Gianreto Manatschal

Evolution of magma-poor continental margins from rifting to seafloor spreading

The rifting of continents involves faulting (tectonism) and magmatism, which reflect the strain-rate and temperature dependent processes of solid–state deformation and decompression melting within the Earth. Most models of this rifting have treated tectonism and magmatism separately, and few numerical simulations have attempted to include continental break-up and melting, let alone describe how continental rifting evolves into seafloor spreading. Models of this evolution conventionally juxtapose continental and oceanic crust. Here we present observations that support the existence of a zone of exhumed continental mantle, several tens of kilometres wide, between oceanic and continental crust on continental margins where magma-poor rifting has taken place. We present geophysical and geological observations from the west Iberia margin, and geological mapping of margins of the former Tethys ocean now exposed in the Alps. We use these complementary findings to propose a conceptual model that focuses on the final stage of continental extension and break-up, and the creation of a zone of exhumed continental mantle that evolves oceanward into seafloor spreading. We conclude that the evolving stress and thermal fields are constrained by a rising and narrowing ridge of asthenospheric mantle, and that magmatism and rates of extension systematically increase oceanward.

A mechanism to thin the continental lithosphere at magma-poor margins

Where continental plates break apart, slip along multiple normal faults provides the required space for the Earths crust to thin and subside. After initial rifting, however, the displacement on normal faults observed at the sea floor seems not to match the inferred extension. Here we show that crustal thinning can be accomplished in such extensional environments by a system of conjugate concave downward faults instead of multiple normal faults. Our model predicts that these concave faults accumulate large amounts of extension and form a very thin crust (< 10 km) by exhumation of mid-crustal and mantle material. This transitional crust is capped by sub-horizontal detachment surfaces over distances exceeding 100 km with little visible deformation. Our rift model is based on numerical experiments constrained by geological and geophysical observations from the Alpine Tethys and Iberia/Newfoundland margins. Furthermore, we suggest that the observed transition from broadly distributed and symmetric extension to localized and asymmetric rifting is directly controlled by the existence of a strong gabbroic lower crust. The presence of such lower crustal gabbros is well constrained for the Alpine Tethys system. Initial decoupling of upper crustal deformation from lower crustal and mantle deformation by progressive weakening of the middle crust is an essential requirement to reproduce the observed rift evolution. This is achieved in our models by the formation of weak ductile shear zones.

Tectonosedimentary evolution of the deep Iberia-Newfoundland margins: Evidence for a complex breakup history

Most of the conceptual ideas concerning sedimentary architecture and tectonic evolution of deep rifted margins are based on either intracontinental rift basins or proximal margins, both of which underwent only small amounts of crustal thinning. In this paper, we investigate the tectonosedimentary and morphotectonic evolution related to continental breakup of the highly extended, deep Iberia-Newfoundland margins. Our results show that continental breakup is a complex process distributed in time and space. On the basis of mapping of dated seismic units and borehole data we are able to identify two major phases of extension. During a first phase, dated as Tithonian to Barremian (145–128 Ma), deformation is related to exhumation of mantle rocks; basins become younger oceanward, and fault geometry changes from upward to downward concave resulting in complex sedimentary structures and basin geometries. A second phase, dated as latest Aptian (112 Ma), overprints previously exhumed mantle and accreted juvenile oceanic crust over more than 200 km leading to the formation of basement highs. The observed complex breakup history challenges classical concepts of rifting and leads to new interpretations for the tectonosedimentary evolution of deep rifted margins.

Kinematics of Jurassic rifting, mantle exhumation, and passive-margin formation in the Austroalpine and Penninic nappes (eastern Switzerland)

The Austroalpine and Upper Penninic nappes in eastern Switzerland represent a passive continental margin and the adjacent ocean of Jurassic-Cretaceous age, imbricated by Late Cretaceous-Tertiary orogenic shortening. Well-preserved, rift-related faults allow reconstruction of the passive margin and ocean-continent transition zone and yield new information on the kinematics of rifting. Rifting evolved from pure-shear stretching to detachment-controlled, asymmetric stretching and resulted in complete exhumation of subcontinental mantle rocks at the sea floor. After precursory normal faulting in the Late Triassic, Jurassic rifting occurred in two phases. During the first rifting phase (Hettangian-Sinemurian), predominantly east-dipping normal faults developed in the upper crust; their dips decreased in the middle to lower crust, where they probably graded into anastomosing shear zones in the lower crust and mantle lithosphere. The resulting overall geometry approximated pure-shear stretching. During the second rifting phase (Toarcian-Middle Jurassic), a system of west-dipping detachment faults formed, penetrating the whole lithosphere and accommodating asymmetric extension. During progressive stretching, subcontinental mantle rocks were tectonically exhumed and exposed at the sea floor in two areas, represented by the Platta and Malenco nappes (Penninic). The intervening Margna and Sella continental nappes are interpreted as an extensional allochthon belonging to the Apulian margin. Finally, a mid-ocean ridge may have formed west of the Margna-Sella allochthon. The Austroalpine realm thus represents the lower-plate margin—and the Brianconnais, the upper-plate margin-of the Piemont-Liguria ocean. This scenario is in qualitative agreement with the subsidence histories of the two margins.

