Christian Gorini

Continental breakup and the dynamics of rifting in back-arc basins: the Gulf of Lion margin

Deep seismic profiles and subsidence history of the Gulf of Lion margin reveal an intense stretching of the distal margin and strong post-rift subsidence, despite weak extension of the onshore and shallow offshore portions of the margin. We revisit this evolution from the geological interpretation of an unpublished multi-channel seismic profile and other published geophysical data. We show that an 80 km-wide domain of thin lower continental crust, the “Gulf of Lion metamorphic core complex”, is present in the ocean-continent transition zone and exhumed mantle makes the transition with oceanic crust. The exhumed lower continental crust is bounded upward and downward by shallow north-dipping detachments. The presence of exhumed lower crust in the deep margin explains the discrepancy between the amount of extension deduced from normal faults in the upper crust and total extension. We discuss the mechanism responsible for exhumation and present two scenarios, a first one involving a simple coupling between mantle extension due to slab retreat and crustal extension, and a second one involving extraction of the lower crust and mantle from below the margin by the south-eastward flow of hot asthenosphere in the back-arc region during slab rollback. In both scenarios, the combination of Eocene crustal thickening related to the Pyrenees, the nearby volcanic arc and a shallow lithosphere-asthenosphere boundary weakened the upper mantle and lower crust enough to make them flow south-eastward. The overall hot geodynamic environment also explains the subaerial conditions during most of the rifting stage and the delayed subsidence after breakup.

Sedimentological and stratigraphic evolution of northern Lebanon since the Late Cretaceous: implications for the Levant margin and basin

This paper presents an updated review of the Upper Mesozoic and Cenozoic sedimentological and stratigraphic evolution of the Levant margin with a focus on the northern Lebanon. Facies and microfacies analysis of outcrop sections and onshore well cores (i.e., Kousba and Chekka) supported by nannofossil and planktonic foraminifers biostratigraphy, allowed to constrain the depositional environments prevailing in the Turonian to Late Miocene. The “Senonian” (a historical term used to define the Coniacian to Maastrichtian) source rock interval was subdivided into four sub-units with related outer-shelfal facies: (1) Upper Santonian, (2) Lower, (3) Upper Campanian, and (4) Lower Maastrichtian. This Upper Cretaceous rock unit marks the major drowning of the former Turonian rudist platform. This paper confirms the Late Lutetian to Late Burdigalian hiatus, which appears to be a direct consequence of major geodynamic events affecting the Levant region (i.e., the continued collision of Afro-Arabia with Eurasia), potentially enhanced by regressional cycles (e.g., Rupelian lowstand). The distribution of Late Burdigalian–Serravallian rhodalgal banks identified in northern Lebanon was controlled by pre-existing structures inherited from the pulsating onshore deformation. Reef barriers facies occur around the Qalhat anticline, separating an eastern, restricted back-reef setting from a western, coastal to open marine one. The acme of Mount Lebanon’s uplift and exposure is dated back to the Middle–Late Miocene; it led to important erosion of carbonates that were subsequently deposited in paleo-topographic lows. The Late Cretaceous to Cenozoic facies variations and hiatuses show that the northern Lebanon was in a higher structural position compared to the south since at least the Late Cretaceous.

Architecture and stratigraphic framework of shelf sedimentary systems off Rio de Janeiro state, Northern Santos Basin-Brazil

A analise sismica de dados sparker das Operacoes GEOMAR permitiu a elaboracao de um primeiro arcabouco estratigrafico da secao rasa (~300 msec) da plataforma continental do Estado do Rio de Janeiro, norte da bacia de Santos. Dois conjuntos de sequencias sismicas foram interpretados como sequencias deposicionais induzidas por oscilacoes glacio-eustaticas. O Conjunto I (SqA-SqC), composto por sequencias dominantemente sigmoidais, reflete condicoes de geracao de espaco de acomodacao sedimentar capaz de preservar seus componentes agradacionais-progradacionais; o Conjunto II (Sq1-Sq5), composto principalmente por prismas de regressao forcada, indica diminuicao relativa de espaco de acomodacao. Dados cronoestratigraficos de pocos permitiram posicionar a secao sismica investigada na janela plio-quaternaria: a maior parte do Conjunto I (SqA e parte inferior da SqB) foi atribuida uma idade Plioceno (indiferenciado); a secao estratigrafica que se estende da porcao superior da SqC ate o Conjunto II foi atribuida uma idade quaternaria. A correlacao entre a base de dados e curvas globais de variacoes isotopicas de δ18O permitiu ainda sugerir que as sequencias Sq1-Sq4 registram sequencias regressivas de quarta ordem (ciclos glacio-eustaticos de cerca de 100-120 ka) durante os ultimos 440-500 ka. A sequencia Sq5 representaria a deposicao holocenica, constituida por depositos transgressivos e de sistemas de mar alto.

