Plate Reorganization Research Articles

Constraints on Trenchward Arc Migration and Backarc Magmatism in the North Patagonian Andes in the Context of Nazca Plate Rollback

AbstractGeochemical and geochronological data reveal that late Oligocene‐early Miocene time is a break point in the evolution of Andean magmatism. The Patagonian Andes registered the onset of arc volcanism since the late Eocene forming part of the El Maitén Belt, whose development was driven by the subduction of the Farallon/Nazca plates beneath the Andean margin. During the Oligocene, the El Maitén Belt shows a change in the geochemical signature of its magmas from tholeiitic to calc‐alkaline compositions, reflecting a more mature stage in the magmatic arc evolution. Toward the early Miocene, a striking event is registered in Andean volcanic sequences as mafic tholeiitic lava flows of the El Maitén are interbedded with marine deposits, suggesting their development in the context of a fast subsiding regime. Geochemical analyses presented in this paper show that these rocks resemble enriched mid‐ocean ridge basalt‐like and ocean island basalt compositions, isotopically depleted, which strongly contrast with previous arc products. By this time, a global plate reorganization event had caused an increase in convergence rates, accelerated rollback, and a more orthogonal geometry of subduction, triggering widespread magmatism and the development of extensional basins in the overriding plate. Arc‐related volcanism during the early Miocene can be found only in the western slope of the Andes, suggesting the retreat of the volcanic front toward the trench. The proposed model highlights a strong linkage between the geochemical signature of magmatic products and changes in the subduction zone configuration and mantle dynamics during the evolution of the Patagonian Andes (41–44°S).

Cenozoic Strike-Slip Tectonics and Structural Controls of Porphyry Cu-Mo and Epithermal Deposits During Geodynamic Evolution of the Southernmost Lesser Caucasus, Tethyan Metallogenic Belt

Abstract The Zangezur-Ordubad mining district of the southernmost Lesser Caucasus is located in the central segment of the Tethyan metallogenic belt and consists of porphyry Cu-Mo and epithermal Au and base metal systems hosted by the composite Cenozoic Meghri-Ordubad pluton. Ore-hosting structures and magmatic intrusions are predominantly confined to a central N-S–oriented corridor 40 km long and 10 to 12 km wide, located between two regional NNW-oriented right-lateral faults, the Khustup-Giratagh and Salvard-Ordubad faults. The anatomy and kinematics of the main fault network are consistent with dextral strike-slip tectonics controlled by the NNW-oriented Khustup-Giratagh and Salvard-Ordubad faults. Dextral strike-slip tectonics was initiated during the Eocene, concomitantly with final subduction of the Neotethys, and controlled the emplacement of the Agarak, Hanqasar, Aygedzor, and Dastakert porphyry Cu-Mo and Tey-Lichkvaz and Terterasar epithermal Au and base metal deposits. The Eocene structures were repeatedly reactivated during subsequent Neogene evolution in transition to a postsubduction geodynamic setting. Ore-bearing structures at the Oligocene world-class Kadjaran porphyry Cu-Mo deposit were also controlled by dextral strike-slip tectonics, as well as porphyry mineralization and its epithermal overprint hosted by an early Miocene intrusion at Lichk. Eocene to early Miocene dextral strike-slip tectonics took place during NE- to NNE-oriented compression related to Paleogene Eurasia-Arabia convergence and subsequent Neogene postcollision evolution. Paleostress reconstruction indicates major reorganization of tectonic plate kinematics since the early Miocene, resulting in N-S– to NW-oriented compression. Early Miocene epithermal overprint at the Kadjaran porphyry deposit and left-lateral reactivation of faults and mineralized structures are linked to this late Neogene tectonic plate reorganization.

Geochemistry and geochronology of mafic rocks from the Jinghe ophiolitic mélange, northwest China: Implications for plume-related magmatism and accretionary processes within the North Tianshan Ocean

