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ABSTRACT

The main objective of the project is the reconstruction of the processes for mass transfer in the convergent margins by the comparison between present-day and fossil examples. This program includes researchs of structural geology, tectonics and stratigraphy in the units representative of different parts of accretionary wedges. The research will be focused on the processes related to the mass transfer from lower plate to accretionary wedge by frontal accretion at shallow structural level, by underplating at medium to deep structural level, by tectonic erosion, by exhumation of previously accreted material.These processes will be
analyzed to determine how mass fluxes change when physical boundary conditions of the margins (thickness, rate and direction of convergence vector) also change, through which tectonic processes these changes occur, how material is transferred to the interior
of the prism up to the surface and how those processes influence the upper plate equilibrium and its evolution. This project includes 5 UO (research teams). The UO I from Pisa University aims to study the structural features of mass tranfer process by accretion al
medium and shallow structural level. The UO II from Genova University is devoted to study the structural features of mass tranfer process by accretion at deep structural level and their subsequent exhumation up to the surface. The UO III from Modena University aims to define the possible causes, mechanisms and effects of the alternanace betewen frontal erosion and frontal tectonic erosion. The UO IV based in Florence will be focused on the stratigraphical analyses of the successions involved in an accretionary wedge in
order to outline the volumes and mechanisms active during mass transfer of sediments.The UO V from Bologna University aims to define the paleogeothermal gradient during the accretion-related structural history in different units involved in the accretionary
processes at shallow level. The program will extend over two years. For the first year the four UO will involved the detailed geological mapping of selected transects in well exposed areas that will be associated to the collection of all the structural data recording the progressive deformation during accretion The second year will be mainly devoted to laboratory analysis of the collected samples..

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KEY WORDS:

CONVERGENT MARGINS ; SUBDUZIONE ; ACCRETION ; TECTONIC EROSION ; EXHUMATION ; TECTONICS ; STRUCTURAL EVOLUTION ; METAMORPHISM ; TECTONIC- SEDIMENTATION RELATIONSHIPS

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PROJECT RESPONSIBLE

MARRONI MICHELE

Full Professor in Structural Geology, Dipartimento di Scienze della Terra, Università di Pisa

