Steffen G. Bergh

Interaction of Basement-Involved and Thin-Skinned Tectonism in the Tertiary Fold-Thrust Belt of Central Spitsbergen, Svalbard

The Tertiary fold-thrust belt in Oscar II Land, central Spitsbergen, consists of three major zones of distinct structural style: (1) a western basement-involved fold-thrust complex, (2) a central zone of thin-skinned fold-thrust units above a decollement in Permian evaporites, and (3) an eastern zone characterized by a frontal duplex system in the fold-thrust belt, bounded eastward by steep, basement-rooted reverse faults (Billefjorden and Lomfjorden fault zones) beneath subhorizontal platform strata. Offshore seismic data from Isfjorden (Statoil) confirm the threefold zonation and document thick-skinned and thin-skinned structural interactions in both the fold-thrust belt and the foreland section. An admissible cross section yields about 45%, or 20 km, of shortening in Oscar II Land. Deeper parts of the seismic profiles show fault-bounded Devonian (central and east) and Carboniferous (west) basins. The structural grain of the Tertiary fold-thrust belt partly coincides with the margin-bounding normal faults of these basins, suggesting that preexisting structures and stratigraphy controlled the Tertiary fold-thrust belt development. A kinematic evolution of the fold-thrust belt is invoked: (1) north-northeast-directed, bedding-parallel shortening, (2) major west-southwest-east-northeast shortening, with in-sequence foreland fold-thrust propagation, (3) basement-involved, west-southwest-east-northeast uplift in the eastern foreland zone, (4) eastward out-of-sequence propagation of thrusts, and (5) west-east extension in the hinterland. Our regional structural compilation map and synthesis of the central Spitsbergen transect advocates structural variation and linked basement-involved thrusting in the hinterland and thin-skinned/thick-skinned reactivation and out-of-sequence thrusting in the east (foreland), and is new compared with previous work of the region. The synthesis also raises several important new structural play concepts for investigating hydrocarbon prospects in Spitsbergen and adjacent regions; for example, inverted Carboniferous basins, and traps produced by Tertiary thin- and thick-skinned contraction and reactivation structures.

Application of a critical wedge taper model to the tertiary transpressional fold-thrust belt on Spitsbergen, Svalbard

The Tertiary opening of the North Atlantic Ocean involved major and long-lived overall dextral transpression between the Svalbard and Greenland plates. On Spitsbergen, this tectonic event is manifest as a 100–200-km-wide contractional fold-thrust belt in the form of an east-pinching prism. This belt can be subdivided into (1) a western, basement-involved hinterland province that reveals more complex deformation, including thrust, transcurrent, and normal faulting, and (2) an eastern thin-skinned fold-thrust belt with structures oriented subparallel (north-northwest–south-southeast) to the transform plate boundary. The time-space distribution and interaction of different structural styles of Tertiary deformation evident on Spitsbergen support a model with linked, long-term and short-term (episodic) dynamic growth of a composite contractional and transcurrent fold-thrust wedge. The growth of a narrow, high-taper (critical-supercritical) contractional wedge occurred during northward-directed crustal shortening (stage 1) in an oblique, dextral transcurrent setting. Crustal thickening in the form of thrust uplift and basin inversion and strike-slip duplexing during the main contractional event (stages 2 and 3) created an unstable, supercritical wedge of basement and cover rocks in the hinterland. At the same time, a broader and more homogeneous frontal part of the wedge developed eastward by in-sequence imbrication in order to reduce the taper angle. Local erosion and lateral wedge extrusion (stages 3 and 4) modified the oversteepened hinterland wedge to a critical taper angle. Continued tectonic activity in the hinterland caused renewed internal imbrication of the frontal wedge, where deformation was accommodated by tear faulting and out-of-sequence thrusting (stage 4). Adjustment toward a stable taper geometry included local extension (stage 5) and erosion and sedimentation. In a transpressional fold-thrust belt, as on Spitsbergen, out-of-plane (orogen oblique to parallel) transport in the hinterland may cause local and lateral supercritical and subcritical wedge tapers. Hinterland geometries could trigger adjustments in a frontal thrust wedge in a decoupled situation, and/or orogen oblique or parallel motions in a coupled situation. Changing kinematics may thus be expected along strike in such an orogen.

