Jacques Charvet

The building of south China : Collision of Yangzi and cathaysia blocks, problems and tentative answers

Abstract This paper aims to give answers to the questions of timing, tectonic style and geodynamic interpretation of south China geologic development. In the middle Jiangnan segment, Yangzi plate and Cathaysia got clearly welded during a Late Proterozoic orogeny of collisional type, marked by HP/LT metamorphism, ophiolite melange obduction, thrusting of greenschist nappes, emplacement of collisional S-type granites. This collision, which built the initial Jiangnan belt, began around 950 ± 40 Ma and was completed at about 770–800 Ma ago. Kinematic study indicates that the Cathaysia plate was underthrust beneath the Yangzi plate. A likely earlier collisional event occurred around 1500 Ma, but is poorly preserved in the studied area. The proposed geodynamic model implies two successive suturings of oceanic domains during the Middle-Late Proterozoic: one about 1500 Ma and one about 950 Ma. A strong remobilization occurred during the Early Paleozoic ‘Caledonian’ orogeny, which induced transpressive ductile deformation. Thin-skinned folding and thrusting took place during the Mesozoic; this intracontinental shortening could be due to collision between the China-Indochina and west Philippines blocks. South China is a composite block, comprising the relics of at least three, maybe four sutures from Jiangnan to the coast.

Nankai Trough and Zenisu Ridge: a deep-sea submersible survey

Abstract Eight submersible dives between 3000 and 4200 m water depth were made off southern Japan in the eastern Nankai subduction zone. Benthic communities associated with chemosynthetic processes were discovered along the 800 m wide active tectonic zone, at the toe of the accretionary prism. A benthic community was also discovered along a zone of active compression, at the foot of Zenisu Ridge, 30 km south of Nankai Trough. Temperature measurements within the sediments below the benthic communities confirm that upward motion of interstitial water occurs there. Studies of water samples indicate advection of methane and light hydrocarbons. Specimens of the benthic community have been shown to have included in their shells carbonate resulting from methane consumption. Thus the benthic communities are related to overpressure-driven fluid advection along tectonic zones with active surface deformation. A 300 m high active scarp at the toe of the accretionary prism is related to relative motion in a 280° direction which is close to the 305° average direction of subduction in this area. The dives establish further that compressive deformation is presently occurring at the foot of Zenisu Ridge. The previous interpretation of the Zenisu Ridge as a zone of recent north-south intraplate shortening, 40 km south of the Nankai Trench, is confirmed. We conclude that tectonic evolution might well lead to future detachment of the Zenisu Ridge and overthrusting of this large piece of oceanic crust over the continental margin. Such a process might be an efficient one to emplace ophiolites over continents.

The eastern and western ends of Nankai Trough: results of box 5 and box 7 Kaiko survey

Abstract Seabeam mapping and detailed geophysical surveying have been conducted over the eastern and western ends of the Nankai Trough. The eastern survey covers the transition between the large Izu-Bonin (Izu-Ogasawara) Ridge collision with Honshu and the Nankai Trough subduction. It includes a northeast trending basement ridge, the Zenisu Ridge, to the southeast of Nankai Trough as well as two large channel systems, one following the trench, the Nankai channel and the other coming from the north, the Tenryu Canyon. The Zenisu Ridge is a zone of recent intra-plate shortening consisting of three distinct segments with an increasing deformation from southwest to northeast. We interpret this gradient of deformation as a way to absorb the kinematic discontinuity between the diffuse shortening prevailing over the Izu-Bonin Ridge and the concentrated shortening along the Nankai Trough. The shortening axis appears to change from north-south along the Zenisu Ridge to northwest-southeast along the Nankai Trough and finally to east-west within Honshu. The western survey covers the junction between the Nankai and Ryukyu Trenches and the Kyushu-Palau Ridge which is subducted without major internal deformation. The ridge appears to act as an indenter upon the margin. The tip of the indenter is presently situated under the upper accretionary prism.

Nankai Trough and the fossil Shikoku Ridge: results of Box 6 Kaiko survey

Abstract Seabeam mapping and detailed geophysical surveying have been conducted over the Nankai Trough where the fossil Shikoku Ridge is subducted below southwest Japan. The geometry of the oceanic lithosphere bending under the margin as well as the three-dimensional structure of the accretionary prism have thus been determined in detail. Three 350° trending, probably transform faults have been identified in the area of the survey. They do not extend further south and appear to be limited to the last phase of spreading within the Shikoku Basin, probably between 15 and 12 Ma; this last phase of spreading would then have been accompanied by a sharp change in spreading direction from east-west to N 350°. The two eastern transform faults limit a zone of reduced Nankai trench fill of turbidites opposite to the Tosa Bae Embayment. This observation suggests that the Tosa Bae Embayment actually results from this reduced supply of trench fill to the imbricate thrusting process. The accretionary prism can be divided into three different tectonic provinces separated by continuous mappable thrusts, the Lower and Upper Main Thrusts. Surface shortening is limited to the lower accretionary prism south of the Upper Main Thrust (UMT) whereas uplift with possible extension characterizes the prism above the UMT. Deformation, due to the relative plate motion, mostly affects the lower accretionary prism south of the UMT.

