Baffin Bay Composite Tectono-Sedimentary Element

Baffin Bay formed as a result of continental extension during the Cretaceous, which was followed by sea floor spreading and associated plate drift during the early to middle Cenozoic. Formation of an oceanic basin in the central part of Baffin Bay may have begun from about 62 Ma in tandem with Labrador Sea opening but the early spreading phase is controversial. Plate-kinematic models suggests that from Late Paleocene the direction of sea floor spreading changed to N-S generating strike-slip movements along the transform lineaments, e.g. the Ungava Fault Zone and the Bower Fracture Zone, and structural complexity along the margins of Baffin Bay. The Baffin Bay Composite Tectono-Sedimentary Element (CTSE) represents a 3-7 km thick Cenozoic sedimentary and volcanic succession that has deposited over oceanic and rifted continental crust since active seafloor spreading began. The CTSE is subdivided into 5 seismic mega-units that have been identified and mapped using a regional seismic grid tied to wells and core sites. Thick clastic wedges of likely Late Paleocene to Early Oligocene age (mega-units E and D2) were deposited within basins floored by newly formed oceanic crust, transitional crust, volcanic extrusives and former continental rift basins undergoing subsidence. The middle-late Cenozoic is characterized by fluvial-deltaic sedimentary systems, hemipelagic strata and aggradational sediment bodies deposited under the influenced of ocean currents (mega-units D1, C and B). The late Pliocene to Pleistocene interval (mega-unit A) displays major shelf margin progradation associated with ice-sheet advance-retreat cycles resulting in accumulation of trough-mouth fans and mass-wasting deposits products in the oceanic basin. The Baffin Bay CTSE has not produced discoveries although a hydrocarbon potential may be associated with Paleocene source rocks. Recent data have improved the geological understanding of Baffin Bay although large data and knowledge gaps remain.

Gulf of Aden spreading does not conform to triple-junction formation

The Gulf of Aden represents an evolving example of a juvenile ocean system and is considered the most evolved rift arm of the Afar triple junction. We have undertaken analysis of recent coupled satellite and marine potential-field data to understand the first-order crustal architecture along the entire length of the gulf. Our interpretation suggests the Gulf of Aden has three domains with distinct free-air gravity and magnetic characteristics. These domains record a progression from active seafloor spreading in the eastern domain, through isolated and discontinuous spreading segments in the central domain, to active continental rifting in the western domain immediately adjacent to the Afar triple junction. Forward models suggest the presence of transitional crust, which displays linear magnetic stripe–like anomalies that bound oceanic stripes in the central domain and covering the majority of the western domain. Magnetic anomalies differ from magnetic stripes sensu stricto because they are discontinuous and cannot be correlated along the length of the gulf. Detection of northwest-southeast extension in the central domain based on magnetic stripe orientation is inconsistent with the regional northeast-southwest extension. Our observations reflect heterogeneous opening of the Gulf of Aden basins, in which spreading is migrating toward Afar as a series of isolated spreading segments, rather than initiating at the junction as proposed by classical platetectonic theory. This mechanism of ocean initiation is inconsistent with transtensional models that involve wholesale tearing of continental crust and contradicts conceptual models that rely on the Afar plume in initiating or driving the extension.

Emplacement of Paleocene-Eocene magmatism under transtensional regime and its evolution to a dynamic equilibrium on the western edge of Colombia