Pleistocene mass-flow deposits of basaltic hyaloclastite on a shallow submarine shelf, South Iceland

A Pleistocene subaqueous, volcanic sequence in South Iceland consists of flows of basaltic hyaloclastite and lava with interbedded sedimentary diamictite units. Emplacement occurred on a distal submarine shelf in drowned valleys along the southern coast of Iceland. The higher sea level was caused by eustatic sea-level change, probably towards the end of a glaciation. This sequence, nearly 700 m thick, rests unconformably on eroded flatlying lavas and sedimentary rocks of likely Tertiary age. A Standard Depositional Unit, describing the flows of hyaloclastite, starts with compact columnar-jointed basalt overlain by cubejointed basalt, and/or pillow lava. This in turn is overlain by thick unstructured hyaloclastite containing aligned basalt lobes, and bedded hyaloclastite at the top. A similar lithofacies succession is valid for proximal to distal locations. The flows were produced by repeated voluminous extrusions of basaltic lava from subaquatic fissures on the Eastern Rift Zone of Iceland. The fissures are assumed to lie in the same general area as the 1783 Laki fissure which produced 12 km3 of basaltic lava. Due to very high extrusion rates, the effective water/melt ratio was low, preventing optimal fragmentation of the melt. The result was a heterogeneous mass of hyaloclastite and fluid melt which moved “en masse” downslope with the melt at the bottom of the flow and increasingly vesicular hyaloclastite fragments above. The upper and distal parts of the flow moved as low-concentration turbulent suspensions that deposited bedded hyaloclastite.

Structural outline of a Tertiary Basement-cored uplift/inversion structure in western Spitsbergen, Svalbard: Kinematics and controlling factors

The Tertiary fold-and-thrust belt of Spitsbergen can be divided into a western basement-involved fold-thrust stack and a central-eastern foreland fold-and-thrust belt. In western Nordenskiold Land the first-order structure is an ENE-verging basement-cored fold and fault complex involving Paleozoic to Tertiary strata. The northern part reveals an upright, monocline geometry of east tilted sedimentary cover units with associated layer parallel to low-angle thrusts and folds. These structures consist of two populations oriented both parallel to (NNW–SSE) and oblique to (WNW–ESE) the general structural trend of the fold complex. In the central and southern parts of west Nordenskiold Land the fold complex involves tilted basement cut by steep, transverse faults and late normal faults. The east limb of the fold complex displays repeated basement and Paleozoic strata (Orustdalen formation) in its core and Mesozoic (Triassic) strata influenced by map-scale chevron folds and two decollement levels, all located above an eastward rotated, major detachment fault, the Kleivdalen Thrust. Establishing fold-fault relations includes a three-stage structural history in the fold complex as follows: (1) a phase of early NNE–SSW shortening associated with WNW–ESE folds and thrusts and (2) a dominant ENE–WSW, basement-involved shortening leading to the first-order, NNW–SSE-striking fold complex, followed by (3) approximately E–W extension. The resulting structures and structural variability along strike as well as across strike appear to have been controlled by basement and Carboniferous basin structures underlying the Permian-Cretaceous platform strata. Restored stratigraphic sections based on thrust-repetition of basement and cover (e.g., within a type section of the Carboniferous Orustdalen formation) support such an interpretation. A tentative inversion tectonic model reproduces the position(s) of local and major thrust ramps and associated folds, as a result of inheritance from Carboniferous basin structures.

Kinematics of Tertiary deformation in the basement-involved fold-thrust complex, western Nordenskiøld Land, Svalbard: tectonic implications based on fault-slip data analysis

