Tectonics of the Palaeogene Forlandsundet Basin, Spitsbergen: a preliminary report

KAREN L. KLEINSPEHN & CHRISTIAN TEYSSIER

Kleinspehn, K. L. & Teyssier, C.: Tectonics of the Palaeogene Forlandsundet Basin, Spitsbergen: a preliminary report. Norsk

Geologisk Tidsskrift, Vol. 72, pp. 93- 104. Oslo 1992. ISSN 0029-196X.

Controversy exists concerning the timing and tectonic setting for the development of the Palaeogene Forlandsundet Basin relative to defonnation of western Spitsbergen and the opening of the Norwegian-Greenland Sea. New data and observations from this basin lead us to advocate abandonment of the concept of the basin as a fault-bounded graben with a simple extensional history. These new data include: ( l ) the Forlandsundet strata are not necessarily in fault contact with the Hecla Hoek basement, and depositional contacts have been recognized; ( 2) vitrinite reftectance data suggest that the western and southem parts of the basin record greater burial and uplift witb palaeotemperatures exceeding 250°C in the southwestem part of the basin; (3) faults affecting the Forlandsundet strata were developed at various stages of sediment consolidation, and a spectrum from synsedimentary fault growth to post-lithification fracturing and folding is observed. Cross-<:utting faults and folds indicate compressional and strike-slip tectonics as well as tensional tectonics, and suggest that a protracted, multiphase defonnational history has shaped the structure of the Forlandsundet Basin, and; ( 4) a macroscopic ductile fabric is observed at various localities in the basin fill. Optical and electron microscopic observations together with thennal data suggest that newly grown mica is likely to define this foliation. These preliminary stratigraphic, structural, and thennal data are unreconcilable with the earlier simple graben model. The basin's modem structural geometry, that of a graben, is a response to late tectonism and should not be used to infer the origin and early development of the basin, an origin that was coeval with Palaeogene shortening and strike-slip defonnation along Svalbard's western margin.

Karen L. Kleinspehn & Christian Teyssier, Department of Geo/ogy and Geophysics, University of Minnesota, 310 Pil/sbury Drive

S.E., Minneapolis, MN 55455, USA .

Ocean-ftoor magnetic anomalies have been used to re­construct the evolution of the Norwegian-Greenland Sea and provide evidence for a Palaeogene dextral transform plate margin between Greenland and the Barents Shelf along the west coast of Spitsbergen (Talwani & Eldholm 1977; Myhre et al. 1982; Eldholm et aL 1984, 1987, 1990; Srivastava & Tapscott 1986; Faleide et aL 1988; Myhre & Eldholm 1988; Rowley & Lottes 1988; Ziegler 1988; Roest & Srivastava 1989; Miiller & Spielhagen 1990). This transform margin persisted from Late Cretaceous ( 80 Ma or chron 25) to Earl y Oligocene time when the pole of rotation shifted and Greenland subsequently separated from Eurasia (36 Ma or Chron 13) (Eldholm et aL 1987, 1990; Faleide et al. 1988; My hre & Eldholm 1988; Roest & Srivastava 1989, using the time scale of Kent & Gradstein 1986). Dextral transpression along that transform plate margin resulted in the West Spits­bergen Orogenic Belt, an east verging, thin-skinned fold­and-thrust belt that permeates much of Spitsbergen, as well as the formation of several sedimentary basins within the orogenic belt (Fig. l) (Atkinson 1963; Har­land 1969; Lowell 1972). The Palaeogene Forlandsundet Basin is one of several sedimentary basins that developed during the initial opening of the Norwegian-Greenland Sea in what is now onshore western Spitsbergen. The basin is of particular interest because it Iies parallel to the plate margin and formed on the axis of the orogen in a position closest to the inferred transform plate margin (Fig. 1). Because of its age and position, the basin is

thought to be linked genetically to transpression between Greenland and Svalbard, although its subsidence mecha­nism and its deformation history are not understood.

The basin has been referred to as the Forlandsundet Graben (Tyrell 1924; Orvin 1940; Atkinson 1962; Hjelle et al. 1979; Rye Larsen 1982; Steet & Worsley 1984; Steet et al. 1985; Ohta 1988), but the term is a misnomer as the basin is not a simple extensional feature (Ry e Larsen 1982; Manby 1986), and kinematic indicators in the basin fill suggest a complex history of syn- and post-sub­sidence shortening and strike-slip displacement as well as extension (Lepvrier 1990). The purpose of this contribu­tion is to present initial results conceming the burial and deformational history of the basin, and to relate those results to the tectonic control on the basin dynamics.

