Evolution Of Margins Research Articles

Morphostructure, emplacement and duration of the Abrolhos Magmatic Province: A geophysical analysis of the largest post-breakup magmatism of the South-Eastern Brazilian margin

The Abrolhos Magmatic Province (AMP), located in the Ocean Continent Transition (OCT) of the Brazilian South Atlantic rifted margin, is spatially extensive covering an area larger than 63,000 km2 and was emplaced in the Early Cenozoic, approximately 40 Ma after rifted margin formation in Early Cretaceous. The explanation for the origin of the AMP is that it forms part of a hotspot track that initiated at ~85 Ma in the Poxoréu region, an igneous province located ~1500 km to the west of the AMP, within Brazilian continental lithosphere and is presently located under Trindade and Martin Vaz Islands at the Vitoria-Trindade seamount chain (VTC). The aim of this work is to investigate the morphostructure, duration and spatial distribution of the AMP, exploring its significance for the margin evolution and possible origins. The magnetic, reflexion seismic and well data analysis show that the AMP consists of an assembly of large volcanic edifices formed during the Paleogene, each with individual diameters in excess of 50 km, sizes varying from 280 to 2100 km2, and formed by multiple volcanic events. The igneous bodies are mainly buried at different depths (from ~2 Km to 8 km below seafloor), while some emerge to form the Abrolhos Archipelago. The spatial arrangement of the igneous edifices of the AMP suggest that in its central part are the main feeders for the outer and shallower bodies at the borders which have a radial distribution. Their distribution seems to partially follow preexisting structural trends like Precambrian and Mesozoic weakness zones, represented by the onshore Paramirin Aulacogen, the Araçuaí and Ribeira belts, and the offshore rifting structures. These observations suggest an influence of lithosphere inheritance on the emplacement of the AMP. The tectono-stratigraphic architecture indicate margin uplift at Early Cenozoic related to the emplacement of the AMP, creating an elevated area. Mapped hydrothermal vents indicate AMP magmatic activity continued into the Neogene, and connect Abrolhos volcanism with that in the VTC located 650 km to the east. This and reported low S-wave velocity from tomographic model and anomalous dynamic topography suggest a distinct mantle thermal structure under the AMC and VTC. However, the spatial extent and duration of the AMP are inconsistent with a fixed hotspot mechanism and linear trail.

Basement-decoupled hyperextension rifting: The tectono-stratigraphic record of the salt-rich Pyrenean necking zone (Arzacq Basin, SW France)

AbstractHalf grabens and supra-detachment basins correspond to end-member basin types of magma-poor rift settings, each of them showing a characteristic stratigraphic architecture. The occurrence of a basement-cover décollement has been shown to drastically change the stratigraphic architecture of half graben basins, however, the effect of such basement-cover décollement remains to be documented in supra-detachment basins formed during hyper-extension. We investigate the tectono-stratigraphic record of the Arzacq Basin (SW France) recording the formation of a salt-rich Cretaceous hyperextended rift system. Combining 2-D and 3-D seismic reflection calibrated from well data, we show that this basin is an asymmetric syn-rift extensional syncline growing above a pre-kinematic salt layer. By mapping the sub-salt basement, we show that the formation of this syncline is controlled by the South-Arzacq Fault (SAF), soling in the sub-salt basement. Based on crosscutting relationships and the observed southward migration of syn-rift depocenters, this N110°-striking, 20°-dipping structure accommodates >10 km of thick-skinned extension. The overlying supra-salt cover coherently glided, following the basement geometry. The 3-D segmentation of the SAF and the sub-salt stratigraphic architecture of the Arzacq Basin suggest a roughly dip-slip kinematic. A post-kinematic kilometer-scale uplift is documented on the southern side of the Arzacq Basin. It may result from the increasing lithospheric thinning and thermal support at the end of asymmetric hyperextension. As salt commonly occurs in extensional settings, we believe that our description of the tectono-stratigraphic record of a basement-decoupled supra-detachment basin has global applicability to unleash the tectono-stratigraphic evolution of worldwide hyper-extended rifted margins.

