Following the India-Asia collision, continental blocks were extruded along large sinistral strike-slip faults. The longest such fault, the Ailao Shan-Red River shear zone (ASRR), separated Indochina ...(Sundaland) from South China. The ~ 1000 km-long, active Red River fault (RRF) extends along the north side of the Ailao Shan and currently exhibits a combination of right-lateral slip and normal faulting. Here, after discussing Tertiary and recentdeformation along and around the RRF system (slip-sense inversion, Oligocene/Quaternary offsets, Holocene slip rates, GPS measurements, earthquake mechanisms, etc.), we focus on its Plio-Quaternary extent and kinematics from SE Yunnan of China into NW Vietnam, the western Gulf of Tonkin, and farther south all the way to Sabah. New data is used to corroborate that, past the triple junction with the Dien Bien Phu fault in NW Vietnam, most ofthe presentday right-lateral movement between South China and Sunda blocks continues chiefly southeastwards of the Day Nui Con Voi along the Da River fault, which is roughly parallel to the RRF and was the site of the 2020, ＭW 5.0 Moc Chau earthquake. We further show that this active fault likely extends much farther south along the western edge of the Oligo-Miocene Yingehai/Song Hong basin and the SE coast of Vietnam (Quy Nhon shear zone), at least to the ‘Ile des Cendres’ volcanic alignment, and possibly farther to the western tip of the Sabah-Brunei thrust belt, offshore the active margin of northern Borneo. Finally, we discuss the kinematic consequences of large-scale tectonicinversion across much of the South China Sea, between the Philippines, Taiwan Island, and Sunda.
We present three‐dimensional numerical models of convection within the partially molten mantle beneath the ridge axis. The modeling takes into account the cavity flow driven by plate spreading, the ...diffuse upwelling due to plate accretion, and the shearing movement generated by large‐scale mantle flow. The ridge axis is free to move in the spreading direction to adjust to the maxima of tension at the lithosphere‐mantle interface induced by the convective circulation. The melt distribution in the mantle and the crustal production at the ridge axis are estimated using the formalism of McKenzie and Bickle 1988. During the experiments the record of the ridge axis positions and crustal production is used to compute synthetic maps of the isochrons and oceanic crustal thickness. Close to the ridge, the ascending convective flow consists of 80‐ to 100‐km‐long hot sheets oriented either roughly parallel or orthogonal to spreading. Most ridge segments fit with the top of hot upwelling sheets, while transient transform faults coincide with the top of cold downwelling flows. The crustal maps display lineations subparallel or slightily oblique to spreading, a few tens of million years long, and separated by ∼60–50 km, resulting from the lithospheric record of the excess crust produced at the junction of hot sheets. When a junction of two hot sheets migrates outside the ridge axial plane, the crustal thickness maximum splits into two maxima along axis, and the induced lineation in the crustal map splits into two branches. The merging of lineations occurs when the ridge plane traps the junction of hot sheets. When the large‐scale mantle circulation moves parallel to the ridge crest, it slowly pushs the spreading‐parallel convective sheets. The resulting lineations form V shapes pointing in the same direction as the large‐scale flow. When the large‐scale flow parallels spreading, it slowly pushes the ridge‐parallel hot sheets in the upflow direction. Thus the ridge segments attached to these hot sheets also migrate in the upflow direction. After several tens of million years the cavity flow driven by the spreading closes most of the transform segments and collects most of the ridge segments to form large continuous lines quasi‐orthogonal to spreading. Using the relationships between crustal lineations and convective flow in the models, we interpret the lineations described in the satellite‐derived gravity maps on the flanks of the Atlantic, Indian, and South Pacific plate boundaries analyzed in the companion paper by Briais and Rabinowicz 2002.
We present the analysis of the deformation in the axial valley of two contrasted regions of the very slow spreading Southwest Indian Ridge based on side‐scan sonar images. Our objective is to ...investigate how the obliquity is accommodated along the system. We show that the robust magmatic segments have axial valleys and major faults subperpendicular to spreading. The other sections show fault populations with various degrees of obliquity, often arranged in left‐stepping echelons, accommodating part of the strike‐slip deformation. Side‐scan sonar reveals the presence of a corrugated surface near 59°E interpreted to be an incipient detachment fault. We show that the large width of the SWIR oblique sections, and the difference in tectonic style between the robust volcanic segments and the magma‐starved sections, is accounted for by large variations in the thickness of the brittle lithosphere. We suggest that the emplacement of mantle rocks in the surveyed amagmatic ridge sections can occur by alternating conjugate faults. Serpentinization of outcropping peridotites might also play a significant role in the development of faults in thin crust regions and the distribution of deformation in space and time.