Architecture and tectonic evolution of nonvolcanic margins: Present-day Galicia and ancient Adria

A comparison of the reconstructed southeastern margin of the Tethys ocean with the present-day Galicia margin shows that although both margins are of different age and had a different fate, their architectures and tectonic evolutions are very similar. Along both non-volcanic margins the site of rifting shifted from a broad area in the future proximal margins to a localized area in its distal parts, accompanied by a change in the mode of extension. During the initial phase of rifting, extension was accommodated by symmetrically arranged listric faults which soled at midcrustal levels, indicating that deformation in the upper crust was decoupled from deformation in the upper mantle along a hot and weak lower crust. During advanced rifting, extension was dominated by simple shear along low-angle detachment faults with a top-to-the-ocean sense of movement. These shallow crustal structures formed a series of breakaways in the continental crust and cut into mantle rocks, indicating that now deformation in the upper crust and in the upper mantle was no longer decoupled. Cooling and strengthening of the lower crust during an initial stage of rifting apparently led to localization of deformation and a different style of deformation, documenting that the tectonic evolution of nonvolcanic margins is largely controlled by the thermal state of the lithosphere. Seafloor spreading initiated only after exhumation and exposure of the subcontinental mantle on the ocean floor and may have been accompanied by a loss of the yield strength of the upper mantle, due to a combination of simple shear extension, asthenospheric uplift, and increased melt production.

Exhumed mantle-forming transitional crust in the Newfoundland-Iberia rift and associated magnetic anomalies

Transitional zones located between Iberia and North America formed during continental rifting and mostly consist of exhumed mantle. In this study we show that ages of exhumed mantle at Ocean Drilling Program (ODP) sites 1068 and 1070 in the Iberia Abyssal Plain and site 1277 in the Newfoundland Basin are similar to ages determined from magnetic lineations created by serpentinization during mantle exhumation. On the basis of paleomagnetic and geological data and a comparison with a fossil ocean-continent transition in the Alps, we envisage a first episode of mantle serpentinization during which a strong component of magnetization was acquired followed by a second episode occurring at the contact with cold seawater, and which only affects the upper tens of meters of the exhumed mantle. The inversion of magnetic data (Euler deconvolution) shows that magnetic sources are N-S trending horizontal cylindrical bodies located within the highly serpentinized upper crust. Therefore the serpentinization process is able to produce magnetic lineations formed in a similar way to those formed by seafloor spreading. Within transition zones, sequences of magnetic anomalies can provide information concerning the timing of the emplacement of crust, but not on its nature (oceanic versus exhumed mantle). This discovery enables us to date the exhumation of mantle rocks in transition zones and allows kinematic reconstructions of the final stages of continental rifting. During rifting the deep distal continental margins and the adjacent transitional zones in the Newfoundland-Iberia rift system were formed by ultraslow extension from early Berriasian to late Valanginian–early Hauterivian and by slow extension from early Hauterivian to the late Aptian–early Albian boundary. Therefore transitional zones share many similarities with slow and ultraslow spreading midoceanic ridges

Formation and deformation of hyperextended rift systems: Insights from rift domain mapping in the Bay of Biscay‐Pyrenees

