Terrestrial Palaeoecology and Global Change

140 Valentin A. Krassilov. Terrestrial Palaeoecology
found on the Zenkevich Ridge and Erimo Seamount outside of the Japan – Nansei trenches
(Vassilyev et al., 1978; Vassilyev & Yevlanov, 1982; Lomtev & Patrikeev, 1983).
In the Eastern Pacific, the Middle American Trench was considered as a typical
subduction zone presently consuming oceanic crust of Miocene age. Instead, the deepsea
drilling data of early 1980s shows an extensional graben-like structure filled with the
Upper Cretaceous to Miocene hemipelagic/turbidite/volcanic deposits onlapped by ophiolites
transported from beneath the continent over a series of reverse faults parallel to
the Benioff zone (Aubouin et al., 1984). Recent data, although interpreted as consistent
with subduction – underplating of Cocos plate off the Nicoya Peninsula, actually indicate
a downfalting of the Mesozoic marginal structures in the Miocene (Vanucchi et al.,
2001). These and similar data support interpretation of Benioff planes as shear zones
conducting the deep-rooted overthrusts of subcrustal material, accompanied by the shallower
overthrusts cutting across the trench slopes.
Given an extensional component to the Middle American trench, a spreading shear
zone of the Gulf of California can be considered as its continuation cutting off a continental
fragment on its way to develop as an island arc. This interpretation would explain
tectonic evolution of American margin without resorting to a highly improbable hypothesis
of a mid-ocean spreading ridge overriden by the continent.
A comprehensive arc-trench model would also account for the Benioff zones dipping
away from the continent as in the case of the Manila trench, the current explanation of
which involves subduction of an imaginary buoyant plateau, as well as of a mid-ocean
ridge (Boutista et al., 2001). Seismic studies of Pacific margins indicate a thick astenospheric
wedge (Rodnikov, 1988) supposedly generated by subcrustal shear. During the
development of the Pacific-type arc-trench systems, a friction-melted subcrustal material
was exhumed from beneath the continental margin and thrust over the Benioff zone
and its parallel reverse faults (Fig. 61). Attenuated by subcrustal shear, the marginal
crust subsides as a backarc basin. The thrust complexes are then reshuffled by backthrusting
resulting in the Manila-type trenches.
The Pacific ophiolites have been interpreted as originating in either the mid-ocean
ridge (MOR) or backarc or else subduction zone (SZ) environments (Pearce et al.,
1985; Edelman, 1988). Actually the equivalents of the Pacific-type onland ophiolites
occur in the Tonga, Puerto Rico and Middle American trenches that develop as extensional
shear zones (Chase, 1971; Tucholke & Ludwig, 1982; Aubouin et al., 1984). However,
the alleged distinctions between these “supra-SZ” ophiolites and those of the “supra-MOR”
Bay of California may not be that profound, because both are confined to the
marginal shear zones, cutting off a continental fragment in the latter example.
Mechanisms of ophiolite emplacement can be sought for in the pulsating strike-slip/
thrust faulting (Fig. 61):
(1) The picritic mantle material is squeezed in the leaky fissure, tectonized by shear
pressure and intruded by dyke swarms and gabbros,