 7 July 2008; accepted 25 August 2008; published 25 December 2008.
[1] Eocene magmatic breakup along the midNorway rifted margin was preceded by extreme Jurassic-Cretaceous crustal thinning in a magma-poor environment. Along the SE borders of the rift, ‘‘top basement’’ detachment faults with heaves on the order of 15–40 km evolved in at least two stages to become the boundaries between moderately thinned (20–30 km thick) crust and a 100–200 km wide, highly extended area with crustal thicknesses generally between 2 and 12 km under the presentday Møre and Vøring basins. In the footwalls of the basin flank detachments, lower and middle crust was exhumed in extensional domes that became incised by a younger set of normal faults. Under the most highly thinned areas, a more distal set of deep-seated (basin floor) detachments incised and extended remnant crust and, probably, the upper mantle, leaving as little as 8000
1450 1800 – 2400 2700 – 3600 3600 – 5000 5000 – 6000 5000 – 6000 5000 – 6000 6000 – 6500 6000 – 6500 6000 – 6500 7000 – 7400 >8000
1.03 2.05 – 2.1 2.3 – 2.4 2.45 – 2.55 2.65 – 2.7 2.65 – 2.7 2.65 – 2.7 2.65 – 2.7 2.75 – 2.85 2.95 – 3.0 3.1 3.3
0 0.0001 0.0002 – 0.003 0.0002 – 0.003 0.0002 – 0.003 0.0002 – 0.003 0.0002 – 0.003 0.005 – 0.01 0.02 – 0.035 0.005 0.005 – 0.0075 0.0025
0 0.4 0.4 0.4 0.4 0.4 0.4 0.5 – 1.0 0.4 – 1.1 0.5 0.5 0.5
a Seismic interval velocities (Vi) have been obtained from wells and from crossing OBS lines (Pf, Terr, Su are platform, terrace, subbasin). LCB, lower crustal body. The errors from the velocity-density relations on the applied densities are in the order of ±0.05 Mg/m3, and ±0.1 Mg/m3 for the upper basement and the deep crustal layers, respectively. The inclination and declination of all remanent magnetic fields are set to 77.0° and 0.5°, approximately parallel to the present magnetic field.
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Figure 4. (a) Depth to top of ‘‘upper,’’ low-magnetic basement, composed of rocks of densities of 2.7 Mg/m3 and with relatively low magnetic susceptibilities. (b) Depth to the denser (2.75– 2.95 Mg/m3) and more magnetic lower basement (see Table 1). (c) Top to high-density, lower crustal bodies (LCB) and continental lower crust (CLC). See Table 1 and text. (d) Depth to Moho as constructed from the 3-D density model based on OBS profiles (gray stippled lines). between magnetic and nonmagnetic material generally runs through the lower continental and oceanic crust. The parameters applied in the model are presented in Table 1, Bouguer gravity and magnetic anomaly maps are presented in Figure 3.
