A 1430 km, deep crustal, seismic reflection profile of the Parnaíba basin shows an asymmetric, structured western margin and a gently dipping eastern margin. The~3 km thick, Phanerozoic sedimentary section overlies a pronounced, planar, regional unconformity that crosses three Precambrian blocks with differing seismic facies: the Amazonian/Araguaia block, the Parnaíba block, and the Borborema block. The blocks are separated by steep crustal-scale boundaries across which seismic facies change abruptly. In the west, the ophiolitic metasedimentary rocks of the Araguaia Group overlie the Amazonian craton. Both craton and metasediments terminate eastward against a steep, east dipping fault zone defining the Amazonian/Araguaia block and Parnaíba block boundary. Reactivation of this Neoproterozoic margin in the Late Triassic and Late Jurassic/Early Cretaceous, folded and elevated basement and basin over 2 km. A second crustal boundary defines the eastern margin of the Parnaíba block with the Neoproterozoic Borborema block. This boundary is interpreted as the extension of the Transbrasiliano shear zone. These data demonstrate that the basement of the Parnaíba basin was formed during Brasiliano orogenesis by west directed collision-related thrusting, succeeded by lateral accretion along steep, crustal-scale boundaries. After formation of a post-Brasiliano peneplain, the Parnaíba basin developed seamlessly across three very different crustal blocks and appears to have been significantly larger than its present outline. No extensive underlying rift system is evident suggesting that basement structure had little to do with basin formation, but that episodic reactivation of the boundary zones and basement fabric has controlled the structuring and preservation of the basin.
Let S be a monoid. A (unital) right S-set A is called (principally) weakly flat if the functor A ® -preserves embeddings of (principal) left ideals into S, and flat if this functor preserves all embeddings of left S-sets. S is called (weakly) (left, right) absolutely flat if all (left, right) S-sets are (weakly) flat. Our paper [3] contains results which will be used in the sequel, and is also suggested as a general reference on flatness.It is easily seen that coproducts of families of (weakly) flat right S-sets are again (weakly) flat. Taking our lead from ring theory, we shall call S (weakly) left coherent if all direct products of non-empty families of (weakly) flat right S-sets are (weakly) flat. (Weak) right coherence is defined similarly and S is called (weakly) coherent if it is both (weakly) left and right coherent. In [5] S. U. Chase proved that the following conditions (among others) are equivalent, if R is a ring with identity:(1) R r is a flat right R-module, for any non-empty set r.(2) Every finitely generated left ideal of R is finitely presented.(3) (a) For every aER the left annihilator (0: a) = {rER I ra = O} is a finitely generated left ideal, and (b) the intersection of any two finitely generated left ideals of R is finitely generated. (4) The direct product of any family of flat right R-modules is again flat.2Research supported by Natural Sciences and Engineering Research Council of Canada grants A4494 and A9241.
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