A three-stage structural evolution of the Quadrilátero Ferriífero: consequences for the Neoproterozoic age and the formation of gold concentrations of the Ouro Preto area, Minas Gerais, Brazil

Abstract The Quadrilatero Ferrifero occurs in the southern part of the Sao Francisco Craton (Brazil), where three major Proterozoic supracrustal units (the “Rio das Velhas”, “Minas” and “Itacolomi” Supergroups) are exposed together with Archaean to Palaeoproterozoic granito-gneissic basement. Microtectonic and kinematic analyses have been undertaken in the area close to Ouro Preto in order to precise the tectonic evolution and to distinguish the tectonic features related to the Transamazonian and Brasiliano orogenies (respectively ∼ 2000 and ∼600 Ma ). Three main tectonic events have been recognized. The earlier event, De, is clearly expressed within basement rocks of the Bacao Complex and surrounding supracrustal units. A stretching lineation (Le lineation), roughly oriented N—S, can be observed. Numerous shear criteria such as drag folds and asymmetric pressure shadows indicate a top-to-the-south sense of movement along the southern border of the Bacao Complex. This event was developed within amphibolite facies conditions, fact confirmed by quartz 〈c〉 axis fabrics. This earlier deformation is related to plutonic activity and reactivation of the Archaean basement expressed by the occurrence of scarce leucocratic granitoid rocks intruded within the Bacao gneisses. It is here attributed to the Transamazonian orogeny in agreement with recent Uue5f8Pb dating ( ∼ 2000 Ma ). The large-scale folds (i.e. Mariana anticline, Dom Bosco syncline) which define the dominant structural geometry of the Quadrilatero Ferrifero were formed during this event. The second tectonic event, Dm, has been defined as a major thrusting towards the WNW related to the Brasiliano cycle. A well-expressed stretching and mineral lineation, oriented E—W to NW—SE (Lm lineation) and associated NW-verging shear criteria characterize this event. Occurrence of muscovite, biotite and more scarcely sillimanite indicate that deformation developed in upper greenschist/lower amphibolite facies. Quartz 〈c〉 axis fabrics confirm the sense of shearing but the large scattering of several fabrics traduces a weak intensity of strain. All supracrustal and gneiss formations were affected by the thrusting which is developed on a folded surface previously formed during the De event. The third event, Dm′, is characterized by hangingwall down movement of the nappe pile in response to relaxation of the Brasiliano compressional Dm forces. This event, developed in greenschist facies, is marked by the occurrence of metre-scale drag-folds, sometimes associated to the development of a new lineation Lm′. Gold mineralization, which occurs along the Main Mineralized Contact (located between the “Minas” and “Rio das Velhas” Supergroups), is found within late veins which comprise large recrystallized quartz, carbonate, sulphide, chlorite and tourmaline. Microscopic analysis and veins geometry allow us to conclude that the veins were emplaced mostly after the ductile Dm deformation. It is proposed that the veins were formed by filling of open cavities created during Dm′, tectonics. This study highlights that the northwest-verging thrusting, which is the main tectonic event of the Quadrilatero Ferrifero, is only relevant of the Brasiliano orogeny. A model of structural evolution which involves plutonic activity at ∼ 2000 Ma within the Archaean Bacao Complex, followed by Neoproterozoic thrust tectonics (Brasiliano) is proposed and discussed. The age of gold concentration, assumed to be achieved during the relaxation that occurred in response to the thrusting event, is re-interpreted and related to the late stage of the Brasiliano tectonics.

Appalachian-style multi-terrane Wilson cycle model for the assembly of South China: COMMENT

Lin et al. (2018) propose a model of Proterozoic-Mesozoic evolution of the South China block (SCB) involving West and East Cathaysia nseparated by the Northwest Fujian fault. They assume a northeastward translation of East Cathaysia after an Early Paleozoic orogeny due to the ncollision of West Cathaysia with a “proposed terrane”. This model ignores most of the available, robust field data.