Oblique subduction and mantle flow control on upper plate deformation: 3D geodynamic modeling

Most subduction zones on Earth are oblique, i.e., the angle between the plate convergence vector and the trench notably differs from 90°. Therefore, modeling and understanding the strain partitioning in the forearc, the development of extensional basins in the back-arc region and the diachronous transition from subduction to collision require a 3D approach. Here, we assess how oblique oceanic subduction and subsequent collision and associated mantle flow around the subducted lithosphere control the thermo-mechanical evolution of active margins. We conducted a series of 3D thermo-mechanical subduction models and discuss the influence of different subduction obliquity angles, the role of mantle flow variations and their connection with sediment transport and back-arc deformation. Numerical models are complemented by scaled analogue models to visualize the mantle flow evolution. Oceanic subduction along an oblique trench results in asymmetric mantle return flow leading to the gradual decrease of the subduction obliquity angle driven by the gradual rotation of the lower plate and the along-trench variation of slab retreat. This creates laterally variable subduction velocities and slab geometries. Back-arc extension is governed by both the toroidal mantle flow along the slab edges and by the oblique subduction induced lateral mantle flow gradient. The diachronous transition from oceanic to continental subduction and collision facilitates the laterally variable trench advance and retreat and back-arc deformation. Tectonically induced lateral sediment transport in the trench and along the subduction interface decreases its strength and viscosity and can alter subduction velocities. Our model results provide critical insights into the evolution of oblique subduction and collisional systems, such as the Arabia-Eurasia convergence zone.

Inherited morphobathymetric controls over contourite drift deposition: A case study from the late Cenozoic Mentelle Basin, Australia

Deep-sea sedimentary deposits are important archives of the geologic past that preserve the records of past environmental changes in earth’s ocean. The detailed analysis of deep-sea sedimentary archives, in particular of contourite drifts, can help elucidate past changes in ocean circulation and the stratigraphic evolution of continental margins. However, the bathymetric profile of an oceanic basin can shape and modify the architecture of contourite drifts via the interaction between down-slope and along-slope processes. The identification of local bathymetric influence on depositional architectures is therefore important to help decipher local versus regional influences on deep-sea sedimentary signatures. Seismic data from Mentelle Basin in the southeast Indian Ocean integrated with deep-sea core data reveal a calcareous-siliciclastic mixed contourite-turbidite system developed during the late Cenozoic, starting in the middle Miocene. Current winnowing led to the formation of regional hiatuses, ferromanganese crusts, and siliciclastic lag deposits. The main locus of sediment deposition occurred on the shallower parts of the basin, whereas sediment preservation remained low in the deeper areas. Seismic analysis shows that inherited topography influenced the architecture of contourite deposits within the basin, with elongate-mounded and sheeted drifts forming preferentially at shallower depths on the continental slope and the Naturaliste Plateau, while channel incision occurred in the deepest parts of the basin. These results suggest that the intensification of current transport occurred preferentially within the deeper and spatially constrained parts of the basin, whereas current deflection around the slope and plateau enhanced drift deposition and preservation at shallower depths. Therefore, the basin topography at the time of deposition controlled the distribution of deep-sea deposits and drift morphologies within the mixed contourite-turbidite system in the Mentelle Basin.

Introduction: Active Margins in Transition—Magmatism and Tectonics through Time: An Issue in Honor of Arthur W. Snoke

Abstract The evolution of active margins through time is the record of plate tectonics as inscribed on the continents. This themed issue honors the eclectic contributions of Arthur W. Snoke (Fig. 1) to the study of active margins with a series of papers that amply demonstrate the broad scope of active margin tectonics and the diverse methods that tectonic geologists employ to decipher their histories. Taken together, this set of papers illustrates the diversity of boundary conditions that guide the development of active margins and the key parameters that regulate their evolution in time and space.

Did accretion of the Caribbean oceanic plateau drive rapid crustal thickening in the northern Andes?

