The role of inheritance in forming and deforming rifted margins and creating collisional orogens
GIANRETO MANATSCHAL – Université de Strasbourg
A long-standing question in Earth Sciences is related to the importance of inheritance in controlling the rheology and deformation history of tectonic systems. In contrast to physical processes that are generally applicable, assessing the role of inheritance suffers from two major problems: firstly, it is difficult to appraise without having insights into the history of a geological system; and secondly, all inherited features are not reactivated during subsequent deformation phases. Therefore, the aim of the presentation is to provide a conceptual framework about how inheritance may control the architecture and evolution of rift systems and their reactivation during subsequent convergence in collisional orogens.
Seaward dipping reflectors along the South Atlantic margins - How do they form, and what implications do they have for hydrocarbon exploration?
IAN DAVISON (PhD) – Director of Earthmoves Ltd.
This talk will examine seismic and well data from the Sergipe-Alagoas and Gabon conjugate margins where seaward dipping reflectors have been observed, and also the major SDR sequences in S. Brasil-Argentina and Namibia-S Africa conjugate margins. The talk will be illustrated with recently acquired high-resolution seismic data and will discuss the possible mechanisms for the formation of seaward dipping reflectors and their composition using well and seismic and outcrop data. Additionally, review the evidence for good shallow marine source rocks developed just above the SDR sequence on the Gabon and Namibian margins, which has implications for good hydrocarbon source rocks on the South American margin.
Evolution of tonian ophiolites in the Brasiliano Orogen
LÉO AFRANEO HARTMANN – Professor Emérito, Universidade Federal do Rio Grande do Sul
Modern, post-Penrose concepts include subduction-related and -unrelated ophiolites. A significant advance in understanding ophiolite evolution in the Brasiliano Orogen is the dating of zircon in metasomatites integrated with the description of aeromagnetometry and aerogamaspectrometry. Widespread occurrences in juvenile terranes in several belts of the orogen led to focusing on Tonian oceanic crust and mantle fragments in Sul-Riograndense Shield. U-Pb dating of oceanic zircon (ɛHf ca. +12, U/Yb <0.1) from chloritite, rodingite, albitite and tourmalinite established the initial rifting of Rodinia at 920 Ma, succeeded by serpentinization events in the oceanic crust, youngest at 720 Ma. Unzoned tourmaline formed in the open ocean (proto-Adamastor) (δ 11ca. +5 and 0) and continent (-8.5) associated with serpentinite and rocks similar to listvenite. Aerogeophysics supports geological mapping and establishes the three-dimensional structure of intensely deformed ophiolites. The beginning of continental collision at 700 Ma is established for SW Gondwana from the youngest age of oceanic crust.
Constraining heat flow, subsidence/uplift and sedimentation histories of rifted margins
MARTA PÉREZ-GUSSINYÉ – University of Bremen
The subsidence and thermal history of margins has been the matter of much debate. Many of this debate revolts around enigmatic observations suggesting that subsidence in the syn- as well as post-rift history of margins does not comply with the Mckenzie model. For example, some margins show too little syn-rift strata, deposited as faults are active, and too much sag strata, deposited soon after fault activity ceases, with an age which pre-dates break-up. Also, borehole and seismic data from many margins like the Atlantic have been interpreted to suggest that the upper surface of the crust remains close to sea-level during rifting, inconsistent with its thickness, and then it quickly subsides as the transition to oceanic spreading occurs. In this work we use numerical models which couple surface processes, i.e. sedimentation and erosion, to plate deformation to investigate the tectonic subsidence, basement heat-flux and thermal history of margins. We systematically simulate different margin architectures, analyze their syn- and post-tectonic sediment distribution and apply back-stripping techniques to the models. Our aim is to use the numerical models as a natural laboratory to elucidate the processes by which enigmatic observations at margins may occur. Our results indicate that 1) in all cases the deformation laterally migrates with time towards the future ocean. The time at which this migration starts depends on the strength of the crust. Rift migration generates thinner than expected syn-rift sequences and sag sequences that predate break-up. 2) The depth reached at the end of syn-rift is larger, and the amount of thermal subsidence during post-rift much smaller than in Mckenzie models. The larger syn-rift depth is related to the strength of the crust and to the slow velocities imposed in our model, both of which are not taken into account in the Mckenzie model. Strong models present more synrift subsidence than weak ones. The lack of thermal subsidence is most probably due to thermal insulation by the sedimentary pile and the slow rifting velocities modelled here which allow to diffuse heat during the synrift. 3) The influence of the sedimentary thickness on top of basement is as important in determining basement temperature in the syn- and post-rift periods, as the structural position of the analyzed location within the evolving rift.
Suturing Gondwana in the Cambrian – the orogenic events of the final amalgamation
RENATA SCHMITT – Universidade Federal do Rio de Janeiro
Gondwana was consolidated in the late Cambrian after 180 m.y of tectonic convergence among Neoproterozoic paleocontinents and smaller fragments. We present a compilation of 55 orogens that record its final amalgamation. Collisional events are registered by the metamorphic peak assemblages and contractional deformational structures. Two main periods of orogenic activity are recognized. The first at ca. 670-575 Ma includes few orogens (ca. 15) but over larger areas. During this stage, the Saharan, West African, São Francisco-Congo and Paranapanema paleocontinents, along with the Arabian Nubian shield that consisted of juvenile Tonian terranes and some East African Orogen microcontinents were accreted to form the proto-Gondwana core. The second stage, at 575-480 Ma, incorporated more orogens, ca. 40, that sutured the Amazonia, Rio de La Plata, Kalahari, Dhawar, East Antarctica and Australian paleocontinents. The collisional orogens pattern throughout both western and eastern Gondwana is similar, indicating that although Gondwana was built up by the convergence of distinct paleocontinents, their approximation might be orchestrated by global geodynamics. In SW Gondwana, opening of ca. 610-570 Ma basins, some with oceanic crust, coincides with the suturing of the proto-Gondwana core. They were rapidly formed but even faster inverted during the second and last 575-480 Ma collisional stage, represented by the major E-W Kuunga and N-S South Atlantic belts. We propose that the 570-500 Ma collision of the Damara-Lufilian-Zambesi belt was coeval with the Cuchilla Dionisio-Saldania-Gariep- Dom Feliciano-Kaoko- Ribeira-Cabo Frio orogens. This South Atlantic orogenic system consists of the main SW Gondwana suture, reactivated 350 m.y. later to form the South Atlantic Ocean. The suture is preserved by old orogenic high pressure and oceanic derived rocks on the actual conjugate continental margins.
Crustal delamination in the Andes: different processes and settings
VICTOR ALBERTO RAMOS – Universidad de Buenos Aires
The last decade displayed through geophysical and geological studies the occurrence of delamination and foundering in specific segments along the Andes. The geological evidence has shown different processes and diverse petrological suites as products of the crustal and lithospheric delamination. Hugh rhyolitic plateau, minor monogenic mafic rocks, as well as large basaltic floods, may alternate in the different tectonic segments. A review of the delamination process and tectonic settings reveals the major geologic facts associated with different lithological suites.