We present geological, whole-rock geochemical and Sr–Nd isotopic, and zircon U–Pb geochronological data for mafic rocks found as angular fragments in the Jinghe ophiolitic mélange of the Northern Tianshan orogenic belt, northwest China. The investigated exotic blocks comprise layered and massive gabbros and pillow basalts that are embedded in a serpentinite-rich matrix. The 380.5 ± 4.9 Ma layered gabbros have chondrite-normalized rare earth element (REE) patterns characterized by significant depletions in the light rare earth elements (LREE) and positive Eu anomalies, suggesting that the petrogenesis of these rocks was controlled by cumulus processes. The 381.5 ± 3.7 Ma massive gabbros have near-flat normal-type mid-ocean ridge basalt (MORB)-normalized multi-element patterns with moderate to strong negative Nb–Ti anomalies. These rocks have initial 87Sr/86Sr ratios (87Sr/86Sr(i)) of 0.703825–0.704758 and high εNd(t) values (+6.7 to +9.0). Two compositional groups of basalts can be recognized: ocean island basalt (OIB)- and enriched MORB-type rocks. Both groups display variable enrichments in large-ion lithophile elements (LILE) in N-MORB-normalized trace element diagrams. Semi-quantitative modelling demonstrates that the parental magmas of the massive gabbros were derived by 5%–10% partial melting of a spinel-bearing lherzolite source influenced by minor inputs of subducted crustal components. The parental magmas of the subalkaline and alkaline basalts were derived by 10%–20% and 1%–5% polybaric melting, respectively, of a lherzolite source containing spinel and garnet in equal amounts. The petrogenesis of the Jinghe basalts can be linked to continental rifting followed by seafloor spreading caused by upwelling of a mantle plume. The petrogenesis of the gabbros is linked to plate re-organization that provided the geodynamic impetus for southwestward subduction in the North Tianshan Ocean, and the onset of incorporation of oceanic materials with different origins into a growing accretionary prism during the Late Devonian. Accretion resulted in chaotic mixing of dismembered pieces of variably deformed oceanic rocks, which are now preserved as an extensive block-in-matrix ophiolitic mélange.

Geochemistry and geochronology of the shallow-level La Esperanza magmatic system (Permian-Triassic), Northern Patagonia

The La Esperanza plutonic-volcanic complex is the largest Late Paleozoic-Early Triassic composite magmatic system of northern Patagonia. This paper reports new SHRIMP U-Pb zircon ages and K-Ar muscovite dating as well as whole-rock geochemical data for selected units. In addition, we present some new and reprocessed whole-rock Sr isotopic compositions. On the basis of the new and published data, three compositionally and isotopically distinct high-K magnesian calc-alkaline series were distinguished. Two of these are characterized by high Ba-Sr: (i) biotite and muscovite bearing rhyolites and granites (265 ± 2 Ma; 260 ± 2 Ma) and (ii) metaluminous amphibole-biotite bearing granodiorites (273 ± 2 Ma), monzogranites (255 ± 2 Ma), dacites (253 ± 2 Ma), and slightly peraluminous granites (dated herein as 251 ± 2 Ma). There is also a low Ba-Sr series of high-silica metaluminous rocks (granites and acid dike swarms; 250 ± 2 Ma and ≈244 ± 2 Ma). Geochemistry coupled with geochronology revealed a pulsatory multi-sourced open magmatic system with mafic magma replenishment and reactivation of crystal mushes that occurred before upward migration to upper crustal levels. Mafic magmas alternated with crust-derived magmas incrementally assembled in subvolcanic levels over 30 Ma. Zircon crystallization and mica cooling ages in the granite units allowed detection of two magmatic lulls, between 270 and 265 Ma and between 260 and 255 Ma. Both episodes coincide with a period of exhumation in upper crustal levels. The new temporal and geochemical constraints allow correlation of the La Esperanza plutonic-volcanic complex with the Los Menucos Group (258-248 Ma), encompassing a volume of magmatism comparable to a moderately sized large igneous silicic province. These mid-to-late Permian to Middle Triassic rocks record the transition between subduction-related magmatism (>273 Ma) and post-orogenic extensional magmatism (<250 Ma) in the Gondwana margin. Even though this magmatism would be coeval with the proposed collision of the Patagonia terrane, no expected syn-collisional magmatism or associated deformation were found in upper crustal levels. However, the different nature and melting conditions of the inferred sources of the magmas that crystallized before 270 Ma, between 265 and 260 Ma, and from 255 to 245 Ma, suggest that the La Esperanza plutonic-volcanic complex was assembled during a 30 Ma period of major plate reorganization.