New deep tow sidescan sonar data from the Southwest Indian Ridge reveal complex volcanic/tectonic interrelationships in the axial zone of this ultra-slow spreading ridge. While some constructional ...volcanic features resemble examples documented at the slow-spreading Mid-Atlantic Ridge, such as axial volcanic ridges, hummocky and smooth lava flows, their distribution and dimensions differ markedly. The largest axial volcanic ridges occur at segment centres, but fresh-looking volcanic constructions also occur at segment ends and in the deep basins marking the non-transform discontinuities. The orientations of the dominant fault population and main volcanic ridges are controlled by tectonic processes such as orthogonal extension in the sections of the ridge perpendicular to the spreading direction and transtensional extension in the obliquely spreading sections of the ridge. Minor faults and small volcanic ridges striking parallel to the axis in the oblique part of the ridge are not controlled by these extensional regimes. This observation suggests that the ridge axis acts as a zone of weakness and that magmatic processes, with associated fractures opening in response to magma pressure, may control local emplacements of axial volcanic ridges at obliquely spreading ridges. This non-systematic pattern of ridge characteristics suggests an along-axis variation between focused and distributed magmatic supply, a model which is supported by our interpretation of low-amplitude mantle Bouguer anomalies calculated for the area. We propose that a change of the axial segmentation pattern, from two segments to the present-day three segments, may have introduced additional instability into the crustal accretion process.
The Béarn range, located to the north of the Axial Zone in the Western Pyrenees, is affected by numerous small‐magnitude seismic events. These events overlap an area characterized by specific ...geological structures which are interpreted to have resulted from multistage extensional and compressional deformation. An analysis of surface geology draped over digital elevation model, together with field investigations, allow identification of two main shortening episodes with differing direction of contraction: D1 represents the inversion of the North Pyrenean Basin, whose Mesozoic infill was detached from highly extended crust and transported southward over the necking zone and the northern margin of the Iberian plate; D2 corresponds to the collision stage and is characterized in the study area by backfolding and backthrusting deformation coeval with uplift in the axial part of the chain due to thickening of the Iberian plate. The microseismicity appears to concentrate along the basal part of the inverted basin units (D1) where this initially low angle thrust has been tilted and steepened during collision (D2). We propose that local steepening of this ancient inversion structure, which should not be named “North Pyrenean Fault,” provided the suitable dip for extensional solicitation in association with the present uplift of the Axial Zone, whatever the driving mechanism of this uplift could be.
Key Points
The microseismicity in the Western Pyrenees reactivates steep collision structures
Paleogene continental subduction and collision built opposite‐vergent structures
Most shortening concentrated along the necking zone of the Iberian margin
The current interpretation of the Messinian Salinity Crisis (MSC) involves the deposition of the so-called “peripheral or marginal” evaporites in onshore basins, as well as the erosion of the margins ...and the formation of thick evaporites in the deep parts of the basins. The present study focuses on intermediate depth basins, i.e. located between the onshore outcrops and the deep basins. Indeed, the Balearic Promontory shows small stepped basins filled with MSC deposits between its western extremity on the Alicante shelf and its eastern end around the Menorca block. New and already available seismic reflection profiles and onshore data allow us to investigate the nature and geometrical relationships of these deposits in the area between the islands of Ibiza and Mallorca.
Our observations suggest the existence of three MSC-related units in this area. The lowermost transparent seismic unit can be interpreted as a 100 to 200-m-thick salt layer deposited in the deepest part of the Central Mallorca Depression. These evaporites pass laterally and upward into a thin-bedded seismic unit, which could be lithologically equivalent to the Upper Evaporites of the deep basins. All around the borders of the Central Mallorca Depression, this bedded seismic unit onlaps a Slope Unit which shows bedded to chaotic and rafted facies. The Messinian continental shelf is eroded and incised by a valley extending from the Palma onshore area to the Mallorca downslope domain.
Our results suggest the presence offshore of at least two generations of diachronous MSC deposits. The observation of salt on a continental high and the widespread MSC deposits on the margins of the Central Mallorca Depression call into question the location of halite preferentially in the deep abyssal basins and the significance of the deep basin MSC units that onlap onto the Margin Erosional Surface (MES), as observed all around the passive margins of the Northwest-Mediterranean Basin. The dual terminology used for peripheral/deep MSC evaporites cannot be applied to any of the MSC units found in the study area.
•We study the MSC-related deposits between Ibiza and Mallorca Islands (Spain).•Thin MSC deposits exist on the slope and thick with halite in the central depression.•A MSC valley is discovered from the Palma shelf to the depression through the slope.•These intermediate depth MSC deposits are affected by post MSC tectonics.•We question their inter-relation and connection with deep and peripheral evaporites.
