The aftermath of the Caledonian continental collision in ...folk.uio.no/torgeir/pdfpapers/Extended.pdf · Cenozoic collision between India and Eurasia and the associated intra-continental

The aftermath of the Caledonian continental collision in the NorthAtlantic Region: A structural template for later events?

Torgeir B. Andersen1), Ebbe Hartz2), Trond H. Torsvik2), Per Terje Osmundsen2),Arild Andresen1), Elizabeth A. Eide2) and Alvar Braathen2)

1) Dept. of Geology, Univ. Oslo, P.O.Box 1047 Blindern, 0316 Oslo, Norway.2) Norwegian Geol. Survey., Leiv Eirikssons vei, 7941 Trondheim

Orogenic belts produced by continental collisions are traditionally viewed as theultimate products of Wilson-cycle tectonics. Modern studies of ancient as well assemi-recent to recent examples have, however, demonstrated the importance of thetectonic activity during the terminal stages of- and immediately after collision. TheCenozoic collision between India and Eurasia and the associated intra-continentaldeformation affecting vast areas in central and SE Asia is the best actualistic example(Fig. 1). The Himalayan collision and the large-scale indentation of India was- andstill is accompanied by regionally distributed deformation involving crustal stackingand thickening in the Himalays, around the Tibetan plateau and in Tien Shan, strike-slip and normal-faulting in the Tibetan Plateau and large scale strike-slip faulting withassociated pull-apart basins affecting vast areas in Central and SE Asia.

While the semi-recent to actualistic examples have precise records of the tectonicprocesses involved and their kinematic boundary conditions, ancient examplesrepresent more evolved systems, which allow more complete examination of theeffects of these processes in crustal sections from the exhumed lower crust to thesurface. The Caledonian collision between Laurentia and Baltica-Avalonia in theSilurian to Early Devonian represents one of the best ancient examples. The plate-tectonic boundary condition of the Caledonian collision is less well constrained,nevertheless, it has been shown that a near orthogonal collision between Baltica andLaurentia occurred when Baltica´s velocity relative to Laurentia was 8 to 10 cm/yr(Torsvik 1998). This is comparable to the velocity of India, which was close to 9

Fig 1. (A) Sketch-map showing theintra-continental deformation of centraland S-E Asia associated with theCenozoic collision of India and Eurasia.Notice that indentation of India isaccommodated by E-ward transport ofthickened and topographically elevatedcrust (B) material above the freesubduction margin of SE-Asia.(Modified from L. Jolivet 2001). A B

cm/yr, during the initial stages of the Himalayan collision. Other importantsimilarities between the two belts are the size of continental Baltica and India, thedouble polarity of the thrust stracking, evidence for extreeme crustal thickening andthe scales of the frontal thrust-belts. The exposed Scandinavian segment of theCaledonides is approximately 1750 km, whereas the Himalayan thrust front on Indiais approximately 2200 km between the eastern and the western syntaxes (Fig. 2)

Fig. 2. The Scandinavian and East Greenland Caledonides in a tight Late-Silurian to EarlyDevonian reconstruction projected onto present day Himalay-Tibetean Plateau topography.

The most important difference appears to be the duration of continental convergence,which in the case of the Caledonides was approximately 30 m.yr. as opposed to theca 55 m.yr. in case of the Himalayas.

Studies of the Caledonides of the North-Atlantic region have revealed that thestructure of the orogen only can be understood in light of tectonic processes whichoutlasted the initial Scandian collision by as much as ca. 100 million years. Moreover,the structure in the exposed parts of the hinterland on both sides of the North-Atlanticand consequently also in the stretched and thinned basement of the shelf areas, aredominated by the late to post-orogenic structures rather than those produced duringthe collision itself. These include large exposed areas characterized by late to post-collisional high-grade ductile fabrics (cf. Western Gneiss Complex of SW Norwayand the allochthonous basement/cover nappes in the Fjord Region of E-Greenland),localized ductile shear fabrics along major detachment- and strike-slip zones andhigh-level brittle faults which in most cases have been reactivated repeatedly (c.f. theMøre Trøndelag Fault Complex). These observations demonstrate the fundamentalnature of the processes related to late and early post-orogenic tectonics.