Relationship of Forlandsundet Basin to other Tertiary tectonic elements

Forlandsundet sedimentation and basin development have traditionally been thought to postdate shortening in the West Spitsbergen Orogenic Belt and to mark the onset of extension between Greenland and Svalbard that per­sists presently (Atkinson 1963; Harland 1969; Harland & Horsfield 1974; Hjelle et al. 1979; Dowdeswell 1988). This view has been enhanced by age interpretations in which the basin is younger than the thrust belt and its associated foreland basin, the Central Basin (Fig. l) (Harland 1975;

94 K. L. K/einspehn & C. Teyssier

Fig. l. Map illustrating the location of the Forlandsundet Basin and other Tertiary basins of Spitsbergen (stippled areas). Stripes indicate the area affected by Late Cretaceous-Tertiary defonnation along the Palaeogene transform margin between Svalbard and Greenland with the axis of maximal defonnation along the western coast of Spitsbergen denoted by the heavier striped pattern. The Hornsund fault marks the approximate transition from continental to oceanic lithosphere and the probable position of the palaeo-transfonn plate margin. Rectangle outlines the location of Fig. 2. Base map is modified after Steel et al. ( 1985).

Livsic [Livshits] 1977) , and by earlier work that over-1ooked interna1 deformation of the Forlandsundet Basin (e.g. Atkinson 1960 , 1962, 1963; Steel et al. 1985) . How­ever, revised pa1aeonto1ogic data suggest that the two basins are in part coeval.

Disparate views exist concerning the age of the For-1andsundet Pa1aeogene deposits. Deposits on the eastern side of the basin (Sarsøyra in Fig. 2) are poor1y lithified but have yie1ded a foraminifera assemb1age of Middle to Late Oligocene age (Fey1ing-Hanssen & Ulleberg 1984) , an age that differs from the Late Eocene age suggested by the contained dinoflagellate assemblage (Manun 1960; Manum & Throndsen 1986) . Age determinations for the western side of the basin (Selvågen area, Fig. 2) are based on macroflora and bivalves, and suggest a provi­sional Late Eocene age for the lower part of the succes­sion whereas an Oligocene age is only inferred for the unfossiliferous upper part of the sequence (Livsic [Livshits] 1974 , 1977) . Livsic [Livshits] ( 1977) points out that the deposits on the two sides of the basin are not entirely contemporaneous and that the rather thin se­quence on the eastern side of the basin ( � 135 m thick; Rye Larsen 1 982) correlates with only the upper part of

R0=0.88- 0.95 n=3

PRINS

KARLS

NORSK GEOLOGISK TIDSSKRIFT 12 (1992)

FOR LAND

o 10 20 km

Fig. 2. Map showing the distribution of Tertiary rocks within the Forlandsundet Basin. Black areas denote Tertiary outcrop beits; shaded areas indicate basement to the basin; stripes represent areas within the Forlandsundet Basin with pre­sumed Tertiary bedrock but with no outcrop exposure and a separate subsurface Tertiary basin at Ny Ålesund. Geology is after Hjelle & Lauritzen ( 1982) and Flood et al. ( 1 971). White asterisk denotes location of outcrop in Figs. 3, SD, 6A-E. R0 indicates vitrinite reflectance values from the labeled areas with n representing the number of coal sites analyzed in each area. Vitrinite reflectance data are from this study and Rye Larsen ( 1 982).

the much thicker succession on the western side of the basin ( > 2000 m thick; Rye Larsen 1982) .

The age of the youngest preserved strata in the Central Basin (Fig. l ) , the Aspelintoppen Formation, is thought to be of latest Palaeocene or Early Eocene age based on pollen and spore assemblages (Head 1989) or Eocene (Late Eocene) based on macroflora, macrofauna (mol­luscs) , dinoflagellates, and foraminifera (Vonderbank 1 970; Livsic 1 974; MatthieBen 1 986) . Therefore, it may be tempting to conclude that Forlandsundet basin fill is

NORSK GEOLOGISK TIDSSKRIFT 72 (I992)

younger than Central Basin subsidence, but the vitrinite reflectance data of Manum & Throndsen ( 1 978) and Throndsen ( 1 982) suggest that 1 .3 -l. 7 km of overburden have been eroded from the eastern Central Basin, making it most likely that deposition and subsidence continued into Eocene-Oligocene time and that the two basins are in part coeval. This view assumes that the missing load was not a tectonic load as no independent evidence for large-scale thrusts or shortening exists in the upper part of the Central Basin (Dalland 1 979; Nøttvedt et al. 1 988; Pershing 1 990) . The tectonic control on the rnissing Eocene-Oligocene part of the Central Basin remains undefined, and therefore, to best evaluate the relation­ship between Eocene-Oligocene sedimentation and tec­tonics, one must turn to the coeval Forlandsundet Basin.

Ear/ier models for deve/opment of For/andsundet Basin

The juxtaposition of Tertiary sedimentary strata adjacent to Caledonian greenschist-facies basement (the pre-De­vonian Hecla Hoek basement) along long continuous lineaments stimulated the concept of a fault-bounded Forlandsundet graben of extensional origin (Tyrell 1 924; Orvin 1 940) . Ocean-floor magnetic anomaly data subse­quently suggested a coeval dextral transform margin between Svalbard and Greenland (e.g. Harland 1 969; Talwani & Eldholm 1 977) , such that more recently the basin has been regarded as genetically related to dextral strike-slip deformation along that Palaeogene plate mar­gin. Rye Larsen ( 1 982) , Lepvrier & Geyssant ( 1 985) , Manby ( 1 986) and Lepvrier (1 990) suggested that the Forlandsundet Basin could have formed as a northward­migrating extensional basin between two parallel right­stepping dextral wrench faults possibly coincident with the present boundary lineaments and thus have a pull­apart origin (sensu Crowell 1 974) . However, a dextral history for the boundary lineaments has yet to be demonstrated. Although Rye Larsen (1 982) points out that clasts of meta-tillite in Tertiary conglomerates are separated dextrally 3 km from an exposed potential Cale­donian source area, he also cautions that Caledonian lithologic boundaries Iie parallel to the boundary linea­ments and direction of presumed strike-slip disp1ace­ment. Therefore, piercing points do not exist a1ong the boundary lineaments, and it is not possib1e to use posi­tions of clasts and source areas to demonstrate a strike­slip component nor quantify offset along the boundary lineaments.