The Cretaceous and Paleogene magmatic arcs of the Central and Western Cordilleras of Colombia have been attributed to the evolution of a subduction system in the Colombian Pacific coast. In this work the distribution and crystallization ages of plutons emplaced between 60 Ma and 53 Ma in the Central and Western Cordilleras are analyzed. From 53 Ma the magmatic arc migrates towards the west of Colombia, installing magmas in a plate edge transitional crust. The crystallization ages analyzed in this work suggest that, within the study area, the plutonic belt is continuous throughout the Western Cordillera. From 40±5 Ma to 26 Ma there was a significant reduction in the convergence velocity of the Farallon plate; as it decreases, also the tectonic loading diminishes resulting in a process of regional stress relaxation. The process of relaxation of the regional stress also occurred in the intra-continental environments producing peneplanization process in the topographic highs of northern Colombia, the reactivation of the piedmont with westwards progradation of sedimentation and the development of a middle- to late-Eocene regional unconformity. In continental shelf environments, the relaxation of the tectonic stress is evidenced by the distribution of reef limestone sequences throughout the Colombian Pacific margin and the Caribbean of Colombia, Ecuador, Panama and Costa Rica, and by a magmatic gap from 33 Ma to 26 Ma. The Paleocene-Eocene magmatic event distributed in the Central and Western Cordilleras took place under a transtensional regime, with the maximum horizontal compressive stress (σ1) oriented SW-NE, product of the oblique convergence between the Farallon and South American plates.

Beyond Plate Tectonics: The Northeast Atlantic Realm

<p>Conventional plate tectonics envisages simple continental breakup with clean splitting of supercontinents and subsequent orderly widening of oceans by seafloor spreading about a central ridge. No sooner was this paradigm proposed when the clear, first-order misfit of intraplate and large-volume volcanism was highlighted. That was quickly accommodated by adding an additional degree of freedom into the theory of Earth dynamics, i.e., ad hoc mantle plumes. Although this simple picture was adequate in the early years of plate tectonics, the subsequent rapid accumulation of vast datasets of ever-more-precise observations has rendered a theory of such simplicity no longer tenable. Simple plate tectonics can now serve only as a basic canvas on which the complexities of the real world must be painted. There is no better region for illustrating this than the Northeast Atlantic Realm which illustrates the full range of complexities. After a history of tectonic unrest spanning several 100 Myr true continental breakup, involving fracture of the entire lithosphere and ocean widening via sea-floor spreading, finally proceeded. However, geological complications are on at least an equal level to features arguably amenable to description by simple plate tectonics. Spreading ridges developed by propagation through continental lithosphere comprising a collage of cratons separated by orogenic belts. Where these propagators met insurmountable barriers the extension demanded by local kinematics could only be accommodated by diffuse continental extension. Continual changes occurred in the direction of regional extension and these resulted in local tectonic instabilities manifest in lateral ridge migrations, jumps, and parallel-ridge-pair extension. Extreme, magma-assisted continental extension, together with intense volcanism, formed lava-capped transitional crust. As a consequence the true extent of continental crust under the oceans is unclear. The geophysical characteristics of transitional crust are ambiguous in terms of physical properties. This presents a challenge to mapping continental material in the oceans, a problem that can be mitigated by joint interpretation with gravity, heatflow and geochemical data. Known continental blocks in the ocean include the array of blocks west of the British continental shelf (the Hatton-, George Bligh-, Lousy-, Bill Bailey’s- and Faroe Bank Highs, and Wyville-Thompson- and Fugløy Ridges), the Jan Mayen Microplate Complex, the Greenland-Iceland-Faroe Ridge and likely others that remain to be found. All of the above complexities in the solid Earth have profoundly affected the natural environment in the region, especially the oceans and the biosphere, and must be taken into account in predictions of future evolution of the natural environment.</p>

Crustal accretion of thick mafic crust in Iceland: implications for volcanic rifted margins

