Intraplate Features Research Articles

On the origin and tectonic significance of the intra-plate events of Grenvillian-type age in South America: A discussion

Abstract The objective of this article is to examine the available evidence of intra-plate tectonic episodes of “Grenvillian-type age”, affecting the South-American continent, assessing their possible causal correlation with the tectonic processes occurring within the orogenic belts active at their margins. For the Amazonian Craton, the active margin is represented by the Rondonian-San Ignacio and Sunsas belts. However, active margins of similar age are not recognized for the Sao Francisco and the Rio de La Plata Cratons, and the intra-plate events over them could be reflections of the Kibaran, Irumide or Namaqua orogenic collisions in Africa. Grenvillian-type age events over the Amazonian Craton can be described in four different aspects: shearing and tectonic reactivation along zones of weakness, cratogenic granitic magmatism, alkaline ring complexes, and pervasive regional heating in some localized regions. The first of them may reflect the compressional stresses at active margins, however the others may have different origins. Within the type-region of the K’Mudku tectono thermal episode, mylonites and pseudotachylites cut across the regional granitoid and metamorphic rocks. These shear belts developed under low-to-moderate temperature conditions, that induced resetting of K–Ar and Rb–Sr mineral ages. In the Sao Francisco Craton, extensional and compressional events of Grenvillian-type age are well registered by the structural features exhibited by the sedimentary rocks of the Espinhaco Supergroup. For example, in Bahia state, an Appalachian-style structure is observed, with large synclines and anticlines extending along hundreds of kilometers. The major difference between the Amazonian and the Congo-Sao Francisco Cratons is related to heat originated from the Earth’s interior. Amazonia exhibits very large areas heated up to 350–400 °C, where the K’Mudku thermo-tectonic episodes were detected. In addition, Amazonia comprises a large amount of cratogenic granitic intrusions, and some alkalic complexes of Mesoproterozoic age, whose origin could be attributed, at least partially, to deeper sources of heat. This is not reported for the Sao Francisco Craton, and also for its African counterpart, the Congo Craton. Moreover, the Grenvillian-type age intra-plate features over South America demonstrate that while many cratonic fragments were colliding to build Rodinia, rifting was already occurring in parts of the Amazonian and the Congo-Sao Francisco Cratons.

Cratons, mobile belts, alkaline rocks and continental lithospheric mantle: the Pan-African testimony

Several late-collision and intraplate features are not entirely integrated in the classical plate tectonic model. The Pan-African orogeny (730–550 Ma) in Saharan Africa provides some insight into the contrasting behaviour of cratons and mobile belts. Simple geophysical considerations and geological observations indicate that rigidity and persistence of cratons are linked to the presence of a thick mechanical boundary layer, the upper brittle part of the continental lithospheric mantle, well attached to an ancient weakly radioactive crust. The surrounding Pan-African mobile belts, characterized by a much thinner mechanical boundary layer and more radioactive crust, were the locus of A-type granitoids, volcanism, tectonic reactivation and basin development during the Phanerozoic. During oceanic closures leading to the assembly of Gondwana, lithosphere behaviour was controlled by its mechanical boundary layer, the crust being much less rigid. We suggest that the 5000 km wide Pan-African domain of Saharan Africa, a collage of juvenile and old reactivated basement terranes, has suffered regional continental lithospheric mantle delamination during the early stages of this orogeny, as has been postulated for the more recent Himalayan orogeny in Tibet. Delamination of the continental lithospheric mantle and juxtaposition of crust against hot asthenosphere can explain many features of the late Pan-African (around 600 Ma): reactivation of old terrains, abundant late-tectonic high-K calc-alkaline granitoids, high temperature-low pressure metamorphism, important displacements along mega-shear zones and mantle-derived post-tectonic granitoids linked to a rapid change in mantle source. Recycling into the asthenosphere of large amounts of continental lithospheric mantle delaminated during the Pan-African, can provide one of the reservoirs needed to explain the isotopic compositions of ocean island basalts. Lastly, the lithospheric control over the location of the alkaline rocks enjoins us to consider the thermal boundary layer (the lower ductile part of the continental lithospheric mantle) as a major mixing source zone for these rocks.