The onset of orogenic shortening in the northern Andes Mountains coincided with latest Cretaceous accretion of the Caribbean oceanic plateau. We present isotopic data (εHf in zircon and εNd) coupled with arc position to test whether accretion led to abrupt crustal thickening in the northern Andes of Ecuador and Colombia. A rapid isotopic excursion toward more evolved crustal compositions was synchronous with ca. 75–70 Ma collision in Ecuador and preceded a similar deviation in Colombia at ca. 70–55 Ma. The rapid but diachronous shift to more evolved isotopic signatures is attributed to progressive northward accretion of the oceanic plateau and associated thickening of continental crust. We emphasize the effects of accretion on the magmatic evolution of Cordilleran-type margins, initially provoking shortening and crustal thickening, and ultimately providing a substrate for subsequent arcs.

Three-stage tectonic subsidence and its implications for the evolution of conjugate margins of the southwest subbasin, South China Sea

Three-stage tectonic subsidence and its implications for the evolution of conjugate margins of the southwest subbasin, South China Sea

French Guiana margin evolution: From Gondwana break‐up to Atlantic opening

AbstractKnowledge of the Guiana Shield evolution during the Gondwana break‐up is key to a better understanding of craton dynamics and margin response to transtensional opening. To improve this knowledge, we investigated the dynamics and thermal evolution of French Guiana, using several low‐temperature thermochronology methods applied to basement rocks, including apatite and zircon (U‐Th)/He and apatite fission tracks. Inverse modelling of results allows us to reconstruct the Phanerozoic thermal history of French Guiana margin and to give a preview of the Guiana Shield evolution. Three main events are inferred: firstly, a long‐term period of relative stability since ~1.2 Ga, with no strong evidence for any erosional or burial event (>5–7 km); secondly, a heating phase between ~210 and ~140 Ma consistent with the Central Atlantic Magmatic Province‐related event. Finally, an exhumation phase between ~140 and ~90 Ma, triggered by the Equatorial Atlantic opening, brought samples close to the surface (<40°C).

Linear response of the Greenland ice sheet's tidewater glacier terminus positions to climate

AbstractGaining knowledge of tidewater glacier (TWG) margin evolution, solid ice flux and their responses to climate over large spatio-temporal scales provides valuable context for the projection of future Greenland ice sheet (GrIS) change. Although studies of sector-wide responses of TWGs exist, studies at an ice-sheet-wide scale have only just become feasible. Here, we present a dataset of 224 annual TWG margins for 1984–2017 (n= 3801), showing that averaged over regional scales, normalised terminus change is linear. Regionally linear retreat trends were identified across most sectors of the GrIS starting in the mid-1990s, although in contrast to previous studies, the northeastern sector is shown to have experienced sustained retreat since the mid-1980s. Through cointegration analyses, individual glaciers are shown to have differing sensitivities to potential climate drivers, though on a sector-wide scale the northwest and southeast are shown to be especially sensitive to annual sea surface temperature and June–July–August air temperature, respectively. Although 92% of the analysed glaciers experience retreat across the GrIS, observed increases in absolute flux for the entire ice sheet can be explained by changes in just 11 of these TWGs.

India‐Elan Bank‐East Antarctica Breakup, Crustal Architecture, and Margin Evolution: Results From Constrained Potential Field and Process‐Oriented Gravity Modeling of Conjugate Margin Segments

AbstractWe present here the results of a seismically well‐constrained potential field modeling along with process‐oriented gravity modeling across the Eastern Continental Margin of India (ECMI) and its conjugate East Antarctica Continental Margin (EACM). The study revealed both along‐strike variations in crustal architecture and effective elastic thickness (Te), showing a locally asymmetric pattern. Crustal stretching factors (β) estimated at these margins range between 1.5 and 4.0 with slightly lower values varying over the narrow zone in the Cauvery basin‐western Enderby basin and relatively higher values in other margin segments. The models reveal typical magma‐poor margin characteristics in the Krishna‐Godavari (K‐G) and Cauvery basin segments of ECMI. The ocean‐continent transition (OCT) in the K‐G basin and its conjugate (central Enderby basin) indicates asymmetric rifting marked by the development of a detachment fault separating the combined India‐Elan Bank margin and the EACM. The higher β values and magmatic underplating at OCT in the Mahanadi basin segment of ECMI may be related to the post‐breakup magmatic emplacement associated with the proximal activity of the Kerguelen plume after the separation of Elan Bank from India by a ridge jump during M2/M0 (124–120 Ma). The pre‐rift reconstruction model through the palinspastic restoration of margins suggests that the central part of ECMI (16–20°) is the ideal pre‐rift location for the Elan bank.