Izanagi-Pacific ridge subduction revealed by a 56 to 46 Ma magmatic gap along the northeast Asian margin

Abstract Recent studies have debated the timing and spatial configuration of a possible intersection between the Pacific-Izanagi spreading ridge and the northeast Asian continental margin during Cretaceous or early Cenozoic times. Here we examine a newly compiled magmatic catalog of ∼900 published Cretaceous to Miocene igneous rock radioisotopic values and ages from the northeast Asian margin for ridge subduction evidence. Our synthesis reveals that a near-synchronous 56–46 Ma magmatic gap occurred across ∼1500 km of the Eurasian continental margin between Japan and Sikhote-Alin, Russian Far East. The magmatic gap separated two distinct phases of igneous activity: (1) an older, Cretaceous to Paleocene pre–56 Ma episode that had relatively lower εNd(t) (−15 to + 2), elevated (87Sr/86Sr)0 (initial ratio, 0.704–0.714), and relatively higher magmatic fluxes (∼1090 km2/m.y.); and (2) a younger, late Eocene to Miocene post–46 Ma phase that had relatively elevated εNd(t) (−2 to + 10), lower (87Sr/86Sr)0 (0.702–0.707), and a lower 390 km2/m.y. magmatic flux. The 56–46 Ma magmatic gap links other geological evidence across northeast Asia to constrain an early Cenozoic, low-angle ridge-trench intersection that had profound consequences for the Eurasian continental margin, and possibly led to the ca. 53–47 Ma Pacific plate reorganization.

Relict subduction initiation along a passive margin in the northwest Indian Ocean

The tectonic evolution of Laxmi basin, presently located along western Indian passive margin, remains debated. Prevailing geodynamic models of Laxmi basin include two mutually competing hypotheses, culminating in either a hyper-stretched continental crust or an oceanic crust overlying an extinct spreading centre. The longstanding conundrum surrounding its precise crustal affinity precludes a complete understanding of the early opening of the Indian Ocean. Here, we present distinct geochemical and geophysical imprints from the igneous crust in Laxmi basin obtained through International Ocean Discovery Program Expedition 355. The geochemical and isotopic signatures of the Laxmi basin crust exhibit uncanny similarities with forearc tectonic settings. Our observations imply a relict subduction initiation event occurred in the Laxmi basin in the Late Cretaceous-Early Cenozoic that marks a significant Cenozoic plate reorganisation record in the northwest Indian Ocean. New findings therefore warrant re-evaluation of the Gondwana breakup to account for the nascent subduction in the northwest Indian Ocean.

Role of strain-dependent weakening memory on the style of mantle convection and plate boundary stability

How plate tectonic surface motions relate to the convecting mantle remains one of the major problems in geosciences. In particular, the cause and consequence of strain localization at plate boundaries remains debated, even though strain memory, i.e. the ability to preserve and reactivate tectonic inheritance over geological time, appears to be a critical feature in plate tectonics. Here, we analyze how a parameterized damage weakening rheology, strain-dependent weakening, affects the time-dependence of plate boundary formation, the transition between mobile and stagnant-lid, and the reorganization of plates in 2-D convection models. The strain-dependent weakening within our models allows for a self-consistent formation and preservation of lithospheric weak zones, which are formed as remnants of subduction zones due to large-scale compressional deformation in the trench region. Such inherited weak zones can be reactivated as intra-plate subduction zones, ridge adjacent subduction, or as spreading centers themselves. Due to the weakening along plate boundaries, the inherited weak zones, and partly the accumulated strain along spreading centers, which weakens the shallow parts of the lithosphere, the longevity of mobile-lid convection increases. Strain-dependent weakening also enhances strain localization along convergent plate boundaries which increases their stability and longevity. As a consequence, tectonic inheritance is an important contribution to understanding the time-dependence of plate reorganization. Strain-dependent weakening results in a shift of the mobile-stagnant lid transition to higher effective yield stresses, if the weak zones fully penetrate the lithosphere and are relatively weakened by at least 20 %.

Reconstruction of Subduction and Back‐Arc Spreading in the NW Pacific and Aleutian Basin: Clues to Causes of Cretaceous and Eocene Plate Reorganizations

