Most of the current views on the evolution of passive margins have roots in ideas that were developed before 1930 in the context of continental drift and geosynclinal theory. These ideas include the concept of an Atlantic type of margin formed by rifting and continental drift; the presence of a thick sedimentary deposit beneath the continental shelves; and subsidence in response to such mechanisms as crustal thinning, igneous underplating, thermal contraction, flexure, and sediment loading. As large amounts of new surface and subsurface data were acquired from modern passive margins after World War II, owing to significant advances in technology for geological and geophysical exploration of the ocean basins, these early ideas were strengthened and modified. With the development of plate-tectonic theory, the origin of passive margins as rifted trailing edges of continents became widely accepted, and significant changes in thinking involved the role of passive margins and their implications for large horizontal displacements in the evolution of geosynclines. Within the past decade, a large number of geophysical models have been developed for passive margins that focus once again on the problem of the mechanisms of vertical movements of the Earth’s crust. At the same time, new developments in the acquisition and processing of data from ocean basins, especially deep-reflection data, have resulted in major new concepts about the deep structure of passive margins, including recognition of the importance of underplating and plutonic activity in the thinned rifted crust and the unexpected degree of faulting and formation of horizontal reflectors in the lower continental crust and subcrustal lithosphere.

Compositions of initial, generally basaltic, rocks of geosynclines are virtually identical regardless of their time and place of formation within the past 3,500 m.y. This constancy of composition through time and place is matched by a similarity of the entire stratigraphic column in each of the studied geosynclines of different age and location. These observations lead to the conclusion that the major element content of the upper mantle has been essentially unchanged during the past 3,500 m.y. except by processes related to geosynclinal development and orogeny. The resulting compositional changes are caused largely by gradual segregation of new continental crust out of the mantle during geosynclinal, orogenic, and postorogenic stages. General enlargement of cratons may also explain the inception of major miogeosynclinal sedimentation in Proterozoic time and platform sedimentation in Paleozoic time.

Abstract Nebgene mineralization throughout the Green Tuff region of Japan, closely related to geosynclinal evolution, is classified mainly into two types: the polymetallic, strata-bound Kuroko-type and hydrothermal vein-type mineralization occurring mostly in submarine volcanogenic sediments of Neogene age. In order to examine their genetic relation, especially the possible origins of their ore fluids, we have analyzed δ 34 S and δ 34 O values of sulfates in more than 100 samples of barite, anhydrite, and gypsum in the Neogene ore deposits, along with some δ 34 O values of quartz and adularia associated with these deposits. Most of the isotopic compositions of Kuroko sulfates collected mainly in the Kuroko (black) and keiko (siliceous) ore zones fall within a relatively narrow range: 19.5 to 25.5 per mil for δ 34 S and 6.4 to 10.9 per mil for δ 18 O. These compositions, combined with fluid inclusion data, indicate that the ore fluids were predominantly Miocene seawater. In contrast, the hydrothermal vein sulfates are characterized by more scattered distributions of δ 34 S (ranging mostly from 16 to 29‰) and δ 18 O (mostly 0 to 14‰) values and were deposited from ore fluids of predominantly meteoric origin. The large spread in the isotopic compositions of these sulfates can be attributed to varying degrees of mixing or meteoric waters with other fluids such as modified seawater or connate water. Barite samples from a single Kuroko deposit, the Ainai-Daikoku deposit, display a tendency for the δ 18 O values of sulfates in the keiko zone to be slightly higher than those in the Kuroko zone. No clear trend exists between the δ 34 S values from these two zones.

Abstract This paper details the evolution of allochthonous salt systems in the Adelaide Geosyncline of South Australia, a divergent continental margin that developed after rifting of the Neoproterozoic supercontinent. It differs from previous studies in that it suggests salt-sediment interaction had a great influence on stratigraphic architecture of the late Precambrian to Early Cambrian succession. This stratigraphic architecture reflects the high subsidence and sedimentation rates in halotectonic mini-basins during divergent margin development of the Adelaide Geosyncline. Sedimentation can be tied to specific stages in the evolution of the various allochthonous salt systems. Lacustrine, fluvial, and evaporite facies of the rift phase, assigned to the late Precambrian Callanna and Burra groups, were deposited in salt-canopy mini-basins that were floored by shallow-water sediments. Debris flows indicated a stage in the development of the divergent margin where gravity flow deposits were associated with marine sedimentation. The transition phase heralded a series of marine incursions into mini-basins, climaxing in the extrusion of a very thick salt canopy that served as a basal detachment surface for the breakup unconformity. Overlying marine sediments of the late Precambrian Umberatana Group were deposited adjacent to reactive diapirs during the passive margin phase. As diapirs passed into the active stage, they separated mini-basins having counter-regional geometries. Progradation and aggradation of the passive margin resulted in salt-canopy mini-basins floored by deep-marine sediments of the late Precambrian Lower Wilpena Group that evolved into counter-regional salt systems. Christmas tree diapirs were characteristic of this final counter-regional phase.