Geochemical, mineralogical and sedimentological analyses of reworked sediments (new) in the syn- to post-rift Middle Cretaceous-Quaternary detrital deposits from western Atlantic margin of Cameroon: evidence from sedimentation-erosion alternation in the context of passive margin evolution

Reworked detrital sediments were discovered in the syn-rift to post-rift Cretaceous to Quaternary (Pleistocene) detrital deposits of the Douala coastal sub-basin in the eastern margin of the South Atlantic Ocean at West Cameroon. This contribution presents the geochemical, mineralogical, and sedimentological data to constrain the origin of these reworked sediments. The investigated samples are characterized by the high values of Plagioclase Index of Alteration (PIA, 97.83–99.90), Chemical Index of Alteration (CIA, 71.9–99.08), and Chemical Index of Weathering (CIW, 98.67–99.90). Data gleaned from these indices suggest that the reworked sediments and their potential source rocks experienced severe weathering. The A-CN-K plot along with the mineralogical indicates a probably post-depositional K-enrichment. The high light rare earth elements (LREE)/heavy rare earth elements (HREE) ratios (15.82–79.13) suggest that the reworked were mainly derived from felsic igneous rocks. This interpretation is further confirmed by the Zr versus TiO2, TiO2/Al2O3, Th/Co versus La/Sc, and La/Th versus Th/Yb plots, which indicate felsic igneous and silicic source rocks. Meanwhile, the high variability of LREE/HREE ratios (15.82–79.13) and the positive Eu anomalies (1.10–1.32) of studied samples could indicate a little contribution of mafic source rocks. Analysis of the distribution of the heavy minerals (e.g. tourmaline, aluminum silicate, and rutile) revealed that these felsic and silicic source rocks are mainly granite and gneiss, probably belonging to the Nyong Group and the Western part of the Neoproterozoic Yaounde Group. The morphoscopic analyses of the host rocks (very angular to angular shapes) have revealed that the reworked sediments have proximal to subproximal onshore origins, probably from former sedimentary deposits, as is indicated by variation of their color. The tectonic discriminant diagram indicates the Oceanic Island Arc and Active Continental Margin which are inconsistent with the geological history of the Douala coastal sub-basin and could be explained by the recycling effect experienced by studied sediments which have been deposited in the coastal plain to the fluvial environment in the Low System Track context.

Evolution and demise of passive margins through grain mixing and damage

How subduction-the sinking of cold lithospheric plates into the mantle-is initiated is one of the key mysteries in understanding why Earth has plate tectonics. One of the favored locations for subduction triggering is at passive margins, where sea floor abuts continental margins. Such passive margin collapse is problematic because the strength of the old, cold ocean lithosphere should prohibit it from bending under its own weight and sinking into the mantle. Some means of mechanical weakening of the passive margin are therefore necessary. Spontaneous and accumulated grain damage can allow for considerable lithospheric weakening and facilitate passive margin collapse. Grain damage is enhanced where mixing between mineral phases in lithospheric rocks occurs. Such mixing is driven both by compositional gradients associated with petrological heterogeneity and by the state of stress in the lithosphere. With lateral compressive stress imposed by ridge push in an opening ocean basin, bands of mixing and weakening can develop, become vertically oriented, and occupy a large portion of lithosphere after about 100 million y. These bands lead to anisotropic viscosity in the lithosphere that is strong to lateral forcing but weak to bending and sinking, thereby greatly facilitating passive margin collapse.