Abstract Kinematic analysis based on the interpretation of small-scale fault and fold data supports a kinematic evolution history involving heterogeneous crustal shortening and uplift, and subsequent extension (collapse) for the major, west coast fold-thrust complex in western Nordenskiold Land, Svalbard. This deformation can be divided into three main kinematic events. Stage 1 represents an early, distinctive, NNE-SSW-oriented contractional episode that generated layer parallel and low-angle thrusts and internal folds. Stage 1 structures (population 1) are arranged in an apparent en-echelon geometry and oblique direction relative to the major NNE-SSW-trending fold-thrust complex of stage 2 affinity. Stage 2 structures include ENE-verging chevron folds and steeply WSW-dipping thrusts and duplexes, that evolved during a progressive WSW-ENE-oriented contractional episode. Initial (pre-fold) stratal shortening (stage 2a) was followed by a continuous buildup of the fold-thrust complex and general crustal thickening (syn-fold, stage 2b), until a supercritical height/thickness was reached (post-fold, stage 2c). At this stage a change in the stress field caused failure of the fold-thrust complex, and continued NE-SW-directed shortening was accommodated as vertical strike-slip faults (population 2c). Late W-E-to WSW-ENE-directed extension, the stage 3 episode, was probably related to collapse of the overthickened stage 2 fold-thrust complex. The detailed reconstruction of kinematic events in western Nordenskiold Land supports initial breakup of the Greenland-Svalbard area in Early Paleocene (?) times during dextral, NNE-SSW-directed transpression (stage 1). The major deformational episode (stage 2), probably of mid-Paleocene to Eocene age, was characterized by progressive WSW-ENE-directed shortening and was related to decoupled deformation where broad zones of convergent strain were linked to narrow strike-slip zones. This strain partitioning is considered to have begun when the Hornsund Fault Zone widened into and reactivated the basin-bounding faults of the Carboniferous St. Jonsfjorden Trough. The latest kinematic episode (stage 3) is likely of Eocene age, and may be ascribed to extensional collapse in the hinterland of the western Spitsbergen orogenic wedge within a regional, dextral transpressive setting.

Svartfjella, Eidembukta, and Daudmannsodden lineament: Tertiary orogen-parallel motion in the crystalline hinterland of Spitsbergen's fold-thrust belt

Within metamorphic basement rocks of the hinterland of Spitsbergens Tertiary fold-thrust belt, a 35-km-long zone of notably deformed Carboniferous strata and Cretaceous intrusives forms a major orogen-parallel lineament from Svartfjella to Eidembukta to Daudmannsodden (SEDL). Orientations and geometries of map-scale fault duplexes and associated fault plane-striae populations, of folds and associated cleavage, and of tension gashes all indicate orogen-parallel motion occurred along the SEDL. Structural analyses indicates three phases: 1) ENE-directed overthrusting, 2) sinistral motion with a backthrust component, and 3) dextral strike-slip motion. This history indicates a change from orogen-perpendicular to orogen-parallel movements. Orogen-parallel movement was likely coeval with orogen-perpendicular fold-thrust transport to the ENE in the foreland. A model where dextral transpressive motion between Greenland and Svalbard was decoupled explains the hinterland-foreland difference. Basement fabric aligned with Carboniferous carbonates on the steep foreland face of an antiformal stack provided a through-going weak surface, a prerequisite for decoupling. Sinistral orogen parallel motion is explicable by short-lived episode of sinistral plate motion or by local wedge extrusion during dextral transpression. The evolution of decoupling patterns has significant implications for deducing plate motions from local kinematic and paleostress studies.

Tertiary or Cretaceous age for Spitsbergen's fold‐thrust belt on the Barents Shelf

Several publications propose that main-phase fold-thrust development on Spitsbergen was Late Cretaceous and not Tertiary as previously thought. The question of timing is crucial to models for crustal response to transpressive plate motions. Involvement of Tertiary strata in fold-thrust structures, the sedimentology of the Tertiary basin strata, and studies of paleo-stress field evolution all indicate Paleocene to Eocene fold-thrust development during opening of the Norwegian-Greenland oceanic basin. A regional angular unconformity of < 1° between Paleocene and Early Cretaceous strata is consistently disconformable to the eye and precludes any significant older deformation in the immediate area. Pre-unconformity deformation was likely strike slip in character and concentrated in the west. The proposal for Late Cretaceous fold-thrust belt formation is inconsistent with the geology.

Fault-controlled alpine topography in Norway

Abstract: Alpine topography in Norway is largely fault-controlled. Linear and asymmetric ranges developed in the footwalls of normal faults that were reactivated after the main phase of Mesozoic rifting, but prior to the Late Cenozoic glaciations. Stark geomorphological contrasts developed across the faults, reflecting differential glacial exploitation of the pre-glacial drainage pattern. Alpine topography developed preferentially in the footwalls. Triangular facets mark the traces of the most recently active faults. At the base of deeply incised, alpine range-front escarpments, the best-exposed faults display metres-thick fault-rock successions and record multiple phases of fault movement. Juxtaposition of Precambrian and Caledonian basement rocks with Jurassic or Cretaceous sedimentary rocks provides evidence for fault activity in or after the Mesozoic for some of the faults. Late Cretaceous or younger reactivation is indicated by jumps in apatite fission-track apparent ages across the faults, and interferometric synthetic aperture radar and earthquake data attest to normal faulting at the present day. Two of the areas described host anomalous clusters of rockslides that may relate to tectonic activity. The most distinct landscape-forming faults in western Scandinavia were probably active in the Cenozoic, and imposed asymmetric landscape patterns from the scale of single mountain ranges to the whole of Scandinavia.