AbstractThe Eocene (~50–45 Ma) major absolute plate motion change of the Pacific plate forming the Hawaii‐Emperor bend is thought to result from inception of Pacific plate subduction along one of its modern western trenches. Subduction is suggested to have started either spontaneously, or result from subduction of the Izanagi‐Pacific mid‐ocean ridge, or from subduction polarity reversal after collision of the Olyutorsky arc that was built on the Pacific plate with NE Asia. Here we provide a detailed plate‐kinematic reconstruction of back‐arc basins and accreted terranes in the northwest Pacific region, from Japan to the Bering Sea, since the Late Cretaceous. We present a new tectonic reconstruction of the intraoceanic Olyutorsky and Kronotsky arcs, which formed above two adjacent, oppositely dipping subduction zones at ~85 Ma within the north Pacific region, during another Pacific‐wide plate reorganization. We use our reconstruction to explain the formation of the submarine Shirshov and Bowers Ridges and show that if marine magnetic anomalies reported from the Aleutian Basin represent magnetic polarity reversals, its crust most likely formed in an ~85‐ to 60‐Ma back‐arc basin behind the Olyutorsky arc. The Olyutorsky arc was then separated from the Pacific plate by a spreading ridge, so that the ~55‐ to 50‐Ma subduction polarity reversal that followed upon Olyutorsky‐NE Asia collision initiated subduction of a plate that was not the Pacific. Hence, this polarity reversal may not be a straightforward driver of the Eocene Pacific plate motion change, whose causes remain enigmatic.

The Jan Mayen microcontinent's Cenozoic stratigraphic succession and structural evolution within the NE-Atlantic

Abstract Cenozoic seismic-stratigraphic units of the Jan Mayen microcontinent (JMMC) are presented in context with kinematic modelling of plate reorganisation correlation with conjugate margins, and their chrono-stratigraphic correlations. Our model of development of the microcontinent, from continental breakup to present-day, is based on the interpretation of new and legacy geological and geophysical data. From these data, we have established eleven Cenozoic seismic-stratigraphic units (JM-70 to JM-01) separated by ten locally varying unconformities and disconformities, six of which are regional and reflect discrete tectonostratigraphic phases in the evolution of the Jan Mayen region. These six main phases are: (1) pre-breakup Paleocene (>55 Ma) extension represented by seismic unit JM-70, which comprises terrigenous and marine sedimentary rocks and a thick plateau basalt volcanic succession; (2) Early Eocene (∼55-52 Ma) syn-breakup (rift-to-drift) represented by seismic unit JM-60 and comprises a mix of intrusive igneous rocks, subaerial lavas and hyaloclastites, and non-marine to shallow-marine sedimentary rocks; (3) Early to Mid-Eocene (∼52-43 Ma) syn-breakup (rift-to-drift) and rift-transfer across the Iceland plateau rift, represented by seismic unit JM-50 that comprises volcanic rocks and shelf-margin deltaic sediment succession; (4) Late Eocene – Early Oligocene (43-30 Ma) ridge transfer and tectonic re-arrangement in proximity of the Iceland hotspot represented in the rock record by shallow-to deep-marine deposits preserved as units JM-40, JM-35, and JM-30; (5) in the Late Oligocene (30-22 Ma), a second breakup occurred along the western JMMC igneous margin, together with the formation of the south-western Jan Mayen igneous province, the proto-Kolbeinsey ridge, and the initiation of proto-Iceland all, of which, is represented in the rock record by seismic units JM-20 and JM-15; and, (6) Miocene-to-present-day separation of the JMMC from East Greenland (since 22 Ma), which is represented by seismic units JM-10, JM-05, and JM-01 that reflect a general deepening of the microcontinent.

Regional upper Albian–lower Cenomanian sequence boundaries, Texas Comanche shelf: Is timing related to oceanic tectonics?

The Albian–lower Cenomanian Comanchean Series on the Comanche Shelf, Texas and northern Mexico is a laboratory to calibrate the numerical ages of mixed carbonate-siliciclastic cycles. The stratigraphic contacts of the long-term cycles formed during subaerial exposure followed by marine drowning. Correlation of these cycle contacts in different basins tests their synchroneity and the hypothesis of eustasy. Petrographic and geochemical evidence of Early Cretaceous meteoric diagenesis differs across the shelf because the Comanche Shelf spanned the subtropical arid belt and the mid-latitude warm humid zone. The contact between the Fredericksburg and Washita groups is Al SB Wa1, which is a composite subaerial unconformity and transgressive drowning surface that is traced from southeastern Oklahoma to El Paso and down-dip to the shelf margin.Chronostratigraphy of the Comanchean Series Fredericksburg and Washita groups is constrained by multiple biostratigraphic data of ammonites, caprinid rudists, and foraminifers. The lower upper Albian Al SB Wa1 is dated by the Dipoloceras cristatum Zone below and the Mortoniceras pricei Zone above. Caprinid zones in carbonate facies are integrated with ammonites and foraminifers to provide a complementary correlation criterion. This contact is coeval with a condensed interval in Europe and flooding sequences in western Asia and northern Africa.Depleted carbon isotope ratios indicate subaerial exposure below sequence boundaries at base of Washita Group, at base of the lower Cenomanian Del Rio Formation and base of the Cenomanian Woodbine-Eagle Ford groups. These contacts are coeval with sequences in Europe, Africa, Asia, and South America. Late Albian–early Cenomanian sea-level changes provide a chronologic scale for significant oceanic plate reorganization dated at about 100 Ma.