The Bay of Biscay and the Pyrenees correspond to a Lower Cretaceous rift system including both oceanic and hyperextended rift domains. The transition from preserved oceanic and rift domains in the West to their complete inversion in the East enables us to study the progressive reactivation of a hyperextended rift system. We use seismic interpretation, gravity inversion, and field mapping to identify and map former rift domains and their subsequent reactivation. We propose a new map and sections across the system illustrating the progressive integration of the rift domains into the orogen. This study aims to provide insights on the formation of hyperextended rift systems and discuss their role during reactivation. Two spatially and temporally distinct rift systems can be distinguished: the Bay of Biscay-Parentis and the Pyrenean-Basque-Cantabrian rifts. While the offshore Bay of Biscay represent a former mature oceanic domain, the fossil remnants of hyperextended domains preserved onshore in the Pyrenean-Cantabrian orogen record distributed extensional deformation partitioned between strongly segmented rift basins. Reactivation initiated in the exhumed mantle domain before it affected the hyperthinned domain. Both domains accommodated most of the shortening. The final architecture of the orogen is acquired once the conjugate necking domains became involved in collisional processes. The complex 3-D architecture of the initial rift system may partly explain the heterogeneous reactivation of the overall system. These results have important implications for the formation and reactivation of hyperextended rift systems and for the restoration of the Bay of Biscay and Pyrenean domains

Hyper-extended crust in the South Atlantic: in search of a model

ABSTRACT The discovery of giant hydrocarbon reservoirs in the pre-salt sequence of the deep-water Brazilian rifted margin together with the new acquisition of high-quality reflection and refraction seismic surveys across many rifted margins worldwide has attracted the interest of industry and researchers to deep-water rifted margins. For the first time, the new data sets enable the imaging and description of the pre-salt structures, which indicate that deep-water rifted margins are very different from what classical models had predicted thus far. Instead of the expected fault-bounded basins and a sharp ocean–continent boundary, the new data suggest the existence of a sag basin lying on hyper-extended crust with little indication for brittle high-angle faulting, a transitional domain between continental and oceanic crust showing neither characteristics of oceanic nor continental material, and very asymmetrical distal conjugate rifted margins. These observations raise significant doubts on the validity of the classical concepts used in rheology, mechanics and isostasy to explain extensional systems leading to seafloor spreading. They also require new concepts and more data in order to understand how these rifted margins evolved in time and space. This has important implications for the exploration and evaluation of petroleum systems in the frontier areas of hydrocarbon exploration. In this study we publish two multi-channel seismic sections across the Angola and conjugate Brazilian rifted margins that we consider as ‘type’ sections for hyper-extended magma-poor rifted margins in the South Atlantic. The aim of this study is to discuss various possible interpretations and models to explain the high-resolution seismic images presented in this paper.

The rift-to-drift transition in the North Atlantic: A stuttering start of the MORB machine?

We report U-Pb and 39Ar-40Ar measurements on plutonic rocks recovered from the Ocean Drilling Program (ODP) Legs 173 and 210. Drilling revealed continental crust (Sites 1067 and 1069) and exhumed mantle (Sites 1070 and 1068) along the Iberia margin and exhumed mantle (Site 1277) on the conjugate Newfoundland margin. Our data record a complex igneous and thermal history related to the transition from rifting to seafloor spreading. The results show that the rift-to-drift transition is marked by a stuttering start of MORB-type magmatic activity. Subsequent to initial alkaline magmatism, localized mid-oceanic ridge basalts (MORB) magmatism was again replaced by basin-wide alkaline events, caused by a low degree of decompression melting due to tectonic delocalization of deformation. Such “off-axis” magmatism might be a common process in (ultra-) slow oceanic spreading systems, where “magmatic” and “tectonic” spreading varies in both space and time.

The role of detachment faulting in the formation of an ocean-continent transition: insights from the Iberia Abyssal Plain

Abstract The Iberia Abyssal Plain segment of the West Iberia margin was drilled during Ocean Drilling Program Legs 149 and 173 and has been extensively studied geophysically. We present new microstructural investigations and new age data. These, together with observed distribution of upper- and lower-crustal and mantle rocks along the ocean-continent transition suggest the existence of three detachment faults, one of which was previously unrecognized. This information, together with a simple kinematic inversion of the reinterpreted seismic section Lusigal 12, allows discussion of the kinematic evolution of detachment faulting in terms of the temporal sequence of faulting, offset along individual faults, and thinning of the crust during faulting. Our study shows that the detachment structures recognized in the seismic profile became active only during a final stage of rifting when the crust was already considerably thinned to c. 12 km. The total amount of extension accommodated by the detachment faults is of the order of 32.6 km corresponding to a β factor of about two. During rifting, the mode of deformation changed oceanwards. Initial listric faulting led to asymmetric basins, accommodating low amounts of extension, and was followed by a situation in which the footwall was pulled out from underneath a relatively stable hanging wall accommodating high amounts of extension. Deformation along the latter faults resulted in a conveyor-belt type sediment accumulation in which the exhumed footwall rocks were exposed, eroded and redeposited along the same active fault system.