[12] Depth-converted seismic reflection profiles provided means of comparison between observed versus modeled gravity response. We also compared our 3-D model to the magnetic anomaly field of the mid-Norwegian margin, which allows correlation of prominent structural features
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Figure 5. (a) Thickness of the crystalline crust and (b) thickness of the highly magnetic lower basement as calculated from maps in Figure 4. Note strong thinning of the crystalline crust to 5 – 12 km under large parts of the Møre and Vøring basins. The lower basement thins from 10 to 14 km under the platform, terrace, and subbasin to less than 5 km over substantial areas. Both the crystalline crust and the lower basement remain thin to the continent-ocean boundary. with high magnetic anomalies, as most magnetic material is expected to reside within basement rocks [e.g., Fichler et al., 1999]. [13] The final 3-D density model of the mid-Norwegian margin has a standard deviation of 2900 kg/m3) with a convex-upward top located at a depth of 15 km, below and northwest of the crest of the interpreted basement culmination (Figure 8b). These rocks must be denser than those of the Western Gneiss region, which occur beneath Mesozoic strata in the Frøya High. In combination, the magnetic rocks of the basement culmination and the deeperseated high-density body define an extensional dome in the
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Figure 14. Seismic expression of the Bremstein Fault Complex (BFC) under the southern Halten Terrace area, as imaged in GMNR-94-104. The main strand of the BFC (white arrows) is a ramp flat, basin flank detachment fault that can be traced as a reflector band to 9 s TWT under the Halten Terrace. The reflector band may represent ductile shear fabrics related to early movements on the BFC. A culmination occurs in the footwall at 4 s TWT. The northward plunging culmination matches well a strong positive magnetic anomaly that can be traced from the southern Halten Terrace area into the Frøya High (Figure 3b). We interpret the culmination to represent an extensional dome in the crystalline basement, related to formation of the BFC. footwall of the BFC, substantiating its role as a largemagnitude extensional detachment. 3.1.3. Klakk Fault Complex [34] The north-south trending Klakk Fault Complex (KFC) separates the southern Vøring Basin from the Halten
Terrace and the Frøya High (Figure 2 and 8). A number of undulations in the mapped fault trace are due to severe erosion into the fault plane [e.g., Blystad et al., 1995]. The southernmost part of the KFC appears to be relatively insignificant, detaching along the Slørebotn detachment,
Figure 15. Detail of GMNR-94-104 showing two deep-seated, rotated fault blocks, a block-bounding fault and an associated half graben basin. Note position of the base Cretaceous unconformity (BCU) and its passing onto what appears as an erosional surface incising into the adjacent fault scarp (EF). Note southeastward dipping reflectors in the reflective seismic basement being cut by (1) a deep-seated, subhorizontal fault (F1) (black arrows) and (2) the block-bounding fault F2 which also cuts F1. 16 of 25
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Figure 16. Seismic expression of basin floor detachment fault, northern Møre Basin (GMNR-94-103, location in Figures 2 and 7). Note convex-upward reflector band (white arrows) incised by narrower reflector band interpreted as a detachment fault (black arrows). Reflectors in the overlying basin are vague, but gently SE dipping reflectors are present below the Lower Cretaceous reflector band. Our interpretation of the top of basement relies on an interpretation of a crossing OBS by T. Raum (personal communication, 2005). The resulting interpretation is that of a high-b basin juxtaposed against deep structural levels across a basin floor detachment fault. Only 2 – 5 km of crystalline crust is preserved in the footwall beneath the basin, depending on the interpretation of the underlying high-density body (see Figure 8). whereas west of the Halten Terrace, the KFC constitutes a moderate-angle normal fault with a heave of 12 km (Figure 8). Farther north, the heave is in the order of 20– 30 km [Osmundsen et al., 2002; Gome´z et al., 2004]. Thus it appears that the character and significance of the KFC changes dramatically from south to north. We note that the largest displacements along the KFC are observed along the northern parts of the Trøndelag Platform, in the area where Jurassic – Early Cretaceous displacements along the BFC appear as less significant than farther south. This may point in the direction of strain transfer between the reactivated portions of the BFC and the Klakk Fault Complex. 3.2. Rotated Fault Block Arrays [35] In the NE Møre Basin to the south central Vøring basin, the large-scale architecture of the deep basins can be described in terms of (1) a rotated fault block array flanking the adjacent domain border fault, (2) an area characterized by flat-lying or gently inclined reflectors to great depth (the flat zones), and (3) a distal ridge, where the Cretaceous succession rises toward the marginal highs, defining structural highs such as the Ra˚n Ridge. OBS data suggest that the thinnest crystalline crust is found under the flat zones, although recent work indicates that very thin crystalline crust appears to continue underneath the Ra˚n Ridge [Raum et al., 2006]. [36] Locally, the long-offset data permits a threefold subdivision of the rotated fault blocks based on contrasting reflector patterns. In Figure 15, a southeastward thickening reflector wedge overlies the top of the seismic basement; we interpret this to represent Triassic and Jurassic prerift and synrift strata preserved in a southeastward tilted half graben basin. Near the crest of the
