Rodrigo Perovano

Two-scale gravitational collapse in the Amazon Fan: a coupled system of gravity tectonics and mass-transport processes

Abstract: Gravity tectonics affects the entire marine sedimentary sequence of the Foz do Amazonas basin, including the topmost Amazon Fan. During the various stages of the margins evolution, the sedimentary loading and bathymetric slope induced the formation of a linked extensional–compressional system in which gravitational fold-and-thrust belts form significant structural features on the continental slope. Seismic analysis carried out in this study shows that sea-floor relief created along these thrust belts can trigger recurrent mass-transport deposits in the upper–middle Amazon Fan, suggesting a long-lasting linkage between gravity tectonics and mass-transport processes throughout the sedimentary evolution of the Amazon Fan. Supplementary material: Uninterpreted seismic sections are available at http://www.geolsoc.org.uk/SUP18393.

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.

Multiple Megaslide Complexes and Their Significance for the Miocene Stratigraphic Evolution of the Offshore Amazon Basin

The interpretation of a new and extended 2D seismic database on the offshore Amazon Basin (Foz do Amazonas Basin) confirms the widespread presence of regional-scale mass-transport deposits (MTDs) that are important architectural elements of the Amazon Deep Sea Fan. These MTDs were deposited since the late Miocene and extend throughout an area of nearly 315,000 km2. They are grouped into three megaslide complexes: the northwestern Amapa Complex, the Central Amazon Fan Complex and the southeastern Para-Maranhao Complex. Each complex has multiple stacked MTDs with various internal seismic facies that are indicative of large downslope modification and disruption of the original stratigraphy. The majority of the MTDs show chaotic or transparent internal seismic facies that we interpret as indicative of debris flow deposits. Although we cannot determine the exact triggering mechanism(s) for the various sediment failures, these events appear to be related to the gravitational compression of fold-and-thrust belts created by gravity-tectonic processes on the upper Amazon Fan and to structurally-induced mobilization of large blocks on the upper continental slope in response to overpressure along impermeable surfaces. These processes were apparently more active during the Pleistocene in response to increased sedimentation rates on the fan.

The Chuí Megaslide Complex: Regional-Scale Submarine Landslides on the Southern Brazilian Margin

The Brazilian Continental Shelf Survey Programme (LEPLAC) identified the occurrence of large-scale mass-transport deposits on the southernmost limit of the Brazilian margin, based mainly on analyses of acoustic imagery. The mass-transport deposits, named the Chui Megaslide Complex, comprise a stack of large translational slides that spread from the shelf break ~650 km downslope to ~4,900 m water depth, cutting into Pliocene-Quaternary sedimentary successions and strongly affecting both the margin morphology and regional depositional processes. The main headwall scarp is U-shaped, 400–500 m high, and extends c. 80 km downslope as a large elongated evacuated scar, 50–85 km wide. Outside this main failure scar, external scarps evidence a large area of erosion and faulted blocks, indicating ongoing retrogressive sediment disruption. Slide masses occur as a combination of variably deformed failed masses and debris flows, covering an area of ~150,000 km2. Main preconditioning parameters and the possible triggering mechanism for the Chui Megaslide Complex are likely a combination of a series of causative factors such as slope failure structurally-induced by gravity tectonics and high sediment influx into the shelf-edge and upper slope during the Early Miocene-Quaternary.

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.