The Iberian peninsula began as a terrane off the north coast of Gondwana some 650 to 550 million years ago during the Late Neoproterozoic era. The Central Iberian island arc undergirding Estremadura merged with the Ossa-Morena and South Portuguese arcs to form what is today western Spain and Portugal.
In the early Paleozoic, sometime from 542 to 488 million years ago, sediments were deposited that hardened into gneisses, schists, quartzites and slates. Volcanoes elsewhere on the planet warmed the Ordovician climate that followed. Sea life left calcium compounds that became limestones.
During the Silurian era, the united group broke away from Gondwana and joined the Amorica terrane. Over the course of millennia Amorica alternated between the northern landmass of Laurussia and the southern Gondwana until those two merged to form Pangaea in the Early Permian period, about 270 million years ago.
The collision that formed Pangaea initiated a period of mountain building. In the Estremadura, molten granite rose through limestone walls that precipitated silver, gold and tin. As they cooled, the minerals were deposited in existing veins of quartz.
About 200 million years ago, in the early Jurassic, the Pangaean supercontinent began breaking up. The Bay of Biscay opened about 126 millions years ago in the Early Cretaceous. The Iberian nucleus rotated counterclockwise to Europe and came back under the influence of Africa during the Cenozoic.
The Alps formed during the Paleocene. Oaks began to differentiate themselves within the beech family about 60 million years ago in that same epoch. They eventually became the characteristic flora of the Estremaduran region.
Tectonic movement stopped in the mid-Eocene and the Pyrenees arose. In the Late Oligocene the peninsula resumed moving with Eurasia. The Tagus basin formed from sediment eroded from the mountains. Tin, silver and gold washed downstream. Limestone now overlays ochre-colored clay and silt.
Today, the two continental plates abut each other under the Mediterranean. The Eurasian continues to pull east away from its boundary with the North American plate under the Atlantic. The African plate is moving northeast about an inch every three years. Less than nine miles separates the surfaces of the two at the Straits of Gibralter.
Judging from current seismic activity, João Duarte Fonseca believes a "continental block formed by Iberia and northern Morocco is being pushed west wards by the convergence."
Notes: João F.B. Duarte Fonseca, Seismicity and Regional Tectonics of the Estremadura, Southwestern Portugal, dissertation abstract, 1989.
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