Trace fossils, depositional context, and paleogeography of the upper Tal Group (upper lower Cambrian), Lesser Himalaya, India: a Gondwanan succession with no affinities to the Avalonia microcontinent – discussion of paper by Singh et al. (2019)

Terminal Ediacaran–late early Cambrian deposition, faunas and passive margin evolution of the north Indian margin are recorded in the Nigali Dhar syncline succession. Restudy of the upper Tal Group (upper lower Cambrian Koti Dhaman Formation, KDF) ichnofauna from the Khud-Drabil section reduces it to 18 confidently named forms. The lower KDF (Lower Quartzite Member) Cruziana-Rusophycus assemblage is in subtidal (not intertidal) sandsheet facies. The overlying black Shale Member (SM) records trans-East Gondwanan deepening, not intertidal facies, in the Palaeolenus Zone. The SM, with low diversity Planolites-Palaeophycus assemblage, is overlain by subtidal (not intertidal) sandsheet facies of the middle KDF (Arkosic Sandstone Member, ASM) with shallow burrowers and furrowers (Gordia marina assemblage, new; Cruziana ichnofacies). KDF faunas with Cruziana and Rusophycus are similar to coeval, shallow marine associations elsewhere in Gondwana and NW Laurentia. Interpretation of a second KDF section 20 km from Khud-Drabil has confused an understanding of Lesser Himalaya geologic evolution as it claims Ordovician Cruziana species in the ASM and an angular SM–ASM unconformity caused by the Cambrian–Ordovician boundary Kurgiakh orogeny. However, upper lower Cambrian microfaunas occur in and above the ASM, while the angular SM–ASM unconformity is consistent with submarine sliding. KDF-type ichnofaunas do not show a tropical location of Avalonia, which has the distinctive lithofacies and biotas of a high-latitude continent unrelated to Gondwana.

South Atlantic passive margin evolution: A thermochronology case study from the Rio de Janeiro-Três Rios section, SE Brazil

The southeastern Brazilian passive margin records a complex post-rift evolution, with two parallel high-elevation features formed after the opening of the South Atlantic. We applied apatite fission track (AFT) and U–Th/He (AHe) low temperature thermochronology to constrain the thermo-tectonic history of the Serra do Mar escarpment in the area of Rio de Janeiro state. New AFT central ages for basement areas collected from a N–S transect orthogonal to the shoreline between the cities of Rio de Janeiro and Três Rios, range between 98.5 ± 5.3 and 54.1 ± 4.2 Ma, with mean track lengths between 12.34 ± 0.40 and 14.63 ± 0.17 μm. Uncorrected AHe ages lie between 68.1 ± 5.9 and 60.2 ± 7.3 Ma and are consistent with AFT results. Inverse thermal history models constrained by AFT and AHe data imply the earliest cooling onset from the Barremian (Early Cretaceous), with steady rates more common for samples closer to coastal areas. Maximum depths of denudation are between 2.5 and 4.5 km. Published thermochronological data from adjacent areas combined with the new results show a seemingly simpler post-rift evolution for the area, although suggesting structural control of age distribution and exhumation.

Early Devonian Ultrapotassic Magmatism in the North China Craton: Geochemical and Isotopic Evidence for Subcontinental Lithospheric Mantle Metasomatism by Subducted Sediment–Derived Fluid

AbstractThe North China Craton (NCC) represents one of the oldest and largest cratons in the earth with a nearly complete record of Precambrian history. In the northern part of the NCC, the earliest phase of alkaline magmatism occurred in discrete pulses in the Early and Middle Devonian; whereas the next episode of alkaline magmatism took place in the early Mesozoic. The Gucheng pluton is exposed in the northern part of the NCC and forms a composite intrusion, consisting of K‐feldspar–bearing clinopyroxenite, clinopyroxene–bearing syenite and alkali‐feldspar syenite. Mineral phases in these lithologies include clinopyroxene (Wo43‐48En19‐35Fs18‐38), sanidine (An0Ab3‐11Or89‐97), and subordinate titanite, andradite and Na‐feldspar. These rocks show homogeneous Sr but variable Nd isotopic compositions, and have relatively high zircon in‐situ oxygen isotopes (δ18O=5.2–6.7). The Gucheng plutonic rocks formed through fractional crystallization and accumulation from ultrapotassic magmas, which were originated from partial melting of metasomatic vein systems in the subcontinental lithospheric mantle of the NCC. These vein networks developed as a result of the reactions of fluids derived from subducted pelitic sediments on the downgoing Palaeo‐Asian ocean floor with the enriched, subcontinental lithospheric mantle peridotites. SHRIMP U‐Pb zircon dating has revealed a crystallization age of 415 Ma for the timing of the emplacement of the Gucheng pluton that marks the early stages of alkaline magmatism associated with the Andean‐type continental margin evolution along the northern edge of the NCC facing the Palaeo‐Asian Ocean.