Seismic geomorphology of Cenozoic slope deposits and deltaic clinoforms in the Great South Basin (GSB) offshore New Zealand

In this study, the Cenozoic sedimentation and infill history of the Great South Basin (GSB), New Zealand, is analysed from a seismic geomorphologic perspective. A suite of sediment types, including mass-transport deposits (MTDs), deltaic clinoforms, contourite-drifts and turbidites, are documented based on high-quality 3-D seismic reflection data and multiple regional 2-D seismic profiles. The MTDs include older, highly compacted and deeper Palaeocene deposits that are markers of late Neogene tectonic reactivation, while the younger MTDs were translated over slopes eroded by drifts. Possible trigger mechanisms for mass wasting may include oversteepened margins, prolonged fluid dissipation and weak geological layers. Sedimentation from the Eocene to Recent was contemporaneous with regional plate reorganisation and syn-orogenic activity. As a result, three distinct Eocene deltaic systems with variably oriented channels and depositional elements provide evidence for changing plate kinematics during the Eocene. The Eocene deltaic systems are river-dominated and were deposited during relative rise in sea level under variable flow regimes. The passage of the Antarctic Circumpolar Current in the GSB from the Late Eocene to Oligocene led to the deposition of three elongate, detached contourite drifts. In a final phase of basin infill, hemipelagic sedimentation and deposition by turbidity currents dominated late Neogene sedimentation in the GSB. The analyses presented here demonstrate the importance of geomorphology in understanding the sediment infill history, their interactions and temporal organisation, which have wider implications for numerous geoscience disciplines.

Punctuated growth of an accretionary prism and the onset of a seismogenic megathrust in the Nankai Trough

Ocean drilling in the Nankai Trough forearc suggests a new scenario for the evolution of the Nankai subduction zone. Continuous subduction since the Late Cretaceous has been a common tectonic scenario, although the plate subduction was transferred from the Pacific Plate to the Philippine Sea Plate during the Miocene. Seismic reflection studies coupled with drilling have demonstrated that two episodes have controlled the recent evolution of the Nankai forearc: a resurgence of subduction at ~ 6 Ma after cessation since ~ 12 Ma and rapid growth of the accretionary prism since ~ 2 Ma because of the influx of large amounts of terrigenous sediments from the Japan Alps in central Japan. Both episodes were synchronous with large-scale plate reorganizations. The westward subduction of the Philippine Sea Plate initiated both in the Ryukyu and the Philippine trenches at ~ 6 Ma. Rifting in the Okinawa and Mariana troughs started at ~ 6 Ma. Compressive tectonics in northeast Japan started at ~ 3–2 Ma, and resultant mountain building with active surface erosion commenced in central Japan at ~ 2 Ma. This recent compressive tectonic phase might be due to the initiation of convergence of the Amurian Plate with the Okhotsk or North American Plate along the eastern margin of the Japan Sea. In addition to this event, the strong collision and indentation of the Izu-Bonin Arc since ~ 2.5 Ma was also enhanced in central Japan.

Late Jurassic intraplate faulting in eastern Australia: A link to subduction in eastern Gondwana and plate tectonic reorganisation

Eastern Gondwana was subjected to subduction processes during the Middle-Late Jurassic, but how these processes affected intraplate deformation in eastern Australia is poorly understood. Here we present 40Ar/39Ar, K-Ar, and Rb-Sr geochronological data from illitic clay-bearing fault gouges associated with the northern part of the 200km long, N-striking, dextral strike-slip, Demon Fault in eastern Australia. We show a major range of geochronological ages at 162.99±0.74–152.1±4.8Ma, indicating that the Demon Fault was active during the Late Jurassic. This period partially coincides with the Middle-Late Jurassic deposition of widespread ash-fall tuffs in the Clarence-Moreton, Surat, and Eromanga basins. We propose that Middle-Late Jurassic intraplate tectonism in eastern Australia was influenced by subduction processes farther east, which produced extensive calc-alkaline magmatism in New Zealand from ~170Ma. A global plate reorganisation event, related to the development of Early-Middle Jurassic sea-floor spreading of the Pacific Plate, possibly acted as the driving mechanism responsible for the intensification of magmatism and intraplate faulting in eastern Gondwana.

