Leg 107: seafloor spreading, sinking.
As the JOIDES Resolution criss-crossed the sea early this year from the coast of Sardinia southeast to the toe of Italy, scientists were looking at the evolution of the basin from two perspectives: They were investigating the history of the young passive margin, or continental edge, on the northwest side of the basin; and they were reconstructing the way the basin has opened at sites in the southeast over several million years.
The passive margin (there is no plate boundary at that point) of the Tyrrhenian provided an opportunity to study a structure common to oceans throughout the world. "It turns out that in the classic places [where] people look at passive margins, like off the coast of New Jersey, the passive margins are old.... [T]he earliest part of the history is very deeply buried and hard to get to by drilling," says Kim Kastens, co-chief scientist on the leg and a marine geologist at Lamont-Doherty Geological Observatory in PalisadeS, N.Y. The Tyrrhenian margin, she says, "was younger and didn't have very much sediment, so we could get all the way down, deeper int the history of opening." The scientists recently made public some of their Leg 107 findings.
Drilling at three sites brought up evidence that the margin rifted and sank at different times in different places. On the upper Sardinian margin, the core showed a "textbook example" of a transition from a continental to a marine environment as the crust subsidied. Overlying the marine sediments, the scientists found "evaporites," sediments laid down during the dessication of the Mediterranean about 5 million years ago. At sites on the lower Sardinian margin, the crust apparently didn't subside as early: The depositions of that age indicate a lake or open-air environment.
"What we wanted to demonstrate is that [the history of such a passive margin] can be a complicated story," says staff scientist Christian Auroux.
Drilling at the Tyrrhenian's two deep basins also indicated that the locus of seafloor spreading apparently jumped from the Vavilov Basin, about halfway down the coast of Italy, into the Marsili Basin to th southeast. The proof of that came when scientists dated basalts brought up from Vavilov Basin at about 3.5 million years, while basalts at Marsili Basin weren't formed until about 1.9 million years ago at the earliest.
The apparent movement of the seafloor spreading site gives some indication of the way basins like the Tyrrhenian evolve. Such basins are generally nestled behind volcanic arcs, on the overriding plate of a subduction zone. It has been suggested that such basins have evolved toward the subduction zone; "We were able to verify that" pattern in the Tyrrhenian, Kastens says.
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|Date:||Mar 22, 1986|
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