Zircon U-Pb geochronology and Nd-Pb isotope geochemistry of Blue Ridge basement in the eastern Great Smoky Mountains, U.S.A.: Implications for the Proterozoic tectonic evolution of the southeastern Laurentian margin

The Mesoproterozoic to Paleozoic history of the southeastern Laurentian margin involved repeated collisional and accretionary tectonomagmatic events that reworked and recycled older continental crust of preceding events. The Great Smoky Mountains Basement Complex (GSMBC) within the southern Appalachian Blue Ridge exposes complexly deformed orthogneiss and paragneiss that preserve a record of Laurentian margin evolution from ca. 1.9 Ga to 450 Ma. The GSMBC consists primarily of: (1) 1.34 to 1.31 Ga (pre-Elzevirian) granodioritic orthogneiss and entrained mafic xenoliths that represent some of the oldest crust in Appalachian Grenville massifs (correlated with pre-Elzevirian crustal components in the Adirondack, Green Mountains, New Jersey Highlands, and French Broad massifs), (2) ca. l.15 to 1.05 Ga augen and granitic orthogneiss produced during Shawinigan and Ottawan phases of Grenville-age magmatism and metamorphism, respectively, and (3) paragneiss derived from protoliths with either Grenville-age (1.1–1.0 Ga) or post-Grenville (Neoproterozoic) depositional ages based on presence/absence of ca. 1.0 Ga metamorphic zircon and 1.9 to 1.1 Ga detrital zircon. All lithologies experienced Taconian metamorphism and variable migmatization. Pre-Ottawan paragneiss exhibits major detrital zircon ages modes at 2.0 to 1.6 and 1.4 to 1.3 Ga that require a component of older Proterozoic crust in the sediment source region. Detrital zircon grains in post-Ottawan paragneiss exhibits the full spectrum of Grenville-age modes that correlate with the five phases of Grenville magmatic/metamorphic events in eastern Laurentia. These paragneiss samples also contain scattered 750 to 600 Ma detrital zircon grains that constrain their maximum depositional age to late Neoproterozoic. The sedimentary protoliths of the latter paragneiss consist largely of detritus from exhumation of all Grenville crustal age components during post-orogenic exhumation and crustal extension leading up to Late Neoproterozoic breakup of Rodinia. Most zircon U-Pb age systematics exhibit variable discordance that can be attributed to disturbance of the U-Pb system and/or new zircon growth during either high-grade Ottawan (ca. 1.04 Ga) or Taconian (ca. 0.46 Ga) regional metamorphism and migmatization. Neodymium TDM model ages for granodioritic orthogneiss and paragneiss range from 1.8 to 1.6 Ga, indicating that the rocks were derived from recycling of Proterozoic crust (that is, they are not juvenile), consistent with the 1.9 to 1.6 Ga detrital zircon grains in pre-Ottawan paragneiss and with 1.8 to 1.7 Ga inherited zircon in some 1.33 Ga orthogneisses and a 1.35 Ga xenolith. Whole rock Pb isotope compositions of GSMBC rocks overlap the field of compositions characteristic of Amazonian crust and of other basement rocks from the south-central Appalachians. The Pb isotopes and geochronology in orthogneiss, mafic xenoliths, and pre-Ottawan paragneiss are consistent with a correlation of the GSMBC with the Mars Hill terrane within the French Broad massif and with the greater Grenvillian south-central Appalachian basement (SCAB) that is considered exotic to Laurentia, and transferred during Rodinian collision prior to ca. 1.2 Ga. Similarities in protolith ages and Pb isotopes point to the Paragua terrane of Amazonia (southwestern Brazil and eastern Bolivia) as a likely match for SCAB. Initial Amazonia-Laurentia collision occurred at ca. 1.35 to 1.32 Ga with final transfer of SCAB to Laurentia occurring after 1.20 Ga within the sinistral oblique collision zone between Laurentia and Amazonia defined by previous workers.