A variation on the pull-apart concept relies on the presence of a major flower structure along the axis of the West Spitsbergen Orogenic Belt as a result of transpres­sion along the Tertiary transform margin (Lowell 1 972) . His model was drawn largely on earlier mapping by others and by analogies with clay experiments and with modem southern California. The axis of that proposed flower structure would coincide with the position of the Forlandsundet Basin, and this coincidence led

Tectonics of For/andsundet Basin, Spitsbergen 95

Steel et al. ( 1 985) to propose a model in which the Forlandsundet Basin originated during dextral transpres­sion as an extensional feature along the divergent core of the fiower structure. However, subsequent structural studies and seismic investigations do not corroborate the flower-structure model (Faleide et al. 1 988; Maher & Craddock 1 988; Manby 1 988; Ohta 1 988) . In the sup­posed axis of the fiower structure where thrusts should steepen, the thrusts display shallow dips (Manby 1 988; Ohta 1 988) . Consequently, the model of Steel et al. (1 985) for the origin of the basin is also spurious, and the tectonic origin of the subsidence and deformation of the basin remains an unresolved question.

Stratigraphy and depositional setting of Forlandsundet Basin

Tyrell (1 924) , Atkinson (1 962) , and Livsic ([Livshits] 1 974 , 1 977) established initial lithostratigraphic units and nomenclature for the Forlandsundet Basin, whereas Rye Larsen ( 1 982) re-examined the entire basin, proposed three new formations, and addressed the basin geometry and depositional settings. Although we support his model in general, we are re-evaluating the stratigraphic relationships in light of probable structural repetition of the Palaeogene sequence.

Conglomeratic alluvial fans and fan deltas prograded from both the· eastern and western margins in to the marine Forlandsundet Basin, which was locally anoxic (Rye Larsen 1 982; Steel et al. 1 985) . Fan-delta front deposits show signs of tidal influence. Away from the margins of the basin, palaeocurrent data suggest north­wardly directed axial dispersal by turbidity currents (Rye Larsen 1 982; Steel et al. 1 985) . In general the deposits are finer toward the center of the basin, although coal, car­bonaceous shale, and lithic arenites are intercalated with the proximal conglomerates and at some localities on the western side of the basin constitute the basal deposits on the Hecla Hoek basement (Fig. 3 ) . The Tertiary deposits in general display a high degree of disturbance in the form of clastic dikes, slump folds, convolute bedding, and general soft-sediment deformation.

Immature Palaeogene detritus can be matched directly with adjacent Ca1edonian (Hecla Hoek) source rocks (Rye Larsen 1 982) , suggesting modem exposure of roughly the same Hecla Hoek horizons as in Palaeogene time. In addition, along the western basin margin, the boundary lineaments do not follow the contact between Palaeogene deposits and the Caledonian basement, and although that contact may display minor faults, the Palaeogene strata also locally occur depositionally, but unconformably (Rye Larsen 1 982) , on regolith developed on the Hecla Hoek basement (Fig. 3 ). Consequently the Cenozoic burial and uplift his tory of the basin also applies to the adjacent Caledonian basement. These observations lead us to question the concept of major dip-slip motion along the boundary lineaments of the basin relative to

96 K. L. Kleinspehn & C. Teyssier NORSK GEOLOGISK TIDSSKRIFT 72 (1992)

N S side of the basin with more mature strata exposed in the south.

Conglomerate (Schistose detritus)

Black shale, coal stringers, and

broadleaf plant fossils

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Fig. 3. Field sketch of an interpreted unconforrnable depositional contact between fossil-bearing Palaeogene strata and Hecla Hoek basement at a locality in the Selvågen region near a peak between Sesshøgda and Thomsonfjellet with altitude 43 1 m. The locality is indicated on Fig. 2 and photos are shown in Figs. 50 and 6A-E. Coal samples from this locality yield vitrinite reHectance data of R0 = 2.55.

the Hecla Hoek basement (e.g. Manby 1 986) as well as the concept that the basin necessari1y originated as an extensional graben.

