Southern Domain Research Articles

Structure profonde du plateau de Madagascar: Relations avec le plateau de Crozet

A study of the crust beneath the Madagascar Ridge and the submarine Crozet Plateau has established clear similarities between these two structures, located in symmetrical positions with respect to the Southwest Indian Ridge. Seismic refraction interpretation confirms the division of the Madagascar Ridge in a northern and a southern domain along latitude 31°S. No argument in favour of a continental nature of the northern domain can be put forward: this domain would rather correspond to a strongly anomalous oceanic crust. The deep structure of the southern domain ressembles that of the submarine Crozet Plateau; seismic velocities beneath these two structures are close to those encountered in the oceanic crust. The southern domain of the Madagascar Ridge and the submarine Crozet Plateau probably have a common origin and were later separated since the early Paleocene by spreading from the Southwest Indian Ridge. Local isostatic equilibrium of the northern domain of the Madagascar Ridge, achieved by crustal thickening, is confirmed by gravity models the depth to the Moho being close to that deduced from seismic refraction results. Gravity also confirms that the southern domain is unbalanced with respect to the northern one, and to the adjacent oceanic basins.

Etude de l'equilibre isostatique dans le sud-ouest de l'ocean Indien a l'aide des resultats de refraction sismique

The aseismic ridges of the Southwestern Indian Ocean are in isostatic equilibrium with respect to the adjacent deep basins. The structural units can be split into two different groups, according to their mean crustal densities. We demonstrate that the crustal thickness within the two groups is a linear function of the water depth: this relationship expresses the Airy isostatic equilibrium. However, the southern domain of the Madagascar Ridge is balanced neither with the northern domain of the Ridge, nor with any other unit of the area of study. The northern domain has undergone only slight subsidence since the Middle Eocene, while the southern domain would have rapidly subsided since the Early Miocene.

Tectonic contrasts in the Southern Uplands

Synopsis The Southern Uplands Fault and the Ettrick Valley Fault define three tectonic domains exhibiting structural contrasts significant in elucidating the Caledonian tectonic history of the Southern Uplands. The Girvan area constitutes the northern domain, and is bounded to the SE by the Southern Uplands Fault. NW-verging asymmetrical folds and two generations of NW-translating thrusts are the dominant structures. The middle domain comprises the Northern Belt and the Central Belt southwards as far as the Ettrick Valley Fault. This domain is characterized by a series of listric faults, the Main thrust faults, associated with outcrops of Moffat Shales. These Main faults apparently cut NW-facing monoforms ( F 1 ), and appear to be associated with SE-verging F 2 monoforms. The faults display south-eastward translation, and culminations and depressions of plunge which may relate to the emplacement of igneous plutons . The southern domain includes the Southern Belt, and the outcrop of the Hawick Group immediately to the N. F 1 folds prevail in this tract, and consist of NW-facing monoforms alternating with tracts of buckle folds, associated with steep strike faults considered to pre-date the Main thrust faults. The significance of the contrasts rests firstly, in the dominance of listric faults in the middle domain, and secondly, in the confinement of these faults to a geographically restricted zone SE of the Southern Uplands Fault. The F 1 structures are considered to have been formed initially by the buckling of a flysch cover in response to its décollement on an oceanic basement which was subducting north-westwards underneath the Laurentian continental plate. Continuing subduction generated an accretionary prism in D 2 , defined by the successive south-eastward propagation of the listric faults, with complementary obduction of the Girvan foundation and its sedimentary cover. Later structures ( D 3 - D 4 ) are of lesser significance, and reflect a progressive deceleration of crustal shortening after the orogenic climax.

Archean rocks from the eastern Lac Seul region of the English River Gneiss Belt, northwestern Ontario, part 1. Petrology, chemistry, and metamorphism

The eastern Lac Seul region of the English River Gneiss Belt is divided into two domains defined by contrasting petrology and structure. The northern domain is underlain by east-trending, steeply south-dipping, migmatized metasediments, intruded by occasional granite sills, and the southern domain by gneissic tonalite and trondhjemite, with abundant amphibolite inclusions, intruded by granite dykes and diapirs: this domain has a complex structure with gently east-plunging open folds of about 5 km wavelength. Field evidence suggests that metasediments of the northern domain have been deposited on the tonalite trondhjemite basement, which was subsequently mobilized, thereby producing the steeply dipping paragneiss belt of the northern domain.The grade of metamorphism throughout the region lies in the upper amphibolite facies, rising locally to the granulite facies. Within 15 km of the southern margin of the gneiss belt, the metamorphic grade decreases to the greenschist facies.U–Pb dating of zircons indicates that the tonalite gneiss was emplaced at least 3040 m.y. ago, and the granite plutons at 2660 m.y., coeval with migmatization and upper amphibolite facies metamorphism. Late pegmatites were emplaced at 2560 m.y.

Archean rocks from the eastern Lac Seul region of the English River Gneiss Belt, northwestern Ontario, part 2. Geochronology

The region is divided into two domains, which are defined by their contrasting petrology, structure, and aeromagnetic anomaly pattern. The northern domain is underlain by east-trending, steeply south-dipping, migmatized garnet–biotite paragneiss. The southern domain is underlain by gneissic tonalite and trondhjemite, intruded by granite plutons, and with abundant amphibolite inclusions.Twelve U–Pb isotopic age determinations of zircons from two gneisses and four granite bodies from the eastern Lac Seul region of the English River Gneiss Belt indicate the following sequence of events. The tonalite gneiss of the southern domain was emplaced definitely more than 3008 m.y. ago and probably more than 3040 ± 40 m.y. ago. Local anatexis or injection associated with regional metamorphism produced a local pegmatite leucosome in the paragneiss 2681 ± 20 m.y. ago. Late- or post-orogenic granites, which invade both the paragneiss and the tonalite, were emplaced 2660 ± 20 m.y. ago, after the major metamorphism in the region. The youngest rock in the area, a cross-cutting pegmatitic granite, was emplaced 2560 ± 40 m.y. ago.