rotated fault block, two unconformities can be recognized; the upper one is interpreted as the BCU. Tracing the BCU southeastward aligns it with a break in slope in the top basement surface in the adjacent fault block, interpreted here as an erosional surface. In Figure 16, subhorizontal to gently NW dipping, narrow reflectors or reflector bands appear to incise and rotate SE dipping, deep reflector packages. Similar to the interpretation in Figure 15, they are interpreted as the deep expression of block-bounding faults. In Figure 16, however, the angles of incision appear as lower than in Figures 6 and 15. The incised, extended and rotated deep reflector package may represent lithological layering, ductilely deformed rocks or a combination of the two, such as would be expected in an intrabasement shear zone, along the top of ductilely deformed middle or lower crust or, alternatively, along the crust-mantle boundary. Comparison of the depth-converted seismic interpretation with the 3-D density model suggests that immediately west of the main Møre margin fault in Figure 7, the deepest reflector band appears to represent the Moho, whereas farther northwest, it defines the top of a high-velocity body above the Moho. 3.3. Basin Floor Detachments [37] In Figure 16, the low-angle normal fault bounding the Vigra High in the NW continues at depth as a narrow reflector band that incises gently SE dipping reflective seismic basement at a very low angle. The persistence of sedimentary rocks to 15 km depth (R. Mjelde and T. Raum, personal and written communications, 2005) in the overlying, flat-bottomed half graben substantiates the interpretation of this fault as a basin floor detachment with a horizontal displacement in the order of 20 km or more.
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The crystalline basement in the two adjacent fault blocks is close to complete separation above the low-angle normal fault, with sedimentary rocks resting upon the fault plane reflector. The 3-D density model indicates a thickness of 2 km of crystalline crust, underlain by a pillow-shaped high-density body. The thickness of the crystalline crust in the footwall of the fault thus depends on the interpretation of the high-velocity body as high-density lower crust, magmatic underplating or low-velocity mantle. [38] In Figure 17, the fault that defines the westernmost limit of the fault block array incises into the laminated reflector band at structurally low levels and then acquires a very gently dipping or subhorizontal attitude at 8.5 s TWT. Although interpretation of the flat reflector band northwest of the fault block array is ambiguous because of sill intrusions and associated multiples, the fault must have a very large displacement (likely >15 km), a very low dip at depth, and clearly must juxtapose sedimentary rocks with the warped, laminated basement unit in the lowermost part of the footwall. We interpret this fault to represent a basin floor detachment in the southern Vøring basin. [39] The age relationships between the faults in the fault block array and the basin floor detachments are not clearly defined. The lack of stratigraphic control in the buried half graben hampers an independent test of the temporal relationships between the half graben bounding faults. The fault planes that incise into the basal reflector band west of the KFC appear, however, to be successively incising into each other from the SE toward the NW, with the youngest faults being located in the westernmost parts of the fault block array (Figures 8 and 15). [40] This is consistent with apparent faulting of units in the Lower Cretaceous across the flat zone boundary (Figures 16 and 17), which would indicate that movements along the basin floor detachments continued after the faults of the fault block array locked up. 3.4. Distal Ridges [41] The geometry of the distal ridges will not be discussed in detail here, but the ridges give evidence for faulting of stratigraphic levels that are postrift with respect to faulting in the SE parts of the basins [e.g., Ren et al., 1998]. On the Gjallar Ridge in the northern Vøring basin, faulting occurred above a fundamental detachment in the latest Cretaceous and early Paleocene, and the Gjallar Ridge may be underlain by an extensional dome or metamorphic core complex [Dore´ et al., 1999; Ren et al., 1998; Gernigon et al., 2003]. This complex may in turn constitute serpentinized mantle rocks [Ren et al., 1998], be inherited from the post-Caledonian extension [Gernigon et al., 2003] or comprise magmatic components. OBS data indicate that parts of the distal ridges are underlain by thick successions of sedimentary rocks [Fernande´z et al., 2005; Raum et al., 2006], and it appears that large, relatively young faults with SE polarity straddle some of the distal ridges [e.g., Mosar et al., 2002; Mjelde et al., 2005]. Faults with SE polarity also appear to displace ocean dipping reflectors in the marginal highs, indicating Cenozoic faulting with a SE polarity and associated rollover formation [Gernigon et
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al., 2006]. Thus, although the distal ridges show multiple evidence for faulting with top-to-the-west polarity, downto-the-SE displacements appear to have taken place in the Late Cretaceous and early Tertiary in the areas close to the line of breakup in the central and northern Vøring Basin [Osmundsen et al., 2002; Mosar et al., 2002; Gernigon et al., 2003, 2006].