Burial history of Forlandsundet Basin

Litt1e attention has been given to the diagenetic and thermal history of the Forlandsundet Basin sediments, but limited vitrinite reftectance data from thin coal beds within the proximal conglomerates on both sides of the basin have yielded results suggesting asymmetric burial and up1 ift histories for the eastern versus western sides of the basin. Six samples from the western side of the basin (Selvågen area; Fig. 2) show values ranging from Ro = 2.55 to R0 = 5.07 with a mean of R0 = 3. 73. In contrast, three samples from 20 km away on the eastern side of the basin (Sarsøyra; Fig. 2) show values of Ro= 0 .56-0 .67. These results suggest profound differ­ences in thermal and uplift histories over fairly short distances. They are also consistent with data reported by Manum & Throndsen (1 986) as well as results from Rye Larsen ( 1 982) , where a western sample (from Sel vågen area) yielded R0 = 4 .01 and two eastern samples yielded Ro= 0.4 3-0 .46. One western sample from coastal expo­sures near Buchananryggen (Fig. 2) produced a value of Ro = 2.65, whereas three samples from the northwest part of the basin ( 30 km away from Sel vågen at Ab­erdeenftya; Fig. 2) produced values of R0 = 0.88-0 .95 (Rye Larsen 1 982) , suggesting a probable gradient in reftectance values from south to north along the western

Under an average geothermal gradient, the western samp1es suggest burial depths of 6-1 O km, whereas the Ro values from 20 km away on the eastern side of the ba sin suggest burial of less than 2 km (Ho od et al. 1 97 5) . This asymmetry may reftect differential burial of coeval strata or differential uplift exposing strata of different ages. Because the coal-bearing units on the eastern side of the basin appear to be younger than the coa1 -bearing units on the western side (Manum 1 960 ; Livsic [Livshits] 1 974 , 1 977; Feyling-Hanssen & Ulleberg 1 984 ; Manum & Throndsen 1 986) , asymmetric uplift is a more like1y exp1anation for the vitrinite reftectance data with the western side of the basin experiencing substantially greater uplift based on age data and degrees of lithifica-tion. Any deformation model for the basin needs to satisfy these thermal data.

Our observations of the degree of lithification of the Tertiary deposits on the eastern versus the western sides of the basin as well as the development of ductile fabrics in the rocks on the western side of the basin are consis­tent with the concept that the western outcrops represent exposure of strata that were buried more deeply. How­ever, vitrinite reftectance data require care in interpreta-tion as they express primari1y temperature and time histories, but not necessari1y absolute burial depth. Be­cause the western margin of Svalbard was an active Tertiary transform and spreading plate margin, it may not be appropriate to assume an average pa1 aeo-geother-mal gradient ( Crane et al. 1 982) . As an alternative to the interpretation above, the differences in reftectance data could reftect spatial variation in the geothermal gradient, but again over short distances, perhaps enhanced by preferred ftow paths of hot connate ftuids that controlled heat distribution. We feel it appropriate to raise this possibility, but prefer an interpretation in which the western side of the basin has suffered greater uplift as it is consistent with seismic reftection profiles (Gabrielsen et al. 1992).

We dismiss Pliocene-Quaternary vertical crustal movements and glacial isostatic rebound as a cause of variation in the thermal history of the exposed basin fill. Although the Forlandsundet region has experienced as much 45 m of post-glacial uplift (Forman et al. 1 987; Landvik et al. 1 988; Forman 1 990) , any glacial subsi­dence would have been suffi.ciently shallow and brief not to have affected the thermal signatures in the Tertiary strata. Thus we regard the reftectance data derived from coal to be of thermo-tectonic origin.

Deformation of Forlandsundet Basin: preliminary results

Mapping on the western side of the Forlandsundet Basin in the vicinity of Selvågen (Fig. 2) a1 so leads us to question the long-standing concept of the basin as a

NORSK GEOLOGISK TIDSSKRIFT 72 (1992) Tectonics of For/andsundet Basin, Spitsbergen 97

Fig. 4. (a) Photo showing a sinistral fault developed in a Palaeogene conglomerate at Geddesfjellet in the Selvågen area on the western side of Forlandsundet Basin. Slickenside striae are visible at a great distance because they are lines gouged by conglomerate clasts moving independently from their matrix during relative motion along the fault. Displacement is inferred to have predated lithification. The surface is oriented 006°, 78°E with slickenside striae plunging l6°N. (b) A sinistral fault displaced clasts and their matrix in the same conglomerate unit as (a) and is inferred to have occurred after lithification. Locality is between Geddesfjellet and Krokodillen in the Selvågen region. Glove for scale is 20 cm.

simple extensional graben. The basin fill displays multi­ple thrusts, strike-slip faults (Fig. 4 ) , open to tight folds, refolded isoclinal folds, and foliated shear zones (Fig. 50) . Thrust orientations suggest shortening approxi­mately parallel to the boundary lineaments of the basin. Some high-angle fault surfaces within the basin display slickenside striae suggesting normal offset, but evidence of major normal faults is strikingly absent. Strata dip dominantly toward the basin axis to the east, but also display dispersion in their orientations and are locally overturned. Such structures are inconsistent with a sim­ple graben interpretation as they record non-extensional strain. Any model for the basin needs to account for a multiphase history of brittle deformation in which the boundary lineaments cut across earlier folds and thrusts (Lepvrier 1 990) . Conjugate sets of normal fractures sug­gesting extension appear to record the latest phase of brittle deformation and clearly postdate rotation of bed­ding (Fig. 5A) .