Late Cretaceous oceanic plate reorganization and the breakup of Zealandia and Gondwana

New 40Ar/39Ar ages of igneous rocks clarify the nature, timing and rates of movement of the oceanic Pacific, Phoenix, Farallon and Hikurangi plates against Gondwana and Zealandia in the Late Cretaceous. With some qualifications, cessation of spreading at the Osbourn Trough is dated c. 79 Ma, i.e. 30–20 m.y. later than 110–100 Ma Hikurangi Plateau-Gondwana collision. Oceanic crust of pre-84 Ma is confirmed to be present at the eastern end of the Chatham Rise, and a 99–78 Ma intraplate lava province erupted across juxtaposed Zealandia, Hikurangi Plateau and oceanic crust. We propose a new regional tectonic model in which a mechanically jammed Hikurangi Plateau resulted in the dynamic propagation of small, kinematically misaligned short-length 110–84 Ma spreading centres and long-offset fracture zones. It is only from c. 84 Ma that geometrically stable spreading became localized at what is now the Pacific-Antarctic Ridge, as Zealandia started to split from Gondwana.

Late Jurassic to Early Cretaceous age of the Daqiao gold deposit, West Qinling Orogen, China: implications for regional metallogeny

The West Qinling Orogen is endowed with more than 100 sediment-hosted gold deposits with an estimated resource of > 2000 t Au. Previous radiometric dating results have shown that most deposits formed during a Late Triassic to Early Jurassic period of contractional deformation over the orogen. However, here we show that the newly discovered Daqiao gold deposit (> 105 t at 3–4 g/t) in the southern belt of the West Qinling Orogen formed in latest Jurassic to Early Cretaceous under a different tectonic regime. The Daqiao gold deposit is hosted in weakly metamorphosed Triassic turbidites and is spatially associated with hydrothermally altered granodiorite and diorite porphyry dykes. Six granodiorite dykes have similar zircon U–Pb ages ranging from 215.0 ± 1.1 to 211.5 ± 1.5 Ma (1σ), whereas one diorite porphyry dyke has a zircon U–Pb age of 187.5 ± 2.1 Ma (1σ). The age of gold mineralization is constrained by two types of sericite: sericite aggregates coexisting with disseminated auriferous pyrite in relatively high-grade breccia ores and sericite coexisting with auriferous pyrite in weakly mineralized granodiorite dykes. Sericite aggregates from the breccia ores have 40Ar/39Ar plateau ages ranging from 150.7 ± 3.1 to 142.3 ± 2.5 Ma (2σ), whereas grains from the altered granodiorite dykes and low-grade breccia ores have 40Ar/39Ar plateau ages of 130.8 ± 3.1 to 127.2 ± 0.6 Ma (2σ). The 40Ar/39Ar ages thus suggest two periods of gold mineralization in the latest Jurassic and Early Jurassic that are likely related to repeated brecciation at Daqiao. These Jurassic-Cretaceous mineralization ages coincide with discounted ages from several other gold deposits in the region and suggest that there is an underappreciated gold event in the West Qinling Orogen that may not have been associated with the orogenic deformation but is genetically related to the far-field effects of plate reorganization during Paleo-Pacific subduction beneath the eastern Eurasian continent.

Formation and evolution of the Chain-Kairali Escarpment and the Vishnu Fracture Zone in the Western Indian Ocean