Megaslides in the Foz do Amazonas Basin, Brazilian Equatorial Margin

Recent analysis of multi-channel seismic data provides new evidence that mass-transport deposits have been recurrent elements in the Foz do Amazonas Basin during the Middle-Miocene to Recent. In regions located to the NW and SE of the Amazon Deep-sea Fan, mass movement processes remobilized thick siliciclastic series (up to 1,000 m) as huge megaslide deposits over areas up to 90,000 km2. The Para-Maranhao Megaslide in the SE shows a displaced block (>104 km2) in association with large mass transport deposits covering an area of more than 105 km2. These deposits are distally bounded by thrust faults, which propagate upwards eventually offsetting the sea floor. In addition, NW of the deep-sea fan, the Amapa Megaslide Complex presents a series of recurrent megaslides in the stratigraphic succession, bounded by listric normal faults and tear zones on the upper slope. Associated remobilized deposits extend from more than 300 km downslope, partially involving the upper channel-levee units of the Amazon Deep-sea Fan.

O processo de colapso gravitacional da seção marinha da bacia da foz do Amazonas - margem equatorial brasileira

As sequencias marinhas da Bacia Foz do Amazonas sao deformadas pela tectonica gravitacional, incluindo aquelas do Leque do Amazonas. As estruturas associadas ao colapso gravitacional, imageadas por sismica multicanal 2D, foram formadas pelo deslizamento gravitacional da cobertura sedimentar sobre niveis basais moveis, induzido pela sobrecarga sedimentar e pelo gradiente do talude. O deslizamento gravitacional ocorreu ao longo de distintas superficies de descolamento e, aparentemente, em diferentes estagios evolutivos da margem. Ao menos tres niveis estratigraficos principais atuaram como superficies de descolamento, em escala regional ou local. Cinturoes de dobramento e cavalgamento em aguas profundas sao as estruturas mais notaveis deste arcabouco; sua geometria e complexidade variam ao longo da margem em reposta a variacao lateral do desenvolvimento dos depocentros do Leque do Amazonas. Estas estruturas sao mais complexas no Compartimento Noroeste onde os principais depocentros estao presentes, formando multiplas frentes compressivas devido ao mais intenso encurtamento da cobertura sedimentar. No Compartimento Sudeste, o sistema e menos desenvolvido e se limita a um cinturao parcialmente inativo, caracterizado apenas por um par de falhas reversas ativas com menor impacto morfologico. Localmente, falhas normais listricas antiteticas se ancoram sobre a superficie de descolamento mais superior, exercendo um controle estrutural sobre a segmentacao dos depocentros do proprio leque.

Radial gravitacional gliding indicated by subsalt relief and salt-related structures: the example of the gulf of lions, western mediterranean

A bacia evaporitica Messiniana do Golfo de Lion constitui um nivel basal movel bastante raso ( ~ 3.6 km de profundidade), o que facilita o imageamento sismico da superficie de decollement e a correlacao entre tipos de estruturas gravitacionais e a forma do relevo subsalifero. O relevo subsal revela morfologias convexas e concavas, sugerindo direcoes radiais regionais de fluxo gravitacional. O deslizamento radial e ainda indicado pelas variacoes de espessura da camada de sal. Enquanto zonas de superespessamento do sal recobrem regioes concavas do relevo subsalifero, e indicam padrao de deslizamento convergente; zonas de afinamento da camada de sal recobrem porcoes convexas da superficie de decollement , indicando um padrao de deslizamento divergente. O padrao de deslizamento radial reflete-se ainda no arcabouco estrutural pela presenca de falhas normais transversais e dobramentos nucleados pelo sal. Falhas paralelas a direcao de mergulho regional da bacia afetam a cobertura sedimentar sobrejacente a areas concavas do relevo subsalifero, atestando a intervencao de componentes de extensao paralela ao strike da margem. Alem disso, uma grande concentracao de estruturas compressionais afeta a cobertura sedimentar sobrejacente a porcoes concavas do relevo subsalifero, atestando um padrao convergente de deslizamento translacional. O Golfo de Lion oferece, deste modo, um interessante cenario para estudos de tectonica gravitacional, assim como um modelo para correlacao entre estruturas mapeadas da tectonica de sal e aquelas previstas em modelos analogicos.