Temporal relations between the various structures are not yet clear in detail, but will be a target of continued study. However, Palaeogene strata onlap progressively

onto separate fault blocks in the Hecla Hoek basement, suggesting that at least part of the deformation was syndepositional (Rye Larsen 1 982) . In addition to growth faults, faults along which intact conglomerate clasts moved relative to their soft matrix (Fig. 4a) , clastic dikes bearing slickenside striae (Fig. 5B) , and soft sediment folding (Fig. 5C) are also consistent with early deforma­tion that was in part coeval with basin subsidence and deposition. Alternatively, man y faults appear to postdate cementation (Rye Larsen 1 982; Lepvrier & Geyssant 1 985; Lepvrier 1 990) , and some faults cut and displace fragments of conglomerate clasts together with the adja­cent matrix suggesting advanced stages of lithification at the time of faulting (Fig. 4b) . Abundant small-scale faults cut Forlandsundet strata throughout the basin and dis­play highly variable senses of slip, including normal, reverse, sinistral, and dextral slip based on slickenside striae and tension gashes. Clearly, only a multiphase deformational history can explain the structural variety and complexity recorded in the Forlandsundet Basin fill, and these types of observations together with cross­cutting relations and ages of recrystallized minerals offer

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the potential to constrain a kinematic history for the basin.

Evidence of ductile deformation in the Forlandsundet strata

Another striking and unanticipated result of the prelimi­nary field work is the widespread evidence of develop­ment of ductile fabrics in the Tertiary strata in the Selvågen and Buchananryggen areas on the western side of Forlandsundet Basin. This ductile deformation occurs in addition to the brittle structures discussed previously and both types of structures are developed within single outcrops suggesting multiphase deformation under differ­ing conditions. Schist-rich conglomerates (Fig. SD) , lithic arenites, and shales display development of foliation on a macroscopic scale. Strain is concentrated in narrow zones that parallel or cut across bedding in which the microscopic foliation is defined by the elongation of detrital mica clasts, the attitude of dissolution films and seams, and the preferred orientation of new phyllosilicate grains (Figs. 6A, B, E) . Foliation in the quartzites is not as continuous and pervasive (Fig. 6F) . Fig. 6E shows an additional typical example of foliation developed along shear zones in the Tertiary lithic arentites, suggesting strain under relatively elevated temperatures.

In the micaceous lithic arenites, mica layers tightly wrap around quartz clasts that show evidence of slight dissolution and reprecipitation. Dissolution at contacts between quartz grains is accompanied by very fine quartz-phyllosilicate beards in the clasts' shadow zones (Fig. 6A, B) . Within these shadow zones, as well as interspersed in the mica layers, are 'optically dean', well crystallized phyllosilicates, commonly 10-20 �m long, which we interpret as being new grains (Fig. 6C, D) . A scanning electron microscope study of these beards re­veals that phyllosilicate grains are intergrown and that the adjacent quartz grain has a serrated boundary, indi­cating dissolution (Fig. 6C) . In addition, lO �m phyllosil­icate flakes impinge into the quartz grain, suggesting that these phyllosilicates are indeed new grains (Fig. 6D) . These textures cannot be attributed simply to com­paction during burial. Foliation in the micaceous beds is locally corrugated in bundles or domains (Fig. 6E) , but this crenulation is not sufficiently penetrative to produce a continuous cleavage. In other samples, shear hands up to 0 .5 cm long locally cut across foliation at a small angle, and deftect it. Further systematic sampling and petrographic analysis will focus on the use of shear hands as potential kinematic indicators.

In the pure, well recrystallized quartzites, no foliation is apparent, but nearly all grains are polygonized and traversed by deformation hands developed along the trace of the quartz c-axis. In rare cases where detrital grain and overgrowth are separated by a rim of 'dust', these deformation hands define an undulatory extinction in optical continuity across the dust rim into the quartz

Tectonics of For/andsundet Basin, Spitsbergen 99

overgrowth (Fig. 6F) . This observation suggests that crystal-plastic processes involving dislocation climb and the formation of optical subgrains (producing deforma­tion hands) (Schmid 1983) were operative in the quartz­ite after formation of the quartz overgrowth. Although absolute values are not known, crystal-plastic processes necessitate elevated temperature to be activated (Pater­son 1 989) . This is consistent with the fact that the quartzites in which deformation hands are well devel­oped were collected in areas where vitrinite reftectance values are R0 = 4.84 and 4.02, indicating palaeotempera­tures higher than 250°C (Hood et al. 1 975; Naeser et al. 1989) . Because white mica occurs by clay-mineral recrys­tallization under the lowest conditions of metamorphism ( Guidotti 1 984) , comparable to those suggested by the vitrinite reftectance data, the basin fill should fall within the stability field of white mica. Therefore, it is likely that the newly grown phyllosilicates are white micas, and future analyses will establish their composition. Meta­morphic fabrics and conditions such as these discussed have not previously been documented in the Tertiary basin and are not consistent with earlier interpretations of the basin as a simple upper crustal graben.

