years. The approximately 3-billion-year-old crystals of the mineral zircon were found in a metamorphosed bedrock outcrop of originally sedimentary rock in northern Ontario, Canada, where the lower crust has been pushed to the surface from depths of 30 kilometers. Like trees in old growth forests, the crystals were found to have roughly circular growth zones that Moser et al. were able to date and analyze with specialized ion probes. From core to rim, the zones track the formation of the early North American continent from its beginning as a series of volcanic island chains, to its eventual fusion into a large, thick continental plate. While crystals of this age were previously known, the longevity of the crystal growth that has been detected is truly remarkable and holds the promise that other crystal micro-records tracking planet evolution exist on Earth and rocky planets including Mars.
Tectonic burial and young (<10 Ma) exhumation in the southern Apennines fold-and-thrust belt (Italy)
S. Mazzoli et al., Dipartimento di Scienze della Terra, Universit di Napoli 'Federico II', Largo San Marcellino 10, 80138 Napoli (NA), Italy. Pages 243-246.
Mazzoli et al. examine the recent tectonic evolution of the southern Apennines, a key Italian area in terms of geological hazards, as well as for petroleum exploration and production. Integrated structural and thermochronological data indicate that, over the past one to five million years, the mountain belt underwent gravitational collapse. This process involved the shallow part (upper 5 kilometers) of the orogen, producing rock exhumation in the axial zone of the chain. Large horizontal extension dismembering the mountain belt was accompanied by forward (eastward) motion of thrust sheets. Linked low-angle extensional faults and frontal thrusts shared a common, shallow detachment. Below the detachment, coeval crustal shortening, though limited (less than 15 percent), led to the gPage: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Related biology news :1
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