The current interpretation of the Messinian Salinity Crisis (MSC) involves the deposition of peripheral or marginal evaporites in onshore basins as well as the erosion of the margin and the ...deposition of thick evaporites in deep basins. The so-called intermediate basins are formed in domains between the onland outcrops and the deep basins. The Balearic Promontory is a bathymetric high located between the deep Algerian and Liguro-Provençal basins and the onland Spanish basin. The SIMBAD project aims to investigate the spatial variability of the MSC-related deposits and to assess the extent of post-MSC reactivation over the Balearic Promontory. We present here the first results of the SIMBAD high-resolution seismic survey (January 2013) which imaged for the first time a thin MSC-related unit widely distributed in small sub-basins over the Balearic Promontory.
Borehole analyses have shown that this unit could be correlated with primary gypsum formations linked to the peripheral evaporites. Locally, in the Central Depression between Mallorca and Ibiza islands, a thicker MSC unit is observed whose lowermost transparent part could correspond to a salt layer. Geometrical relationships suggest that the MSC in the Central Depression could postdate the primary gypsum. The occurrence of a halite layer in the Central Depression, at depths of 1000 to 1500 m, favours the hypothesis that the evaporites precipitated passively in closed or partially closed perched sub-basins, possibly as a result of evaporative drawdown at different depths and possibly diachronously, at least with respect to the deep-basin evaporites.
•We present high-resolution seismic images of MSC deposits of the Balearic Promontory.•A thin MSC unit is widely spread on the Promontory in several connected sub-basins.•The MSC unit is correlated to the primary gypsum on the Alicante shelf.•Halite is suspected in the Central depression, resembling that of the deep basin.•Deposition of evaporite occurred in variously perched basins on the Promontory.
Using a combined approach of seafloor mapping, MAPR and CTD survey, we report evidence for active hydrothermal venting along the 130°‐140°E section of the poorly‐known South‐East Indian Ridge (SEIR) ...from the Australia‐Antarctic Discordance (AAD) to the George V Fracture Zone (FZ). Along the latter, we report Eh and CH4 anomalies in the water column above a serpentinite massif, which unambiguously testify for ultramafic‐related fluid flow. This is the first time that such circulation is observed on an intermediate‐spreading ridge. The ridge axis itself is characterized by numerous off‐axis volcanoes, suggesting a high magma supply. The water column survey indicates the presence of at least ten distinct hydrothermal plumes along the axis. The CH4:Mn ratios of the plumes vary from 0.37 to 0.65 denoting different underlying processes, from typical basalt‐hosted to ultramafic‐hosted high‐temperature hydrothermal circulation. Our data suggest that the change of mantle temperature along the SEIR not only regulates the magma supply, but also the hydrothermal activity. The distribution of hydrothermal plumes from a ridge segment to another implies secondary controls such as the presence of fractures and faults along the axis or in the axial discontinuities. We conclude from these results that hydrothermal activity along the SEIR is controlled by magmatic processes at the regional scale and by the tectonics at the segment scale, which influences the type of hydrothermal circulation and leads to various chemical compositions. Such variety may impact global biogeochemical cycles, especially in the Southern Ocean where hydrothermal venting might be the only source of nutrients.
Key Points
Intense and contrasted hydrothermal activity has been evidenced along the South‐East Indian Ridge in the Furious Fifties
Ultramafic circulation is evidenced in the George V FZ, which is the first observation of this type along an intermediate‐spreading ridge
Chemical compositions of the plumes reveal various regional and local controls on the hydrothermal circulation
Multiple geological processes affect the distribution of hydrothermal venting along a mid‐ocean ridge. Deciphering the role of a specific process is often frustrated by simultaneous changes in other ...influences. Here we take advantage of the almost constant spreading rate (65–71 mm/yr) along 2500 km of the Southeast Indian Ridge (SEIR) between 77°E and 99°E to examine the spatial density of hydrothermal venting relative to regional and segment‐scale changes in the apparent magmatic budget. We use 227 vertical profiles of light backscatter and (on 41 profiles) oxidation‐reduction potential along 27 first and second‐order ridge segments on and adjacent to the Amsterdam‐St. Paul (ASP) Plateau to map ph, the fraction of casts detecting a plume. At the regional scale, venting on the five segments crossing the magma‐thickened hot spot plateau is almost entirely suppressed (ph = 0.02). Conversely, the combined ph (0.34) from all other segments follows the global trend of ph versus spreading rate. Off the ASP Plateau, multisegment trends in ph track trends in the regional axial depth, high where regional depth increases and low where it decreases. At the individual segment scale, a robust correlation between ph and cross‐axis inflation for first‐order segments shows that different magmatic budgets among first‐order segments are expressed as different levels of hydrothermal spatial density. This correlation is absent among second‐order segments. Eighty‐five percent of the plumes occur in eight clusters totaling ∼350 km. We hypothesize that these clusters are a minimum estimate of the length of axial melt lenses underlying this section of the SEIR.
Key Points
Most extensive hydrothermal plume survey anywhere on the mid‐ocean ridge
Hydrothermal frequency tracks magmatic budgets at regional and segment scales
Plume clusters and MBA lows imply a melt lens beneath 14–33% of the ridge axis
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