4. Discussion [42] In the Porcupine Basin, the Rockall Trough and the Møre and Vøring basins, extreme prebreakup crustal thinning took place in the Jurassic and earliest Cretaceous [Reston et al., 2001; O’Reilly et al., 2006; Klingelho¨fer et al., 2005; Mjelde et al., 2002; Fernande´z et al., 2005]. We suggest that our interpretations from the mid-Norway margin may hold clues to how continental crust can be thinned to the critical thickness for embrittlement in magma-poor environments, hitherto, a significant problem in the interpretation of magma-poor margins. On the basis of our observations, we suggest that three principal types of fault systems developed along the mid-Norway rifted margin during the magma-poor, Jurassic-Cretaceous extension phase. These were (1) basin flank detachments or detachment complexes which evolved to define the areas of maximum crustal taper, (2) block-bounding fault arrays, and (3) basin floor detachment faults. 4.1. Basin Flank Detachment Faults and the Initial Reduction of Continental Crust [43] The main Møre boundary fault, which defines the basinward taper of the crystalline crust along the southeast Møre Basin margin, incised the Slørebotn detachment prior to the end of the Cenomanian, when the Slørebotn detachment had already accommodated a minimum of 10 km of Jurassic extension and fault block rotation. Although Jongepier et al. [1996] suggested that the detachment(s) under the Slørebotn Subbasin were related to footwall collapse of the southeastern border fault of the Møre Basin, we argue that the Slørebotn detachment represents early stages of a detachment fault complex that became fundamental to the development of the Møre Basin. We propose that the Slørebotn detachment and the MMF represent a warped splay and an incising splay, respectively, of a basin flank detachment fault. Thus, the Slørebotn Subbasin likely preserves remnants of a Jurassic structural configuration that had a continuation under the present deep basin areas. The tilting and deactivation of the Slørebotn detachment and the overlying subbasin after late Volgian time may relate to isostatic footwall uplift, incision by the MMF or, possibly, rotation on fault strands of the Møre-Trøndelag Fault Complex. These scenarios would be consistent with classical models for extensional detachment faulting, such as those of Lister and Davis [1989], where early detachments become excised or incised by new detachment faults that develop along the distal flank of the evolving core complex and cause rotation and deactivation of the earlier detachment(s). Additional rotation may be imposed by lateral accommodation faults [Brun
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Figure 17. Detail of seismic lines showing fault boundary between fault block array and the flat zone in the southern Vøring Basin. (a) An extensional rider block rests on the fault plane; black arrow points to top of seismic basement. The base Cretaceous unconformity (BCU, stippled white lines) appears to be downfaulted. (b) Note ‘‘small-scale’’ deformation of Cretaceous strata below the lower Cretaceous reflector band (LC) adjacent to the flat zone bounding fault (F3). Note also warping of reflective seismic basement in the footwall. The flat zone boundary fault dips 16° in depth-converted cross section (see Figure 8) but appears to flatten with depth. Displacement at top basement level exceeded 15 km in Figure 17b, where sedimentary rocks are juxtaposed with the reflective seismic basement along the lower portions of the fault. Parts of the flat zone would then tentatively belong to a high-b basin. S, sill, M, multiple. Compare with Figure 16. et al., 1994] or by the development of a deeper detachment fault as the crust cools and the ductile-brittle transition moves downward. In the case of basin flank detachments at continental margins, the youngest incising fault would develop into the top basement detachment that controls the crustal taper in the direction of the basin and the COB. [44] For many continental margins, the maximum taper area is a principal boundary zone between normal and very thin crust. In the present case, we interpret the combined Slørebotn detachment/main Møre margin fault as fundamental in the evolution of the larger-scale rift architecture, not just in the vicinity of the mapped fault, but in a 150 km wide area that borders the line of breakup. Northwest of the MMF, the crustal thickness remains less than 10 km for
