Thrust structures and transport direction at Revneset, Svalbard

Harmon D. Maher Jr., Dept of Geography and Geology, University of Nebraska at Omaha, NE 68182-0199, U.S.A.

Steffen Bergh, Institute of Geology, University of Tromsø, Tromsø, Norway

Location: This point of land is almost due north of and easily accessible from Longyearbyen, and northeast of the airport across Adventfjorden (Fig. 1). If approaching by boat beware of extensive shallow water west of the point. The outcrops are primarily several meter high shoreline erosion features, with some outcrops in stream beds near to shore.

Figure 1: Geologic map of Revneset area modified from Major et al. (1992). The thrust symbol shows the location of the deformed Janusfjellet Subgroup shales.

Description of geology: The rocks exposed along the shore are Jurassic and/or Cretaceous age black shales and siltstones of the Janusfjellet Subgroup, which are in excess of several hundreds meters thick. Elsewhere on Spitsbergen they represent an incompetent horizon in which Tertiary deformation has been concentrated (e.g. Dallmann et al., 1991). At this locality a zone of thrusts and folds is exposed for some 300 meters along shore, and is tens of meters in thickness. It is structurally overlain by relatively flatlying and undeformed Cretaceous and Tertiary sandstone-dominated strata in the above mountain slopes. Further along the northeast striking shore, on the north side of the point, large glide blocks exist. Such glide blocks of Helvetiafjellet and Carolinefjellet sandstone blocks elevated above the soft Janusfjellet Subgroup shales along a fjord wall are also seen on the south shore of Midterhuken, Bellsund.

Both east and west verging thrusts are common in the deformation zone, as are rotated thrusts and duplexes. Associated folding has a wavelength of cms to several meters, is anywhere from open to tight, and has a geometry consistent with formation by small scale ramps and fold duplexes. The eastern margin of the disturbed zone has predominantly east verging thrusts, while the interior is dominated by west verging thrusts. The zone is proximal to and at the same structural position as an east verging detachment mapped to the southeast south of Adventdalen (Haremo et al., 1990). However, it may also be connected to west verging thrust structures to the east along the southern strike extension of the Billefjorden fault zone (Ringset, 1988).

Transport direction: Plots of poles to bedding (Fig. 2), and of fault planes with striae (Fig. 3) are consistent with S70W and N70E transport directions. The girdle pattern of slip directions evident in Figure 3 is likely due to rotation during development of the observed small scale thrust duplexes.

Figure 2 (to left): Superimposed scatter plot and contoured stereoplot of poles to bedding with statistical best fit girdle and associated interpreted fold axis.

Figure 3 (to right): Plot of 48 fault planes with striae and slip direction shown. Note the N70E subvertical girdle of slip lines. This pattern is consistent with bidirectional thrusts and rotated thrusts. A S70W direction predominates in the data set. Minor oblique directions seen are relatively symmetrically distributed around the N70E plane, and are likely due to anomalous directions at the lateral tips of thrust surfaces.

Geologic significance: This locale lies between the fold-thrust front on the north side of Isfjorden (Bergh & Andresen, 1990) and thin-skinned structures east of the Central Tertiary Basin (e.g. Haremo et al., 1992), and is best interpreted as an exposure of a continuous roof detachment beneath the CTB within the Janusfjellet shales. It helps to further document the extensive distribution of thin-skinned Tertiary deformation in Spitsbergen. The roof detachment and the CTB's foreland position, best explain why some Cretaceous strata (Helvetiafjellet and Carolinefjellet Fms.) and Tertiary strata are less deformed than underlying rocks (Maher et al., 1995). The transport direction is very close to orogen-perpendicular, in keeping with formation during decoupled Tertiary dextral transpression. It is inconsistent with a coupled flower like model. The reason for prevalence of backthrusts is not clear.

Acknowledgements: Dave Burger and Teresa Castelhano helped gather data at Revneset. The Petroleum Research Fund helped to support this fieldwork.

References:

Bergh, S. G. and A. Andresen, Structural development of the Tertiary fold-and-thrust belt in east Oscar II land, Spitsbergen. Polar Res. 8, 217-236, 1990.

Dallmann, W. K., A. Andresen, S. G. Bergh, H. D. Maher Jr., and Y. Ohta, Tertiary fold-and thrust belt of Spitsbergen, Svalbard, Medd. 128, 46 p. + 2 map sheets, Nor. Polarinsti. Oslo, Norway, 1993.

Haremo, P., Andresen, A., Dypvik, H., Nagy, J., Elverhøi, A., Eikeland, T. A. and Johansen, H., Structural development along the Billefjorden Fault Zone in the area between Kjellstromdalen and Adventdalen/ Sassendalen, central Spitsbergen. Polar Research, 8, 195-216, 1990.

Maher, H.D., Jr., Braathen, A., Bergh, S., Dallmann, W., and Harland, W. B., Tertiary or Cretaceous age for Spitsbergen's fold-thrust belt on the Barents Shelf, Tectonics, 14, 1321-1326, 1995.

Major, H & Nagy, J., 1972, Geology of the Adventdalen map area; Norsk Polarinstitutt Skrifter # 138, 1-58.

Ringset, N. & Andresen, A., 1988, The Gipshuken fault system - evidence of Tertiary thrusting along the Billefjorden Fault Zone; Norsk Polarinstitutt Rapportserie # 46, 67-70.


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