An analysis of the altimetric geoid in various wavebands in the Central Pacific Ocean: constraints on the origin of intraplate features

SEASAT profiles were filtered in three different wavebands and interpolated to elaborate maps of the very short (25–110 km) and short (80–320 km and 280–820 km) wavelengths of the geoid anomalies for the South Pacific. For the very short wavelengths, we mapped the geoid roughness i.e. the variations of amplitude of the anomalies. Two lines following present-day plate motions and separated by 800–1000 km, were identified. The first is formed by the Tuamotu Archipelago, the Pitcairn-Crough alignment and the Easter-Sala y Gomez chain. The second comprises the Cook-Austral Islands, a seamount cluster mapped close to the East Pacific Rise, south of 30°S and the Chile Rise. Alignments with a N150°E direction and corresponding to traces of old thermal activity, were mapped on lithosphere older than 40 m.y. These features are also spaced of 800–1000 km. The map for the 80–320 km waveband shows a pattern of 200 km wavelength undulations. Several trends are superimposed, but the N95°E direction is dominant in the area. We associate these geoid undulations to lithospheric phenomena, such as fracturations and/or small magmatic ridges. The 280–820 km waveband reveals linear anomalies, roughly following the direction of present-day absolute plate motions. The linear anomalies have a 400–500 km wavelength and are nearly continuous between the Pacific and the Nazca plates. The topography is located on the highs, but the anomalies generally precede the active ends of the volcanic chains. This result favours the hot-line hypothesis previously suggested by several authors. We associate these bands to thermal heterogeneities probably due to a convective regime. Since the main topographic features shall be emplaced over the ascending limbs of two neighbouring cells, the 800–1000 km spacing of the topography imposes a minimum size of 400–500 km for the cells. So, the convective cells cannot be limited to a thin layer beneath the lithosphere, but will affect part of the upper asthenosphere. The existence of hot lines aligned parallel to plate motions, can explain the intermittent magmatic activity observed for the volcanic lineaments of the South Pacific. Volcanic activity can be the result of the interaction between the hot lines and lithospheric zones of weakness and/or of instabilities in the convective cells. The regular spacing observed for topography formed before and after 40 m.y. suggests that the size of the cells is rather constant through time.

The geoid roughness: A scanner for isostatic processes in oceanic areas

Maps of bandpass‐filtered geoid height anomalies deduced from Seasat altimetry data – which are referred to as “geoid roughness maps”– are contoured over the Indian Ocean both in the short (30–90 km) bandwith and in an intermediate one (90–230 km). Interpretation of these maps essentially provides a guide as to where further detailed studies should be conducted. The roughness maps can thus be used as a fast and efficient “scanner” for isostatic processes at the scale of an entire ocean. Additionally, roughness maps provide a qualitative appraisal of the emplacement mechanisms of intraplate features: this can for example be appreciated by comparing the interpretations of the maps with the results of admittance studies over such features.

The Geology of Plate Margins

Copies of this attractive chart will be hung in the halls of many earth science departments. Types of plate margins and intraplate features are presented in semicircular array in logical order from maximally divergent to maximally convergent: intraoceanic spreading (fast, moderate, slow); passive continental margins; intracontinental rifts and basins; transtensional, transform, and transpressional systems; convergent oceanic arcs (“migratory,” “detached,” “stationary”); convergent continental margins (“noncontracted,” “contracted”); and continent‐continent collision. Each type is illustrated by a cross‐sectional diagram, examples of the type are named, and its attributes are described briefly in wedged boxes narrowing toward the center of the semicircle: volcanism and plutonism, seismicity, metamorphism, structures, mineral deposits, and sedimentary basins and sediments.

Viscoelastic membrane tectonics

The theory of membrane tectonics proposed by Turcotte and Oxburgh [1973] to explain intraplate tensional features and earthquakes, and developed by Turcotte [1974] for an elastic lithospheric membrane, is extended to the case of a viscoelastic lithosphere. Such an extension may be expected to seriously modify the theory since, for all lithospheric viscosities except the very highest, the time scale for viscous relaxation is comparable to the time scale for changes in plate curvature. It is found that for most viscosities considered, the membrane stresses generated by changes in plate curvature do not exceed ∼10² bars. We conclude that either the effective viscosity of the lithosphere is much higher than indicated by most studies, or membrane tectonic stresses play a secondary role in the generation of intraplate features and earthquakes.