more than 150 km in the direction of the continent-ocean boundary [Olafsson et al., 1992; Gome´z et al., 2004; Fernande´z et al., 2005]. Thus, the basin flank detachments were probably fundamental in the production of severely thinned continental crust under the evolving Møre and Vøring basins. We suggest that a similar scenario may also apply to other margins. [45] Offshore mid-Norway, the severe crustal thinning accommodated by the basin flank detachments appears to have been associated with a significant redistribution of rocks in the lower and middle crust. Our map of the lower basement shows that the Platform area is enriched with respect to rocks with high magnetic susceptibilities and densities >2.75, whereas this material appears to wedge out
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underneath the basin areas. The dome-shaped accumulations of high-density lower crust or highly magnetic, less dense rocks in the footwalls of the MMF and the BFC (e.g., Figures 10 and 11) are interpreted here as extensional gneiss domes or metamorphic core complexes, features that are commonly associated with large-magnitude normal faults in other settings (Figure 10) [Lister and Davis, 1989]. Changes in the physical properties of rocks during deformation, burial or denudation can, of course, affect such patterns. The correlation between rocks of high magnetic susceptibility with dome-shaped reflector patterns in the seismic reflection data does, however, appear to support this interpretation regarding the margins of the Trøndelag Platform [e.g., Skilbrei and Olesen, 2005; Ebbing et al., 2006]. The footwall of the Slørebotn detachment in the Gossa High area appears to have been exhumed as a composite core complex or dome-shaped mylonite front and eroded in the Early Cretaceous. [46] We suggest that in the principle, the basin flank detachments or detachment complexes represent a fault type that has the potential to thin continental crust from 25 to 40 km to the critical value required for embrittlement of the entire crust (100 km wide area of strongly thinned crust between the basin flank detachments and the COB. The deep-seated, strongly laminated levels in and under the rotated fault block arrays (Figures 15 and 16) may represent sheared rocks that became incised by brittle faults as ductility was reduced in the middle and lower crust, the ductile-brittle transition moved downward, and the crust approached complete embrittlement. Whether or not this stage was actually reached under the mid-Norway margin remains uncertain. Basin floor detachment faults did, however, become localized in the area of highly thinned crust 50– 100 km NW of the basin flanks. The crustal thinning accommodated by the basin floor detachments resulted in drawer-style, high-b extensional basins that juxtaposed rotated prerift and synrift sediments with rocks exhumed from the middle/deep crust or upper mantle (Figures 7 and 8). A similar explanation may apply to the flat zones between the fault block arrays and the distal ridges. If basement-cored fault blocks are present in the flat zones, they must be small, very deepseated and underlain by very thin (5 km or less) continental crust. The seismic data give the impression of extensional allochthons and high-b basins floored by material exhumed from very deep structural levels. The structural evolution of the mid-Norway rifted margin may serve as an analog to other passive margins where similar patterns of crustal thinning are observed but the structures are more poorly imaged.
[56] Acknowledgments. We are indebted to Thomas Raum and Rolf Mjelde for permission to use an interpretation of OBS data from the Møre Basin in the upgrading of our 3-D density model and in depth conversion. Discussions with Laurent Gernigon, Erik Lundin, and Gianreto Manatschal are greatly appreciated. Tim Redfield commented on an early version of the manuscript. We thank BP Norway, Norske Shell, Statoil a.s.a., and the Geological Survey of Norway for financial support. The GMNR-94 and VMT-95 surveys were recorded by Schlumberger Geco-Prakla and TGS Nopec, respectively. The ST 8705 survey was acquired by Stratoil. The Norwegian Petroleum Directory provided part of the seismic data. The above interpretations remain the sole responsibility of the authors. We thank Tim Reston and an anonymous referee for constructive comments.
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J. Ebbing and P. T. Osmundsen, Geological Survey of Norway, Crustal Processes, Leiv Eirikssonsvei 39, N-7491 Trondheim, Norway. (
[email protected])