Movimentos de massa multiescala na bacia da foz do Amazonas - margem equatorial brasileira

Analysis of 2D multi-channel seismic data provided evidences that sediment slides are recurrent elements in the sedimentary succession of the Foz do Amazonas Basin. In the Amazon Fan, a linked extensional-contractional system glides along weak levels, forming gravitational fold-and-thrust belts of regional scale. Gravity-related fault scarps and sea-floor uplifts (up to 500 m high) induced surficial sediment slides along thrust-faults (average thickness between 200-300 m), resulting in structurally-driven mass movement deposits, as large as 104 km2. We document, as well, the occurrence of two thick large-scale megaslides along the steep margin to the NW and to the SE of the fan. Megaslides can remobilize siliciclastic series downslope as allochthonous masses over areas as large as thousands of km2: the Para-Maranhao Megaslide (up to 1000 m thick) extends downslope to the Para-Maranhao Basin over an area of about 90, 000 km2. To the Northwest of the fan, the Amapa Megaslide Complex consists of a series of recurrent megaslides (each one as thick as 300-700 m), forming a thick stratigraphic succession related to mass transport processes, across an area of about 80, 000 km2. The instability that triggers megaslides is most likely created by pore overpressure as indicated by the seismic attributes of their respective base of slides, characterized by seismic signals of negative polarity.

Inversión tectónica reciente de la zona central de Alborán

Inciativa Iberica para el Estudio de las Fallas Activas, Segunda Reunion Iberica sobre Fallas Activas y PaleosismologiaI IBERFAULT 2014, 22-24 de octubre de 2014, Lorca, Espana.-- 4 pages, 3 figures, 1 table

Arcabouço estratigráfico e ciclicidade deposicional dos sistemas sedimentares do Pleistoceno Médio-Holoceno da plataforma sul da Bacia de Campos, Brasil

A analise sismica de ~ 3.000 km de dados sismicos sparker de reflexao monocanal permitiu a proposicao de um arcabouco estratigrafico englobando a secao rasa (~ 300 milissegundos) da plataforma continental sul da Bacia de Campos. Cinco sequencias sismicas foram reconhecidas (Sq1 – Sq5), limitadas por superficies erosivas de escala plataformal (superficies S1 – S5), interpretadas como sequencias deposicionais formadas por oscilacoes glacioeustaticas e limites de sequencias (discordâncias regionais) esculpidos durante longos periodos de exposicao subaerea total da plataforma, indicando condicoes de diminuicao de nivel de base e destruicao parcial de espaco de acomodacao sedimentar. As sequencias Sq1 – Sq4 sao dominantemente regressivas, compostas principalmente por prismas de regressao forcada de borda de plataforma/talude superior. A sequencia superficial Sq5 e constituida essencialmente de unidades retrogradantes restritas a plataforma continental, interpretadas como compondo uma sequencia eminentemente transgressiva. A correlacao entre dados sismicos e cronoestratigraficos de um poco exploratorio disponivel na area permitiu posicionar a deposicao desta sucessao estratigrafica entre o Pleistoceno Medio-Holoceno (ultimos ~ 500 ka). Alem disso, a correlacao entre a base de dados com curvas globais de variacoes eustaticas, baseadas nas razoes isotopicas de δ 18 O (estagios isotopicos marinhos), permitiu ainda sugerir que a sucessao estratigrafica Sq1 – Sq4 registra sequencias regressivas de quarta ordem, refletindo deposicao durante ciclos glacioeustaticos de ~ 100 – 120 ka de duracao e de alta amplitude de oscilacao eustatica (± 100 – 145 m), que caracterizam o sinal eustatico nos ultimos ~ 500 ka. A sequencia Sq5 seria uma sequencia ainda em formacao, constituida por depositos transgressivos e de mar alto formados durante o Pleistoceno Tardio-Holoceno.

The Pará-Maranhão and the Amapá Megaslide Complexes – Characteristics of Giant Submarine Landslides in the Foz do Amazonas Basin – Brazilian Equatorial Margin

Recently, through 2D seismic analyses, great megaslide deposits were identified in SE and NW steeper slopes of the Foz do Amazonas Basin. Now in this work, previous acquired seismic data was reinterpreted and time correlated with well data to better investigate two mass transport complexes (MTCs). These two MTCs are developed at steep slope settings adjacent to the Amazon Fan: to SE the Para-Maranhao Megaslide Complex and to NW the Amapa Megaslide Complex. Such deposits can mobilize thick siliciclastic series (up to 1,000 m thick) as a combination of debris flows, slided blocks and compressivelly-deformed allochtonous masses downslope over areas as large as thousands of km2, leaving behind large scars (as high as ~700m) that strongly impact the sea-bottom morphology. In the context of megaslides, the instability is most likely induced by levels of weak overpressured shales revealed by previous studies on gravity tectonics across the Foz do Amazonas Basin.