Continental Collision Processes Research Articles

Charnockites and the genesis of gem minerals

Charnockitic rocks that occur in many Proterozoic high-grade metamorphic terrains correspond to regions of crustal thickening and are often associated with continental collision processes. The charnockites of the Highland Group of Sri Lanka represent this type and show genetic relationships to the central High-land Group. Geochemical investigations indicate that charnockites play a key role in the formation of the gem minerals. It seems probable that the contact metamorphic effects of charnockitic plutonic activity in the highly aluminous metasediments such as those found in Sri Lanka result in the formation of an assemblage of gem minerals such as sapphirine, cordierite, spinel, ruby and corundum. The pegmatites with which gem minerals like beryl, chrysoberyl and tourmaline are associated can also be considered as being derived from a charnockitic parent.

Proterozoic links between Northeastern Brazil and West Africa: A plate tectonic model based on gravity data

Using the Bullard predrift reconstruction for the southern Atlantic, analysis of the major gravimetric trends of the West African craton (2000 Ma) and the surrounding mobile belts allows us to delineate the upper Proterozoic shield boundary and to put forward an evolutionary model for late Precambrian time. Over the cratonic area gravimetric anomalies are related to a basement framework or to sedimentary basins whose geometry is linked to reactivation of ancient faults. Gravity anomalies and LANDSAT imagery show a NE-SW lineament in West Africa which can be correlated with a variety of geologic features, the largest being the late Precambrian Gourma aulacogen. At the margins of the craton the deformation patterns and gravity signatures associated with the 600 Ma orogeny were largely controlled by the geometry of the stable craton itself. On the eastern edge (Pan-African belt and Brazilian belt east of the São Luis and Amazonian cratons), the distribution of the anomalies and deduced structures can be explained by a continental collision process. On the western and southern edge (Brazilian belt between the São Luis and Amazonian cratons, Rokelide belt, Mauritanide belt) the gravity signature is different, suggesting a large amount of intensely tectonized basic rocks. A limited spreading process followed by an intense compressional episode is suggested.

“Exposed cross-sections through the continental crust: implications for crustal structure, petrology and evolution”, by D.M. Fountain and M.H. Salisbury—a discussion

Fountain and Salisbury [1] examine geological and geophysical evidence for the nature of the deep continental crust, and emphasise the significance of deep crustal profiles uplifted and exposed along major overthrusts such as the Ivrea and Strona zones (North Italy), Pikwitonei belt (Manitoba), Kasila Group (West Africa) and Fraser and Musgrave Ranges (Australia). On the other hand these authors state that Archaean terrains not usually expose a cross-section through the crust and therefore do not provide data concerning changes in the crust with depth [1, p. 263]. To explain the overthrusts the authors refer to possible obduction, subduction and continental collision processes. The authors view Precambrian continental evolution in terms of repeated crustal recycling and reworking. The purpose of this comment is to point out evidence for (1) the exposure of vertical crustal sections in Archaean cratons; (2) the stability and little resetting of large sectors of the crust since the Archaean, and (3) for limits on the applicability of Mesozoic /Cainozoictype plate tectonics to Precambrian situations. Seismic refraction studies in the Yilgarn [2] and Pilbara [3] blocks in Western Australia indicate regional dips of the intracrustal (Conrad type) and Moho discontinuities. In the Yilgarn and eastward slope is accompanied by a general decrease in metamorphic grade in this direction [4]. In the southern Indian Shield an overall northward

Combined thrusting and wrenching in the Ibero-Armorican arc: A corner effect during continental collision

This paper is an attempt to reconcile strain pattern and deformation history, in an overall arcuate structure, with continental collision processes. Geological data are first presented for the early history (400 Ma) of the Variscan orogenic belt in France. Principal stretch trajectories are, on a large scale, more or less parallel with the trend of the belt. They became strongly oblique to this trend near the basal surfaces of the nappes. A kinematic model combining two orthogonal shear components is proposed to explain such a strain pattern. It is concluded that the observed strain trajectories result from a combination of a southward thrusting and a sinistral wrenching. This conclusion is in good agreement with previous conclusions gained by several local microtectonic studies. The geodynamic significance of combined thrusting and wrenching at the scale of the Ibero-Armorican arc is discussed, and a progressive deformation model is proposed.

Geophysical and geodetic observations in the messina straits

The present note is intended as a contribution to the clarification of the collision process, generally assumed to be active between the African and the Eurasian plates in the area of the Messina Straits. For this purpose detailed analyses are made of the surveys carried out in the Straits of Messina in order to investigate the 1908 earthquake. The magnitude and intensity are established; limits are set for the linear dimension of the fault and of the dislocation that gave rise to the earthquake and an estimate is then given of these parameters. The results are then set out of the investigation into the focal mechanism of the earthquake of 1908. The results are also studied of geodetic surveys carried out at the time for the purpose of measuring the variations in height that occurred during the earthquake. A study is also made of the results of geodetic surveys (some of which are still under way) designed to measure the horizontal movements of Sicily with respect to Calabria in order to study locally the continental collision process in the area of the Messina Straits. It is found that the old and recent geodetic measurements as well as the fault-plane solution of the old and recent earthquakes of that area and the slips associated with those earthquakes indicate a motion towards the north of Sicily with respect to Calabria.

Charnockite genesis and the Proterozoic crust

Charnockitic rocks that occur in many Proterozoic high-grade metamorphic terranes have crystallized (or recrystallized) under high-temperature and low water pressure conditions: they usually contain two pyroxenes and mesoperthite as well as orthoclase, plagioclase, quartz and traces of hornblende and biotite. Estimates of physical conditions based on subsolidus reactions independant of water pressure show that crystallization occurred in the range 800 to 900°C at pressures varying from 7 kbar in the Grenville province to 10 kbar in Africa and in Norway. These physical conditions correspond to a depth of charnockite genesis of about 25–35 km and a gradient of 25 to 30°C per km. Charnockite terranes might therefore correspond to regions of crustal thickening as do those which are associated with continental collision processes. If such is the case, their actual distribution might help in understanding Proterozoic plate motion.