These preliminary results are consistent with evidence of a post-Mesozoic (younger than ca. 200 Ma) thermal overprint suggested by discordant -w Ar/39 Ar age spectra on detrital micas from Hecla Hoek basement rocks on the eastern side of the Forlandsundet Basin (Dallmeyer 1 989; Dallmeyer et al. ·1990) . Although they have been unable to constrain the age of that thermal overprint with precision, these authors propose a likely Tertiary age. The therma1 overprint in the Caledonian rocks appears to have been variable and everywhere significantly lower than the initial Palaeozoic closure temperatures for white mica (Dallmeyer 1 989) . Our Tertiary samples also contain evidence of a Tertiary thermal history with temperatures sufficiently high to deform ductilely and to produce abundant recrystallized white mica.

The temporal relationship between development of these ductile fabrics and the multigenerational brittle structures is not yet known, although their occurrence suggests that the basin was at the brittle-ductile transi­tion at some time in its history. The sense of shear in these ductilely deformed zones is also y et to be docu­mented, such that the ductile fabrics could be associated with any of the post-depositional deformational phases, including the extensional youngest phase. Ductile defor­mation is of particular interest because the brittle-duc­tile transition is thought to serve as a major detachment horizon in other continental transform plate boundaries that experience transpression (e.g. Namson & Da vis 1988; Lettis & Hanson 1 991 ).

Discussion of the history of Forlandsundet Basin

This preliminary report, based upon limited field and

100 K. L. Kleinspehn & C. Teyssier NORSK GEOLOGISK TIDSSKRIFT 72 (1992)

Fig. 6. The sarnple location for A-E is shown in Fig. 2. A and B. Plane polarized and crossed nicols view, respectively, of fibrous, quartz-phyllosilicate beards in a pressure shadow zone around a quartz clast. Note that the quartz grain in the upper teft of the photograph appears to be truncated at its base by a dissolution surface subparallel to the phyllosilicate fibers. These microstructures cannot be the result of sedimentary compaction only, and indicate dissolution and neocrystallization at low metamorphic grade. Scale bar is O.l mm. C. Scanning Electron Microscope image of an elliptical quartz grain showing fibrous pressure shadows. (D) is an enlargement of the right end of this quartz grain. Scale bar is 100 J.llll. D. A detailed image of the pressure shadow reveals the presence of intergrown phyllosilicates. Note the serrated aspect of the quartz grain contact and the presence of phyllosilicate flakes grown into the quartz (white arrow). Scale bar is lO 11m. Numbers on figure have no meaning here. E. The weak foliation (parallel to the long side of the photograph) in lithic arenite is corrugated. Scale bar is 0.5 mm. F. The dark, quartz grain in the center of the photograph is parti y surrounded by an overgrowth zone. Deformation bands (white arrow) are continuous between the detrital grain and its overgrowth, suggesting that at least some dislocation movement {climb) postdated the overgrowth. Scale bar is 0.5 mm. Sample is from a locality at the east end of Krokodillen in the Selvågen area.

laboratory data, points out several key features which, although not fully understood at this stage, depart from earlier concepts about the Forlandsundet Basin and should be taken into consideration in any model for the initiation and the development of the basin.

Relationship between basement and basin strata

The Forlandsundet strata are not necessarily in fault contact with the Hecla Hoek basement, and depositional contacts have been recognized. Earlier basin models have

NORSK GEOLOGISK TIDSSKRIFT 72 (1992)

consistently viewed the basin as fault-bounded and have suggested a basin geometry in which detritus was shed across boundary faults to be deposited in a basin whose subsidence was controlled by motion on those faults. We emphasize that the boundary lineaments extend to the north and south beyond the limit of Tertiary exposures (Fig. 2). (Hjelle et al. 1979; Manby 1986), and we raise the question: If they experienced substantial normal offset along their entire length, why are Tertiary deposits absent farther to the north and south of their present outcrop distribution (Fig. 2)? Our observations suggest that although some boundary lineaments occur in the vicinity of the contact between Palaeogene basin fill and the basement, the lineaments do not follow the contact and also occur well within the basin itself (Fig. 2). Movement on those lineaments was not coeval with ambient deposition, but postdates sedimentation, and those lineaments do not coincide with the original basin margins.

Structural complexity of the basin

Faults affecting the Forlandsundet strata were developed at various stages of sediment consolidation, and a spec­trum from synsedimentary fault growth to post-lithifica­tion fracturing is observed. In addition, sense of slip on faults varies greatly, suggesting that a protracted, multi­phase deformation history has shaped the structure of the Forlandsundet Basin. However, a crude chronology of basin development emerges with several stages requir­ing further refinement:

l . 0/dest phase: The origin of the basin The tectonic regime during initial basin subsidence remains an open question. Previously the basin was thought to have an extensional origin along normal boundary faults. The observation of depositional con­tacts between basin strata and Caledonian basement, the observation that the boundary lineaments do not necessarily follow the contact between basin and base­ment, and the lack of direct evidence of normal displacement on the boundary lineaments make the graben concept less tenable and raise the question as to the origin of the basin. The deformational struc­tures discussed in this paper are mainly post-deposi­tional, some even postdate lithification, and cannot be used to address the question of initiation of the basin subsidence. An extensional origin remains among many possibilities, but we stress that it is essential to entertain alternative interpretations in the absence of rigorous data concem ing the basin's origin.