The Chaparral shear zone: Deformation partitioning and heterogeneous bulk crustal shortening during Proterozoic orogeny in central Arizona

The Proterozoic Chaparral shear zone of central Arizona is one of a network of subvertical, northeast-striking shear zones that divide the Proterozoic orogenic belt of Arizona into tectonic blocks. The zone is several kilometers wide and contains variably developed mylonitic foliation that is subparallel to the regional subvertical foliation. Stretching lineations plunge shallowly northeast, and displacement across the zone was dominantly right-lateral. Several lines of evidence constrain displacement across the zone to be greater than 5 km, but probably less than tens of kilometers: (1) stratigraphic and plutonic rocks can be correlated across the zone, (2) structural and metamorphic histories of tectonic blocks on opposite sides of the zone are similar, and (3) integration of shear strain (estimated by deflection of earlier fabric) suggests greater than 5 km of strike slip. More important than scale of movement across the zone, structural studies have clarified several aspects of the assembly history of the orogen. Early north- to northwest-striking S 1 foliation, on both sides of the Chaparral shear zone, records one or more deformational events that took place between 1.75 and 1.7 Ga. Early fabrics record northeast-southwest or east-west shortening and west-verging thrusting that we interpret to have formed during development of a primitive arc complex. In contrast, intense deformation in the Chaparral shear zone took place during regional northwest-southeast shortening, D 2 , that produced the dominant subvertical northeast-striking foliation and the present block architecture of the orogen. D 2 is interpreted to have been in part synchronous with emplacement of 1.70 Ga granitoids and to record assembly of volcanic belts to North America. Of general interest is the character of D 2 deformation partitioning in and across the shear zone. Two types of high-strain domains were generated during shortening. One type, represented by rocks southeast of the shear zone, involved extreme shortening and transposition by folding. The other type, represented by the shear zone, involved simple shear deformation in a complicated array of anastomosing and conjugate shear zones. Overprinting relationships suggest that the second type may have nucleated on the first, implying an important component of deformation partitioning in time, as well as space. Mylonitic fabric in the shear zone developed by progressive heterogeneous simple shear followed by brittle fracturing. Conjugate shear bands suggest a shallow southeast-plunging σ 1 late during strike-slip deformation. This is consistent with regional D 2 shortening but not with the incremental shortening direction expected during right-lateral simple shear. This supports a regional kinematic model in which the Chaparral shear zone (right-lateral) and a temporally related left-lateral shear zone were regional-scale conjugate shear zones that accommodated heterogeneous D 2 northwest-southeast orogenic shortening via escape-block tectonics. Thus, orogen-parallel strike-slip displacements in the Chaparral shear zone were a response to partitioned shortening in the late stages of accretion rather than transpressional accretion during oblique subduction.

Palaeovolcanology and tectonic setting of a proterozoic metatholeiitic sequence near the baltic shield margin, Northern Norway

Abstract The Kvenvik Greenstone Formation ∼ 1500 m thick, is a very well preserved, 2.0–1.8-Ga-old? sequence of MORB-type, tholeiitic metabasaltic lavas and volcaniclastic rocks. They were deposited as cyclically repeated couplets in a continental, shallow-water to terrestrial environment near the margin of the Baltic Shield. The effusive facies comprise massive and amygdaloidal lava, pillow lava, pillow breccia and hyaloclastite. Volcaniclastic facies are: ash and accretionary lapilli tuff, lapilli tuff, and tuffaceous sedimentary rocks. Lithofacies associations and primary structures in the tuffs indicate air-fall, base-surge, and pyroclastic-flow deposits, with little input of epiclastic material. The prevalence of shallow-water to subaerial emplacement throughout the Kvenvik Greenstone Formation and correlative sequences to the south indicates deposition in a spasmodically subsiding basin, almost certainly formed by taphrogenic tectonism. The rifting may have been incidental to coeval plate convergence tectonism evidenced by calc-alkalic magmatism in the Repparfjord-Komagfjord area, 50–100 km northeast of the Kvenvik area.