Rifting near hotspots results in mantle melting to create thick mafic igneous crust at volcanic rifted margins (VRMs). This mafic crust is transitional between rifted continental crust with mafic intrusions landward and oceanic crust into which it grades seaward. Seismic velocities, crustal drilling, and exhumed margins show that the upper crust in these areas is composed of basaltic lava erupted in subaerial to submarine conditions intruded by downward increasing proportions of dikes and sparse gabbroic intrusions. The lower crust of these regions is not exposed but is inferred from seismic velocities (Vp > 6.5 km/sec) and petrological constraints to be gabbroic to ultramafic in composition. Limited access to crustal sections generated along VRMs have raised questions regarding the composition and structure of this transitional crust and how it evolves during the early stages of rifting and subsequent seafloor spreading. Active processes in Iceland provide a glimpse of subaerial spreading with the creation of a thick (40–25 km) mafic igneous crust that may be analogous to the transitional crust of VRMs. Segmented rift zones that propagate away from the Iceland hotspot, migrating transform fault zones, and rift-parallel strike-slip faults create a complex plate boundary zone in the upper, brittle crust. These structures may be decoupled from underlying lower crustal gabbroic rocks that are capable of along-axis flow that smooths-out crustal thickness variations. Similar processes may be characteristic of the early history of VRMs and volcanic hotspot ridges related to rifting and seafloor spreading proximal to hotspots.

CHARACTERIZATION OF THE OCEAN-CONTINENT TRANSITION IN THE PARAÍBA BASIN AND NATAL PLATFORM REGION, NE BRAZIL

ABSTRACT. Several studies have tried to address the evolution of the Atlantic conjugate margins, including Brazil and West Africa. However, past researchadvances has been hindered by a lack of data for the marginal region in the eastern portion of northeastern Brazil, extending from the Pernambuco Shear Zone tothe Touros High. This situation has imposed serious limitations on the development of a regional view of the paleotectonic and paleogeographic evolution of the marginin this area and on correlations with regional counterparts in Africa. Here, we present an investigation using regional seismic and potential field data. The results showthat this region represents a basement high forming a narrow platform with a thin sedimentary cover (0.8-2.5 km) and an abrupt shelf break, which created a large bypasszone towards the slope. The analysis of a deep seismic section revealed that thinned continental crust (transitional crust) occupies a narrow zone and that the continentaloceanicboundary (COB) is located approximately 100 km to the east of the present coastline. Geophysical modeling integrated with interpretation of the seismic datasuggests that this region is characterized by an abrupt thinning of continental crust, with an accompanying sudden rise of the Moho. There are also indications for theexistence of a zone of extremely thinned continental crust, which was interpreted as proto-oceanic crust. Our findings suggest that the study area shows strong similaritiesto non-volcanic rifted margins.Keywords: Paraíba and Natal Platform Basins, continental-oceanic transition, northeastern Brazilian continental margin, Atlantic rift. RESUMO. Vários trabalhos têm tentado abordar a evolução das margens conjugadas do Atlântico, incluindo o nordeste do Brasil e o oeste da África. Entretanto,o avanço de pesquisas anteriores tem sido dificultado em razão da falta de dados na região marginal da porção oriental do nordeste do Brasil, a área entre a Zonade Cisalhamento de Pernambuco e o Alto de Touros. Este fato tem imposto limitações ao desenvolvimento de modelos regionais sobre a evolução paleotectônica e paleogeográfica desta região, assim como na correlação com sua contraparte na África. Aqui é apresentada uma investigação realizada com base em dados sísmicos e de métodos potenciais regionais. Os resultados mostraram que esta região representa um alto do embasamento que forma uma plataforma estreita com uma coberturasedimentar pouco espessa (0,8-2,5 km) e uma quebra abrupta da plataforma, o que criou uma grande zona de by pass através do talude. A análise de uma seçao sísmica profunda revelou que a crosta continental afinada (crosta transicional) representa uma estreita zona, e que o limite crosta continental-oceânica (COB) está localizadoa aproximadamente 100 km a leste da atual linha de costa. A modelagem geofísica, integrada com a interpretação sísmica, indica que esta região é caracterizada porum afinamento abrupto da crosta continental, com a consequente ascensão súbita da Moho. Também há evidências da existência de uma zona de crosta continental extremamente afinada, a qual foi interpretada como crosta proto-oceânica. Estes novos dados demonstram que esta área apresenta fortes similaridades com margens rifteadas não vulcânicas.Palavras-chave: bacias da Paraíba e da Plataforma de Natal, transição crosta continental-oceânica, margem continental do nordeste Brasileiro, rifte Atlântico.