2. Soft-sediment deformation Evidence that the basin was tectonically active derives from brittle structures of tectonic origin that predate lithification, and it is likely that deformation of older basin sediments ensued white younger basin fill was

Tectonics of Forlandsundet Basin, Spitsbergen 101

still being deposited. Discrimination between soft-sed­iment deformation attributable to tectonism and soft­sediment deformation that is inherent in depositional systems remains a hurdle in interpreting the early kinematic his tory of the basin in general and the sense of tectonic displacement in particular.

3 . Post-lithification deformation This phase actually consists of several phases whose chronology is not yet clarified. These phases include shortening roughly parallel to the basin axis, strike­slip displacement of basin strata, and isoclinal and broad open folding. Although some structures display cross-cutting relations, these phases may be essentially coeval and constitute partitioning of strain. A com­pressional and/or strike-slip setting appears consistent with the developed structures although the orienta­tions of strain axes remain to be determined. The observed ductile fabrics may have developed during these phases in association with observed shear zones and isoclinal folds. Phases 2 and 3 probably represent a continuum with progressive burial of the basin. The overburden suggested by temperatures necessary for ductile behavior and the vitrinite reftectance data may have been due to continued basin subsidence and sediment accumulation or it may have been due to tectonic load of thrust origin.

4. Youngest phase: Extension Evidence for this phase derives from abundant small­scale and mesoscale extensional structures that cut across the structures of phase 3. The orientations of the palaeostress axes are still indeterminate, but it is this phase that probably generated the modem basin geometry and boundary lineaments that stimulated the concept of a Forlandsundet graben. The present limit of Palaeogene exposures, which locally coincides with the young boundary lineaments, may not repre­sent the original basin extent. The observed ductile fabrics may have developed during this phase and may record extension at depth. We also speculate that this phase may have resulted in uplift of the western basin margin and its basement on Prins Karls Forland with the istand representing part of a metamorphic core complex exhumed as the result of extension. We emphasize that such a concept remains uncorrobo­rated in the absence of data for the age of uplift and recognition of a major detachment in the Caledonian basement. However, it raises a question about the possible location and dip direction of a detachment, a question that will focus future mapping.

Crane et al. (1991) assume a detachment that dipped steeply eastward beneath Spitsbergen in Tertiary time and, although they assume the dip direction, they make it clear that some simple-shear detachment and asym­metric Tertiary extension are necessary to satisfy their heat ftow and topographic data for the Norwegian­Greenland Sea. An east-dipping detachment might be

102 K. L. Kleinspehn & C. Teyssier

resolved in the face of data suggesting greater uplift of the western margin of Forlandsundet Basin, but would suggest that Prins Karls Forland may be the uplifted half of a single fault block with Forlandsun­det Basin on the other subsided side and a listric normal fault bounding the basin on its eastern mar­gin. In such a model, the West Spitsbergen Orogenic Belt would be the exhumed core complex. The role of the boundary lineaments on the western side of the Forlandsundet Basin is not clear in such a model, and the elements of such a model require additional sup­portive data to move beyond speculation.

The modem erosional topography of the Forland­sundet Basin on Prins Karls Forland consists of east­west oriented ridges and valleys whose positions appear structurally controlled by faults. Structures with similar orientations in the basement on the east side of the basin are considered younger than the boundary lineaments (Ohta 1 988) . Development of those structures also may have been coeval with uplift of the island and the western basin margin.

Therma/ history of the basin

Based upon vitrinite refiectance data, it appears that strata exposed on the southwestern side of the basin record greater burial and uplift than those currently exposed in the eastern and northern parts of the basin. In addition to the vitrinite refiectance data, the observation of ductile deformation features in the southwestern For­landsundet Basin suggests that, at least locally, palaeotemperatures were elevated. These limited data contrast with earlier models for basin development, and suggest an asymmetric uplift and thermal history that needs again to be acknowledged in any model for devel­opment of the Forlandsundet Basin.

Relationship of Forlandsundet Basin history to the evolution of the Norwegian-Greenland Sea and Tertiary deformation of Svalbard

Reconstructions of the early opening of the Norwegian­Greenland Sea have consistently suggested a multiphase Tertiary evolution of the plate margin between Green­land and Eurasia with an early Palaeogene transform phase of transpressive and compressive nature followed by a passive-margin phase and transtension beginning in Oligocene time to open the modem sea (Talwani & Eldholm 1 977; Myhre et al. 1 982; Faleide et al. 1 988; Ziegler 1 988; Eldholm et al. 1 990) . In a revised recon­struction, the transform phase with associated strike-slip, compression, and transpression began at 80 Ma (Campa­nian or Late Cretaceous) and continued to 36 Ma (Rupe­lian or earliest Oligocene) with the final passive-margin

.phase and transtension beginning at 36 Ma and continu­ing to the present (Roest & Srivastava 1 989; Muller &

NORSK GEOLOGISK TIDSSKRIFT 72 (I992)

Spielhagen 1 990 , using the time scale of Kent & Grad­stein 1 986) . Late-stage extension and uplift of the basin may correspond to the final transtensional passive-mar­gin phase. Although maximum shortening in western Spitsbergen is thought to have occurred during 59-56 Ma (Late Palaeocene time), the magnetic sea-fioor anomalies suggest that compression was a significant but less dominant component a1ong the plate margin throughout the entire transform phase of 44 million years duration (Muller & Spielhagen 1 990) . Thus it is likely that the Forlandsundet Basin originated under transpression or compression during the transform phase (Stee1 et al. 1985; Muller & Spielhagen 1 990) .