Published models for the plate tectonic evolution of the Western Indian Ocean suggest that the Southern Mascarene Basin opened by oceanic crustal accretion between the continental margins of southwestern India and southeastern Madagascar. However, with the cessation of the Mascarene Basin spreading centre followed by a ridge jump resulting in the opening of the Carlsberg Ridge, almost all the traces of India-Madagascar divergence were carved away from the Indian Plate and attached to the African Plate. According to some recent studies, the Chain-Kairali Escarpment, a prominent feature on the southwestern continental margin of India, is possibly the only trace of India-Madagascar divergence that remained on the Indian Plate. But the exact conjugate correspondence of this feature on the Madagascar side is uncertain. Published plate tectonic reconstructions imply that the Vishnu Fracture Zone on the Indian side and the Mauritius Fracture Zone on the Madagascar side are aligned at chron C22ny (∼49.04 Ma). Based on the near collinearity of gravity anomaly trends, the Chain-Kairali Escarpment appears to be the landward extension of the Vishnu FZ. However, at chron C34ny (∼83.0 Ma), the Chain-Kairali Escarpment was in close proximity to the incipient Mahanoro Fracture Zone. In this study we investigate this incompatibility, using an up-to-date compilation of the tectonic elements from the conjugate regions of India and Madagascar and the latest available rotation parameters that describe India-Madagascar separation through a direct India-Seychelles-Madagascar plate circuit. Our revised plate reconstruction model suggests that the Chain-Kairali Escarpment was formed due to the strike-slip motion between the southeast coast of Madagascar and the then southwest coast of India during the initial stages of India-Madagascar breakup. The migration of the Chain-Kairali Escarpment from the proximity of the Mahanoro FZ and aligning with the Vishnu FZ was the result of several successive events. The first among those events was asymmetric crustal accretion in the Mascarene Basin due to ridge propagation, between chrons C34ny (83.0 Ma) to C33ny (∼73.62 Ma). The Chain-Kairali Escarpment and associated crustal weak zones offshore India appear to have facilitated subsequent initiation of the Mauritius FZ and its conjugate Vishnu FZ during a plate reorganization at about chron C33ny (∼73.62 Ma). The cessation of spreading in the Mascarene Basin and development of full extent of the Carlsberg Ridge, shortly after chron C27ny (60.92 Ma), resulted in the initiation of a long transform fault, coinciding with the Vishnu FZ, which connected the Carlsberg Ridge with the spreading centre of the Madagascar Basin. Therefore, the Vishnu FZ and the Chain-Kairali Escarpment are two independent features created during different episodes of evolution of the Western Indian Ocean and the Chain-Kairali Escarpment is not a landward extension of the Vishnu FZ.

Examining the influence of tectonic inheritance on the evolution of the North Atlantic using a palinspastic deformable plate reconstruction

Abstract To accurately reconstruct plate configurations, there is a need for a quantitative method to calculate the amount and timing of crustal extension independent of any one model for the formation of rifted margins. This paper evaluates the suitability of the various plate modelling methods for structural inheritance studies and proposes a classification scheme for the methods that are currently in use. A palinspastic deformable margin plate kinematic model is most suitable for tectonic inheritance studies, particularly at hyperextended margins. This type of plate model provides a valuable analytical tool that can be used to show the temporal and spatial relationship between pre-existing orogenic structures, evolving rift axes and global plate reorganization events. We use a palinspastic deformable margin plate model for the southern North Atlantic and Labrador Sea to quantitatively restore up to 350 km of Mesozoic–Cenozoic extension. This provides us with a pre-rift restoration of the Proterozoic and Paleozoic terranes and structural lineaments on the conjugate margins that helps us to analyse their relationship to evolving rift axes and global plate reorganization events through time. Interpretation of these modelling results has led to a clearer understanding of the relationship between inherited structural features and their control on rifting and the break-up history.

Structures within the oceanic crust of the central South China Sea basin and their implications for oceanic accretionary processes

Internal structures in mature oceanic crust can elucidate understanding of the processes and mechanism of crustal accretion. In this study, we present two multi-channel seismic (MCS) transects across the northern flank of the South China Sea basin to reveal the internal structures related to Cenozoic tectono-magmatic processes during seafloor spreading. Bright reflectors within the oceanic crust, including the Moho, upper crustal reflectors, and lower crustal reflectors, are clearly imaged in these two transects. The Moho reflection displays varied character in continuity, shape and amplitude from the continental slope area to the abyssal basin, and becomes absent in the central part of the basin where abundant seamounts and seamount chains formed after the cessation of seafloor spreading. Dipping reflectors are distinct in most parts of the MCS data but generally confined to the lower crust above the Moho reflection. These lower crustal reflectors merge downward into the Moho without offsetting it, probably arising from shear zones between the crust and mantle characterized by interstitial melt, although we cannot exclude other possibilities such as brittle faulting or magmatic layering in the local area. A notable feature of these lower crustal reflector events is their opposite inclinations. We suggest the two groups of conjugate lower crustal reflector events observed between magnetic anomalies C11 and C8 were associated with two unusual accretionary processes arising from plate reorganizations with southward ridge jumps.