Depending on the plate-tectonic boundary conditions such processes include: [1]Indentation and regional crustal thickening, [2] Large-scale strike-slip and escapetectonics; [3] Major vertical movements accompanied by exhumation of high- andultra-high-pressure rocks; [4] Formation of penetrative fabrics related to late- andpost-collisional extension and accompanied by formation of sedimentary basins.

Fig. 3. A schematic maximum tight-fit reconstruction for the Scandian collison taking intoconsideration the crustal thickening as documented by high- and ultra-high metamorphiccomplexes in Norway and Greenland. Palaeozoic sinistral strike-slip as well as post-orogenicextension of the shelf areas have been removed. This has particular significance to the areasoutboard of the exposed high- and ultra-high pressure rocks shown with shaded ellipses inGreenland and Norway, where the shelf areas presently are particularly wide (see Fig. 4)

In both the Scandinavian and Greenland Caledonides it has been demonstrated thatmajor re-structuring of the collisional infra-structure related to points [2]; [3] and [4]occurred in the interior of the orogen throughout the Devonian. The main effectswere, exhumation of HP and UHP rocks, crustal thinning on major detachments andnormal faults; basin formation and a large-scale orogen-parallel strike-slip(transtension?) tectonics. The sinistral strike-slip with releasing and restraining bendsapparently affected the entire orogenic belt from the British sector to Svalbard (Fig.3). The Mid to Late Devonian extension and crustal thinning in parts of Greenlandwas dramatic and resulted in a high heat-flow, magmatism and a thermally weakenedcrust (Hartz et al. 2000). Evidence for indentation and regional crustal thickening of ascale similar to that observed north of the Himalayan orogen in central Asia iscurrently not documented. It is likely that indentation of Baltica came to a halt alreadyin the Lower Devonian because the trailing, free margins of Baltica developed intoactive margins soon after Iapetus had closed; hence there was no driving force forcontinued convergence (Fig. 4). The “free” Herzynian and Uralian margins of Baltica-Avalonia entered subduction zones with polarity away from Baltica-Avalonia. It issuggested that the free margins were important elements that permitted and perhapseven drove large scale relative motions across the mountain belt.

BALTICA

LAURENTIA

E-AVALONIA

In conclusion: (1) Late- to post-orogenic processes produce fundamental crustalstructures. (2) Collapse-related structures rather than those produced by collisiondominate the hinterland of orogenic belts and provide the structural template for laterevents. (3) The late-to post-orogenic collapse structures are particularly important as atemplate for renewed extension and rifting that affected the N-Atlantic region. Forthese reasons, it is critical to establish the Late Paleozoic structural framework on theEast Greenland and Mid-Norway conjugate margins, in order to place both kinematicand precise time constraints on subsequent, basin-controlling tectonic activity.

Fig. 4. A) Pre-breakup reconstructionmodified from Mosar et al. (2002),showing the large and highly extendedshelf-areas (magnetic anomaly map)adjacent to Caledonian high-gradeareas onshore. Areas affected by lateCaledonian high- and ultra-highpressures are shown as shaded areas.The red arrows show the principaldirections of extension in the Hangingwalls of extensional detachments inGreenland and Scandinavia.

B) A 400 Ma (Emsian) reconstructionfrom Torsvik & Cox (2001). Noticethat the ”free” Uralian and Herzynianmargins of Baltica are subductionmargins with polarity away from thecontinent. There is apparently noplate-tectonic driving force that couldsustain convergence between Balticaand Laurentia. The Lower to MiddleDevonian is consequently a timeduring which extension and majorstrike-slip movements dominated themountain belt.