In other earlier interpretations, subsidence and deposi­tion in the Forlandsundet Basin were thought to post­date shortening and have been correlated to the passive-margin transtensional phase (e. g. Har1and & Horsfie1d 1 974) . However, it is clear from the contained structures that shortening and/or strike slip deformation continued as the basin evolved (Lepvrier & Geyssant 1 985; Lepvrier 1 990) , and it is inappropriate to restrict the basin history to younger extension (post-36 Ma). Although limited and imprecise, palaeontological data suggest that the older basin fill on the western side is of Late Eocene age (Livsic [Livshits] 1 974 , 1 977) , and there­fore initial basin subsidence may have been coeval with a phase dominated by strike slip along the transform mar­gin (Muller & Spielhagen 1 990) . The paucity of high-pre­cision age data for the oldest strata will continue to hamper models for the origin of the basin. The age of basin origin is not likely to be resolved with ease as palynologica1 collections from the basin's western margin have proven fruitless due to the supermaturity of the sediments (M. J. Head, pers. comm. , 1 990 ; A. Nøttvedt, pers. comm. , 1 990) . However, the basin clearly predates the termination of shortening and strike-slip deforma­tion, but the local tectonic control on the initial basin subsidence is unresolved.

We emphasize that if the basin originated during the transform marg in phase, several hypotheses for the origin of the basin should be entertained. Both extension and shortening are typical of strike-slip margin s. The basin may have originated as a transtensiona1 pull-apart basin at a releasing bend or stepover in a strike-slip fault system (sensu Crowell 1 974 ; Rye Larsen 1 982; Lepvrier & Geyssant 1 985; Manby 1 986; Lepvrier 1 990) . This model is rather attractive because a basin in such settings often experiences later shortening as deformation progresses. Such a history would be consistent with part of the post-lithification structures observed in the basin. A problem with this interpretation is the absence of candi­dates for the major strike-slip faults that should be adjacent to the basin and extend to the north and south of the basin. Seismic pro files to the south at the mouth of Isfjorden lack evidence of major faults in the appropriate positions (Eiken & Austgard 1 987) . Also, no discrete data or evidence of strike-slip displacement on the boundary lineaments has been presented, although such

NORSK GEOLOGISK TIDSSKRIFT 72 (1992)

an interpretation has been offered (e. g. Lepvrier & Geyssant 1 985; Manby 1 986; Lepvrier 1 990). An addi­tiona1 shortcoming is that lithofacies suggest deposition in progressively deeper water northward in the basin (Rye Larsen 1 982), and no northern basin margin is known. Such a basin architecture is inconsistent with a pull-apart model.

Alternatively, the basin may have originated in a com­pressional regime, due to lithospheric flexure or in a piggyback position on top of transported thrust sheets that constitute the core of the deformational zone in western Spitsbergen. This view is enhanced by the pro­posal by Man by ( 1 986) that the Tertiary thrusts on Prins Karls F orland are all east vergent, placing the Forland­sundet Basin in a position between and overlying east­northeast verging thrusts. In this alternative, deposition would have been succeeded by folding and thrusting of the basin strata, followed by a period of extension (most probably transtension) that produced the 'graben' struc­ture observed today. This deformational sequence is consistent with that proposed by Harland & Horsfield (1 974), Hjelle e t al. ( 1 979), and Lepvrier ( 1 990), and may also hetter satisfy facies architecture in which gravelly non-marine and deltaic facies are confined to the south­em part of the basin (Rye Larsen 1 982). We offer these alternative hypotheses not as an attempt to answer the question of the o ri gin of the basin, but to provoke e l oser examination of the basin, carefu1 review of existing data, and vigorous discussion of the basin evolution.

Acknowledgements. - Kleinspehn gratefully acknowledges the financial and logis· tical support of Statoil and Den Store Norske Spitsbergen Kulkompani. Later stages of this study were supported by grant DPP-9022689 of the National Science Foundation. William Maze is thanked for his superb assistance and discussions in the field. Scanning electron microscopy was done with the assis­tance of John Humenansky at the University of Minnesota Electron Microscopy Center. Vitrinite reftectance analyses were provided by Norsk Hydro. An earlier manuscript benefited considerably from comments and reviews by Claude Lep· vrier and Roy Gabrielsen. Arild Andresen and Winfried Dallmann are thanked sincerely for organizing the Symposium on Post-Devonian Tectonic Evolution of Svalbard.

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