Una nuova Classe di Limiti di Placca: le Megatrasformi Oceniche

Responsabili di progetto

Marco Ligi, Sergey Skolotnev

Accordo

RUSSIA - RFBR-suspended - Russian Foundation for Basic Research

Bando

CNR/RFBR triennio 2018-2020 2018-2020

Dipartimento

Scienze del sistema terra e tecnologie per l'ambiente

Area tematica

Scienze del sistema Terra e tecnologie per l'ambiente

Stato del progetto

Nuovo

Proposta di ricerca

According to the theory of Plate Tectonics one of the three types of boundary between adjacent plates is the transform boundary, first introduced by Wilson (1965), where two lithospheric plates slide across each other, moving in opposite directions. Transform faults are observed commonly offsetting the axis of mid-ocean ridges; they are also observed in active regions at continent edges where they form complex zones of deformation (examples are the St. Andreas and the North Anatolian faults, in California and Turkey, respectively). Oceanic transform faults, especially large-offset ones, have been thoroughly investigated by different teams around the world (Bonatti et al. 1977; Bonatti 1978; Karson & Dick 1983; Sinha & Louden 1983; Detrick et al. 1984; Cormier et al. 1984; ten Brink & Brocher 1988; Tucholke & Schouten 1988; Pushcharovskj et al. 1989, 1991, 1995; Detrick et al. 1993; Wolfe et al. 1993; Bonatti et al. 1994; Mueller et al. 2000; Bonatti et al. 2005). These works show complex plate boundaries that could deform under the influence of far field stresses, especially changes in plate motion. It appears that not all transforms react equally to equivalent stress changes, suggesting the influence of parameters such as offset length, spreading rates and mantle heterogeneities (Fornari et al. 1989; Pockalny et al. 1997; Bonatti et al. 2003; Maia et al. 2016).
A team from the Institute of Marine Sciences (ISMAR) of the Italian National Research Council has acquired a strong experience in the study of oceanic transform boundaries. Much of this work has been carried out in collaboration with the Institute of Geology (GIN) of the Russian Academy of Sciences (RAS) in Moscow. Based on this experience we propose a joint Russian-Italian project to study a poorly known area of the Equatorial Atlantic, where the axis of the Mid Atlantic Ridge (MAR) is offset by a major transform system at ~7-8° N (Doldrums Fracture Zone). This work hopefully will add to our understanding of a new and important type of plate boundary that we defined as a "megatransform" plate boundary (Ligi et al., 2002).
The notion of mega-transforms applies to oceanic transforms with exceptionally long age offset, such as the Romanche (equatorial Atlantic) and Andrew Bain (South West Indian Ridge) transforms. Megatransforms display particularly complex morphologies, reflecting their evolution through time. Most oceanic transform boundaries consist of a single narrow (a few km) strike-slip zone offsetting two mid-ocean ridge segments. In contrast, the slow slipping Romanche and Andrew Bain megatransforms are characterized by a broad (>100 km) and complex multifault zone of deformation similar to that of some continental strike-slip systems. These studies have brought forward the complexity of these major plate boundaries and their role on fundamental processes building the oceanic lithosphere, such as fluid circulation, mantle exhumation and mantle flow. One question to be addressed concerns the way extra-long, slow-slip transform faults react to both far field and near field stress changes. It has been found that even minor changes in plate motions can induce stress changes at transforms, producing either extension or compression within the transform domain (Maia et al., 2016). Yet, how the length of the offset and the amplitude of the stress changes control the transform response is still poorly understood. Relative motion along mega-transforms involves the deformation of extraordinarily thick and cold lithosphere. Ligi et al. (2002) proposed that the extreme thickness of the lithosphere, hence its rheology, must be a factor in determining the unusual width and complex geometry of mega-transforms. They found that long age-offset (> 30 Ma) faults produce two major symmetrical faults joining the two ridge segments, with a lens-shaped area in between, as observed at the Romanche and Andrew Bain transforms. Sclater et al. (2005) also supported the basic concept that yield stresses associated with varying plate thicknesses around megatransforms control the type of deformation, arguing that whether or not the transform is in "transtension" or "transpression" has bearing on the stress field and that horizontal, as well as vertical stresses, need to be considered in yield stress calculations.
We plan to explore the Doldrums transform system, that offsets the MAR axis by about 600 km and includes three 40 km-long intra-transform spreading centres (ITRs). This transform system displays features similar to those of the St Paul system to the South, suggesting that the birth of intra-transform spreading centres in this part of the MAR may represent an evolutionary stage of mega-transforms. This region of the MAR is still poorly known; in order to close this gap, we propose to carry out multibeam, gravity, magnetics, reflection seismic and rock dredging surveys with the R/V Strakhov of RAS. We have considerable experience with this vessel, having sailed on it for several major expeditions in the Atlantic and Indian Oceans. The project will involve the following steps a) field work along the entire Doldrums transform, involving two expeditions within the first two years of the Project; b) processing the geophysical data obtained at sea; c) laboratory analytical program on a large collection of ocean floor rock samples, including mineralogy, major and trace element composition as well as Nd, Sm, Sr and Pb isotopic chemistry; d) rock age determinations by U/Pb isotope geochronology in zircons; e) theoretical modelling incorporating lithospheric rheology and regional stress regime in order to assess megatransform evolution through time; f) quantitative modelling of magma genesis and differentiation in crustal magma chambers beneath ITRs.
This study will provide a better understanding of geologically active zones characterized by intense seismicity and high casualty risk and property destruction, if located close to populated regions.

Obiettivi della ricerca

The scientific main goals of our study of the Doldrums transform system are:
1. Define the morphology and structural framework and compare both with other slow-slip megatransform faults and large continental strike-slip zones. We propose to fill in the existing scattered bathymetry and geophysical data (from cruises S6 and S9, R/V Strakhov) expanding coverage and rock sampling in order to include transform walls and ITRs. Once the detailed morphological and structural framework of the Doldrums system has been established, it will be compared to that of the better studied Romanche, Andrew Bain, St Paul, North Anatolian and San Andreas fault systems.
2. Determine rock composition within the transform domain and investigate how this megatransform fault systems affects mantle upwelling and melting along the Mid Atlantic Ridge. Given the very large total age-offset and the short length of ITRs, we expect a very strong "cold edge effect" at ridge-transform intersections with a reduced mantle melt production, extensive exposures of mantle-derived ultramafic rocks and limited outcrops of basaltic crust.
3. Assess if the cold thick lithosphere alone controls the history of a megatransform or if small oscillations of the rotation pole need to be considered.
4. Develop 3/4D numerical models, including both lithospheric rheology and regional stress regime to explain the observed morpho-tectonic structure and assess megatransform systems temporal evolution.

Ultimo aggiornamento: 19/04/2024