A thin orogenic wedge upon thick foreland lithosphere and the missing foreland basin

Abstract

Along the Caledonian front in central Scandinavia, the expected peripheral or pro-foreland basin is neither physically present nor are there any significant traces in the sedimentary record. In order to explain and quantify this situation, the authors assess the major geometric and mechanical constraints on the Caledonian orogenic wedge and model the orogenic load and its influence on the foreland lithosphere of Baltica. Geologic and geophysical data show a strong foreland lithosphere with a flexural parameter (α) of approximately 100 km. The shape of the orogenic wedge and its critical taper angle are dependent mainly on basal friction and wedge strength. In the external part organicrich black shales provide a low-friction horizon both at the basal detachment surface and within the wedge itself. The more internal part of the wedge is composed of metamorphic and crystalline rocks, which cooled and strengthened prior to thrusting. As a result, the external part of the wedge had a lower strength and a smaller critical taper angle than its internal part, so the orogenic load is upward concave. Modelling of the effect of such a load on the Baltica lithosphere shows a very small depression in front of the load (2 km). The flexural depression produced by the main part of the orogenic load is filled up by the thickening thrust-andfold belt, so that there is little space left for a foreland basin. These results imply that the missing foreland basin in front of the central Scandinavian Caledonides is not due to subsequent erosion, but is a primary feature.

Depositional wedge tops: interaction between low basal friction external orogenic wedges and flexural foreland basins

“…In this way, an orogen may develop with little or no foreland basin. This scenario has been proposed for the Norwegian Caledonides (Garfunkel & Greiling, 1998). (c) A fold and thrust belt becomes inactive and is buried below the sediments of the continuously subsiding foreland basin.…”

“…3b). Such a model has been proposed for the Caledonides of Central Scandinavia (Garfunkel & Greiling, 1998), which developed on the Precambrian Baltic craton.…”

Restoration of the external Scandinavian Caledonides

“…1) to c. 120 km east of the present-day eroded thrust front; this is very similar to the 110 km proposed by Hossack & Cooper (1986). In contrast, Garfunkel & Greiling (1998) estimated that the pre-erosional thrust-front lay c. 80 m east of the eroded thrust front in the Västerbotten area, considerably less than the 120 km inferred by Hossack & Cooper (1986).…”

“…9, sections 2.1–2.8). This value has been used, rather than the c. 80 km proposed by Garfunkel & Greiling (1998) but, as shown later in this section, the actual value chosen makes little difference since the fully restored section length is controlled by the position of the Børgefjell Window-Basement.…”

Scandinavia's North Atlantic passive margin

“…The development of intramontane, detrital Devonian basins [ Osmundsen et al , 1998; Steel et al , 1985; Steel , 1976; Ziegler , 1996] followed the orogenic climax and is linked to the collapse of the mountain belt. The present‐day thrust front of the overriding upper plate constitutes the remainder of the original thrust front which was farther to the east, and has been eroded [ Andréasson , 1994; Garfunkel and Greiling , 1998; Hossack and Cooper , 1986] (Figure 1). The geodynamics causing the mountain chain collapse and the transition to the purely rift‐related mechanisms are still debated [see, e.g., Andersen and Jamtveit , 1990; Fossen , 2000; Hartz and Andresen , 1997; Koyi et al , 1999; Milnes and Koyi , 2000; Milnes et al , 1997; Mosar , 2000; Wilks and Cuthbert , 1994].…”