Post-rift submarine volcanic complexes and fault activities in the Baiyun Sag, Pearl River Mouth Basin: New insights into the breakup sequence of the northern South China Sea

Post-rift volcanisms and fault activities are significant for understanding the tectonic-sedimentary evolution of rifted continental margins and associated hydrocarbon entrapments. In this study, a series of post-rift fault-associated volcanic activities have been found in the northern continental margin of the South China Sea (SCS). However, their episodes and relationships with the progressive continental breakup are still doubtful. Here, new 2D/3D seismic, borehole, dating, and biostratigraphic data are used to investigate the characteristics of the post-rift volcanism and fault activities in the Baiyun Sag (BYS), Pearl River Mouth Basin. The newly-found fault-associated volcanic complexes include thirteen submarine volcanic mounds and associated lava flows, together with igneous sills that are mainly composed of basalts and tuffs. Seismic stratigraphic analysis combined with KAr dating from cores confirms that post-rift volcanism in the BYS climaxed along with the breakup of the SCS mainly in three episodes: (1) the first episode near Oligocene/Miocene boundary (23.3 Ma); (2) the second at the middle Early Miocene (17.6 Ma); and (3) the new-found third episode at the late Early Miocene (17.1 Ma). Also, fifty-three syn-sedimentary faults are interpreted, which are characterized by a WNW trending normal fault group separated by an ENE trending transcurrent fault group. And there is evidence that these fifty-three faults were long-lived from ca. 32–17.1 Ma and served as important conduits for the upward migration of the magma. The three episodes of volcanism and tectonic fault activities are related to the progressive continental breakup in the northern SCS margin, and they represent the tectonic signature of the early stage of the post-rift subsidence after the lithospheric breakup which extends to the top of a hypothetical breakup sequence (32–17.1 Ma) in the BYS. Furthermore, this study proves that the diachronous continental breakup in the SCS margin can result in corresponding changes in depositional architectures, along with a breakup sequence characterized by the deposition of a ‘sandstone‑carbonate-shale’ regressive-transgressive cycle. The sedimentological changes and associated tectonic features recorded in the Baiyun Sag suggest an exceptionally active tectonic setting in the time of the formation of the breakup sequence (Early Oligocene-Early Miocene) along the northern SCS margin.

Lateral variability of shelf-edge and basin-floor deposits, Santos Basin, offshore Brazil

ABSTRACTConstruction of continental margins is driven by sediment transported across the shelf to the shelf edge, where it is reworked by wave, tide, and fluvial processes in deltas and flanking clastic shorelines. Stalling of continental-margin progradation often results in degradation of the outer shelf to upper slope, with resedimentation to the lower slope and basin floor via a range of sediment gravity flows and mass-movement processes. Typically, our understanding of how these processes contribute to the long-term development of continental margins has been limited to observations from broadly two-dimensional, subsurface and outcrop datasets. Consequently, the three-dimensional variability in process regime and margin evolution is poorly constrained and often underappreciated. We use a large (90 km by 30 km, parallel to depositional strike and dip, respectively) post-stack time-migrated 3D seismic-reflection dataset to investigate along-strike variations in shelf-margin progradation and outer-shelf to upper-slope collapse in the Santos Basin, offshore SE Brazil. Early Paleogene to Eocene progradation of the shelf margin is recorded by spectacularly imaged, SE-dipping clinoforms. Periodic failure of the outer shelf and upper slope formed ca. 30-km-wide (parallel to shelf-margin strike) slump scars, which resulted in a strongly scalloped upper-slope. Margin collapse caused: 1) the emplacement of slope-attached mass-transport complexes (MTCs) (up to ca. 375 m thick, 12+ km long, 20 km wide) on the proximal basin floor, and 2) accommodation creation on the outer shelf to upper slope. This newly formed accommodation was infilled by shelf-edge-delta clinoforms (up to 685 m thick), that nucleated and prograded basinward from the margin-collapse headwall scarp, downlapping onto the underlying slump scar and/or MTCs. Trajectory analysis of the shelf-edge deltas suggests that slope degradation-created accommodation was generated throughout the sea-level cycle, rather than during base-level fall as would be predicted by conventional sequence-stratigraphic models. Our results highlight the significant along-strike variability in depositional style, geometry, and evolution that can occur on this and other continental margins. Coeval strata, separated by only a few kilometers, display strikingly different stratigraphic architectures; this variability, which could be missed in 2D datasets, is not currently captured in conventional 2D sequence stratigraphic models.

