The crustal evolution of the northern Variscan foreland is characterized by a complex history of colliding continents and terranes as well as opening and closure of oceans. A wide variety of facies developments and several tectonic phases are recorded in the deep boreholes of northern Germany and neighbouring countries. Analyses of cored well sections together with results of deep seismic and magnetotelluric surveys allow the distinction of several crustal domains that underlie the deformed, thrusted and folded deep-sea sedimentary rocks of the Rhenohercynian realm and their undeformed, but faulted Devonian and Carboniferous successions of dominantly shallow marine origin in the northern foreland. The northernmost part of these foreland deposits rests on basement rocks of southern Baltica representing different crustal domains, e.g. the Svecofennian and Danopolonian in the east and the Sveconorwegian in the west. These granites and gneisses and their Cambro-Silurian cover deposited on a passive margin were overthrust by Caledonian deformed Ordovician to Silurian strata and Cadomian rocks that belong to the Tornquist Sea and the microcontinent Avalonia, respectively. This continental fragment was separated from Gondwana during early Ordovician times and collided with Baltica and Laurentia in the late Ordovician and early Silurian. This Caledonian collision resulted in the formation of the Old Red continent named Laurussia (Euramerica) which was divided by the Rheic Ocean from Gondwana. The beginning oceanic closure in the Devonian was accompanied by the accretion of several smaller terranes, e.g. Armorica and Saxothuringia, all representing a Cadomian domain along the northern margin of Gondwana. The collision of Laurussia and Gondwana lasted until the early Carboniferous (Mississippian) and led to the formation of the Variscan orogenic belt. This is documented by nappes of Devonian and Lower Carboniferous olistostromes as well as turbidites that were thrust onto the Rhenohercynian rocks lying just north of the Mid-German Crystalline Zone. Further to the north, the distal flysch facies (Kulm) intercalated with coal-bearing limestone facies (Kohlenkalk), the latter deposited in the northern part of the Variscan foreland, especially in the Rügen Basin or Strelasund Depression. This local basin, which already subsided since middle Devonian times, was subsequently filled with erosional material of the rising Variscan Orogen during the late Carboniferous (Pennsylvanian).

Terra Incognita—The Hidden Basement of Northern Central Europe Revealed by Deep Boreholes

章节
发表日期：

2025年

查看全文

Karsten Obst

摘要

The crustal evolution of the northern Variscan foreland is characterized by a complex history of colliding continents and terranes as well as opening and closure of oceans. A wide variety of facies developments and several tectonic phases are recorded in the deep boreholes of northern Germany and neighbouring countries. Analyses of cored well sections together with results of deep seismic and magnetotelluric surveys allow the distinction of several crustal domains that underlie the deformed, thrusted and folded deep-sea sedimentary rocks of the Rhenohercynian realm and their undeformed, but faulted Devonian and Carboniferous successions of dominantly shallow marine origin in the northern foreland. The northernmost part of these foreland deposits rests on basement rocks of southern Baltica representing different crustal domains, e.g. the Svecofennian and Danopolonian in the east and the Sveconorwegian in the west. These granites and gneisses and their Cambro-Silurian cover deposited on a passive margin were overthrust by Caledonian deformed Ordovician to Silurian strata and Cadomian rocks that belong to the Tornquist Sea and the microcontinent Avalonia, respectively. This continental fragment was separated from Gondwana during early Ordovician times and collided with Baltica and Laurentia in the late Ordovician and early Silurian. This Caledonian collision resulted in the formation of the Old Red continent named Laurussia (Euramerica) which was divided by the Rheic Ocean from Gondwana. The beginning oceanic closure in the Devonian was accompanied by the accretion of several smaller terranes, e.g. Armorica and Saxothuringia, all representing a Cadomian domain along the northern margin of Gondwana. The collision of Laurussia and Gondwana lasted until the early Carboniferous (Mississippian) and led to the formation of the Variscan orogenic belt. This is documented by nappes of Devonian and Lower Carboniferous olistostromes as well as turbidites that were thrust onto the Rhenohercynian rocks lying just north of the Mid-German Crystalline Zone. Further to the north, the distal flysch facies (Kulm) intercalated with coal-bearing limestone facies (Kohlenkalk), the latter deposited in the northern part of the Variscan foreland, especially in the Rügen Basin or Strelasund Depression. This local basin, which already subsided since middle Devonian times, was subsequently filled with erosional material of the rising Variscan Orogen during the late Carboniferous (Pennsylvanian).