New age constraints on the palaeoenvironmental evolution of the late Paleozoic back-arc basin along the western Gondwana margin of southern Peru

The tectonic evolution of the western Gondwana margin during Pangaea amalgation is recorded in variations in the Permo-Carboniferous back-arc basin sedimentation of Peru. This study provides the first radiometric age constraints on the volcanic and sedimentary sequences of south-central eastern Peru up to the western-most tip of Bolivia, and now permits the correlation of lateral facies variations to the late Paleozoic pre-Andean orogenic cycle. The two phases of Gondwanide magmatism and metamorphism at c. 315 Ma and c. 260 Ma are reflected in two major changes in this sedimentary environment.Our detrital U-Pb zircon ages demonstrate that the timing of Ambo Formation deposition corroborates the Late Mississipian age estimates. The transition from the Ambo to the Tarma Formation around the Middle Pennsylvanian Early Gondwanide Orogeny (c. 315 Ma) represents a relative deepening of the basin. Throughout the shallow marine deposits of the Tarma Formation evidence for contemporaneous volcanism becomes gradually more pronounced and culminates around 312 - 309 Ma. Continuous basin subsidence resulted in a buildup of platform carbonates of the Copacabana Formation. Our data highlights the presence of a previously unrecognized phase of deposition of mainly fluvial sandstones and localized volcanism (281–270 Ma), which we named ´Oqoruro Formation'. This sedimentary succession was previously miss-assigned to the so-called Mitu Group, which has recently been dated to start deposition in the Middle Triassic (∼245–240 Ma). The emersion of this marine basin coincides with the onset of a major plutonic pulse related to the Late Gondwanide Orogeny (c. 260). Exhumation lead to the consequent retreat of the epeiric sea to the present-day sub-Andean region, and the coeval accumulation of the fluvial Oqoruro Formation in south eastern Peru. These late Paleozoic palaeoenvironmental changes in the back-arc basins along the western Gondwana margin of southern reflect changes in tectonic plate reorganization in a long-lived Paleozoic accretionary orogeny.

Rift-associated ultramafic lamprophyre (damtjernite) from the middle part of the Lower Cretaceous (125 Ma) succession of Kutch, northwestern India: Tectonomagmatic implications

Mineralogical, geochemical and isotopic (Sr and Nd) studies on the recently reported ca. 124 Ma ‘anorogenic lamproite’ dyke from the Palanpur area, Kutch seismogenic rift zone, northwestern India, are presented. We propose a new classification for the dyke as a damtjernite (ultramafic lamprophyre; UML) based on its porphyritic-panidiomorphic texture, abundance of phlogopite, presence of nepheline in the groundmass, and the composition of liquidus phases such as olivine, phlogopite, magnetite, and clinopyroxene (diopside). The Palanpur UML is primitive (Mg# = 74–77), silica-undersaturated (SiO2 <40 wt.%), potassic to slightly sodic in nature, and is strikingly similar to the ∼69 Ma UML dykes and sills of the Tethyan Indus suture zone, which are considered as the earliest yet known manifestations of the Deccan Large Igneous Province (LIP). Bulk-rock (87Sr/86Sr)i (0.70460–0.70461) and ɛNd(t) (+2.56 to −0.69) of the Palanpur UML signify derivation from a slightly depleted mantle source similar to that of asthenospheric magmas such as OIB. This is further attested to by the high incompatible trace element ratios (viz., La/Ba, Nb/U, Nb/La and Ta/Yb) that are typical of plume-type magmas. However, the Neoproterozoic TDM depleted mantle Nd model ages (∼655–919 Ma) also necessitate some involvement of a lithospheric mantle component in its genesis. High bulk-rock Fe2O3t and TiO2 contents require the involvement of a fertile peridotitic mantle source, whereas high La/Yb (60–80) implies a control by residual garnet. Higher Rb/Sr and lower Ba/Rb suggest phlogopite as a residual phase and high Nb and lower La/Sm favour carbonatite, rather than silicate melt as metasomatising agent. Low degrees of partial melting of a primitive garnet lherzolite mantle can account for the observed REE patterns in the Palanpur UML. The Palanpur UML shares a temporal similarity to the Kerguelen plume-derived Rajmahal basalts and associated alkaline rocks from the eastern India. The tectonomagmatic significance of its emplacement during the mid-Cretaceous vis-à-vis various models involving the timing of eruption of the Deccan and the Rahmahal Traps and the rifting in the Kutch basin induced by far-field plate reorganization is evaluated.