A Quantitative Tomotectonic Plate Reconstruction of Western North America and the Eastern Pacific Basin

AbstractPlate reconstructions since the breakup of Pangaea are mostly based on the preserved spreading history of ocean basins, within absolute reference frames that are constrained by a combination of age‐progressive hotspot tracks and paleomagnetic data. The evolution of destructive plate margins is difficult to constrain from surface observations as much of the evidence has been subducted. Seismic tomography can directly constrain paleotrench locations by imaging subducted lithosphere in the mantle. This new evidence, combined with the geological surface record of subduction, suggests that several intraoceanic arcs existed between the Farallon Ocean and North America during late Mesozoic times—in contrast to existing quantitative models that typically show long‐lived subduction of the Farallon plate beneath the continental margin. We present a continuously closing plate model for the eastern Pacific basin from 170 Ma to present, constrained using “tomotectonic analysis”—the integration of surface and subsurface data. During the Middle to Late Jurassic, we show simultaneous eastward and westward subduction of oceanic plates under an archipelago composed of Cordilleran arc terranes. As North America drifts westward, it diachronously overrides the archipelago and its arcs, beginning in the latest Jurassic. During and post‐accretion, Cordilleran terranes are translated thousands of kilometers along the continental margin, as constrained by paleomagnetic evidence. Final accretions to North America occur during the Eocene, ending ~100 Myr of archipelago override. This model provides a detailed, quantitative tectonic history for the eastern Pacific domain, paving the way for tomotectonic analysis to be used in other paleo‐oceanic regions.

Seismic stratigraphic framework of the continental shelf offshore Delmarva, U.S.A.: Implications for Mid-Atlantic Bight Evolution since the Pliocene

Understanding how past coastal systems have evolved is critical to predicting future coastal change. Using over 12,000 trackline kilometers of recently collected, co-located multi-channel boomer, sparker and chirp seismic reflection profile data integrated with previously collected borehole and vibracore data, we define the upper (< 115 m below mean lower low water) seismic stratigraphic framework offshore of the Delmarva Peninsula, USA. Twelve seismic units and 11 regionally extensive unconformities (U1-U11) were mapped over 5900 km2 of North America's Mid-Atlantic continental shelf. We interpret U3, U7, U9, U11 as transgressive ravinement surfaces, while U1,2,4,5,6,8,10 are subaerial unconformities illustrating distinct periods of lower sea-level. Based on areal distribution, stratigraphic relationships and dating results (Carbon 14 and amino acid racemization estimates) from earlier vibracore and borehole studies, we interpret the infilled channels as late Neogene and Quaternary courses of the Susquehanna, Potomac, Rappahannock, York, James rivers and tributaries, and a broad flood plain. These findings indicate that the region's geologic framework is more complex than previously thought and that Pleistocene paleochannels are abundant in the Mid-Atlantic. This study synthesizes and correlates the findings of other Atlantic Margin studies and establishes a large-scale Quaternary framework that enables more detailed stratigraphic analysis in the future. Such work has implications for inner continental shelf systems tract evolution, the relationship between antecedent geology and modern coastal systems, assessments of eustacy, glacial isostatic adjustment, and other processes and forcings that play a role in passive margin evolution.