marroni@dst.unipi.it

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PROJECT AIMS

The proposed project aims to study and understand the mass fluxes in convergent margins through the analyses and the comparison between fossil and active margins. We will try to address two fundamental science themes:
1) how do mass fluxes change when physical boundary conditions of the margins - thickness, rate and direction of convergence
vector - also change? Through which tectonic processes do these changes occur?
2) How material is transferred to the interior of the prism up to the surface? How those processes influence the upper plate
equilibrium and its evolution?
The project focuses on the investigation of key areas selected in peculiar fossil complexes (Northern Apennines, Northern Corsica, Northern California and Alaska), combining the field work, devoted to the reconstruction of the geodynamic and structural evolution and to the definition of the tectonic and sedimentation relationships, with analytical techniques. The reconstructed processes will be then verified and compared with those analyzed in the Costa Rica margin, chosen as an active analogue.
Main criteria to select key examples comprise high degree of preservation and exposuere of thicvk portions of prism, easy access, boundary conditions variation along the complex, possibility of making detailed analyses in tranects across the complex, good level of geological background database.
The project is arranged between the different research units so that all the different aspects of the mass cycle will be investigated, from the input of material into the system, to the lower-upper plates mass exchange (accretion and tectonic erosion processes), up to the upward migration of material to the surface. Particularly the following aspects will be analyzed:
- how do mass flux and tectonic processes during accretion change when the sediment thickness of the lower plate changes;
- relationship between physical conditions of sediments (degree of compaction, lithification and diagenesis) and deformation
partitioning (mélange vs. coherent units);
- analysis of the frontal part of the prism where unlithified sediments from the lower plate are transferred to the upper plate through frontal accretion. The frontal zone also represents the most unstable part of the prism with gravitative deposits due to fluid pressure activity and to dynamic re-equilibrium phenomena of the prism. In this part of the prism occur most of the material recycling of the upper plate, and here the same material can be transferred to the trench and be involved again in the subduction and accretion processes;
- analyses of the underplating processes, that is those processing comprising transfer of lithified material from the lower to the upper plate at intermediate to deep levels. At the intermediate level we will also try to decipher tectonic erosion processes, that can be hard to identify because they do not usually leave traces into fossil margins.
- analysis of the exhumation paths and the associated deformations. As mentioned before key areas have been selected so that they represent different boundary conditions, show good outcrops and a well documented geological background, and, above all, they cover all the above cited different aspects.
The different localities selected in the Northern Apennines, for example, are representative of accretion at different structural levels, from shallow to medium-deep, and record both frontal accretion and underplating. Recently has been documented in the Northern Apennines the occurrence of tectonic erosion episodes, that would alternate in time with episodes of accretion.
The shallow to intermediate level accretion recorded in the Northern California and Alaska complexes is characterized by different structural features from that of the Apennine (e.g. thicker and well developed mélange units), moreover, the Californian margin is also representative of subduction of a sediment-starved type oceanic plate. The comparison between these three different margins allow the understanding of the sedimentary input influence on prism evolution.
The units outcropping in the Northern Corsica represents some of the deepest units accreted to the margin during alpine subduction, showing metamorphic paragenesis that can reach the eclogitic facies. The reconstruction of their evolution can help to quantify the percentage of deep underplating for the mass balance calculation of the alpine-appenninic system, as well as to define the deep mechanisms of deformation.
The Tertiary prism exposed in the Osa Peninsula (Southern Costa Rica) represents a portion of the inner slope, of the active Costa Rica subduction, exposed above sea level, so it is a perfect chance to look at a portion of the margin usually under the sea level. The peninsula is a natural laboratory where slope instability processes and turbiditic and hemipelagic sedimentation are strongly connected to the subduction of punctual anomalously thick oceanic crust (i.e. seamounts). The analysis of these deposits allow the definition of the morphological and sedimentary effects of these processes on the margin, and also how the margin can recover the dynamic equilibrium. These deposits also record transition of tectonic processes from accretion to tectonic erosion episodes.
At the end of the project we expect to define and constrain all the physical processes determining mass flux in the selected analogues, and, more general, in convergent margins. The main goal is to summarize all the proposed kinematic models and propose an unique dynamic model addressing how mass flux during subduction and exhumation may vary in space in time, and constraining precisely the possible factors interplaying during continental crust growth. This experiment, although conducted in active margins, has never been attempted in fossil analogues. The "Subduction Factory" experiment, part of the American NSF-"Margins" project, focuses on the study of subduction zones mass cycles, essentially from a geochemical point of view. These kind of projects, although fundamental for the geological knowledge, are extremely expensive and essentially based on indirect observations (GPS, geophysical surveys etc.). The project we propose will combine field and laboratory works to give e modern key-lecture of the processes accompanying convergence, and can be used as a reference model while studying mass cycles and mass balance, with important consequence on the understanding of processes as seismicity and volcanism.

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PREVIOUS STUDY

Convergent margins are characterized by movement of material: sediments, rocks and fluids exchanges between upper and lower plates, mass movement in and above the upper plate, subduction of lithosphere under the mantle, magmatic products coming from arc volcanism (MARGINS, 2004 and references therein). In fact, the subduction process either builds new crust through arc volcanism or, mechanically through the piling up of sedimentary deposits and fragments of crustal material in structures known as accretionary prisms (Auboin, 1989; Moore & Sample, 1986; Sample & Fisher, 1986; Sample & Moore, 1987; von Huene & Scholl, 1991; Davis, 1996). During the same process crustal material can be removed by the prism by tectonic erosion processes (von Huene & Scholl, 1991; Fisher et al., 1998; Ranero & von Huene, 2000; Vannucchi et al., 2003; Fisher et al. 2004). So, material can either be re-cycled to the prism, after being metamorphosed at different structural levels, or be subducted down to the deep mantle, or be transferred from the mantle to the crust. Moreover, the shallowest part of the prism is characterized by deposition of gravity-driven, terrigenous and detrital deposits (e.g. turbidity currents, debris flows) coming from the erosion of the continental block (eg. the siliciclastic turbidites) and from the remodelling of the accretionary prism during the evolution of the convergent system (Di Marco et al. 1995; Marroni et al., 2001; Vannucchi et al., 2003). These deposits can also be recycled into the system and participate to the subduction cycle.

This complicated mass circulation is associated to energy exchange, so that subduction and collision are characterized by seismic and volcanic activity, and to movement of elements that enrich the earth, the oceans and the atmosphere around us (MARGINS, 2004). An important potential new form of energy, the gas hydrates, is generated mainly by the convergent margins. Finally, although only a little percentage of continental growth is due to non magmatic processes, as accretion and exhumation, must of Italian surface and its basement are the results of mass exchange between two different plates. These processes have a direct impact on society because they cause part of the seismic and volcanic phenomena affecting, for example, Central and Southern Italy. A better recognition of those processes is needed in order to prevent and control natural risks, and for a better utilization of natural resources.

Studies of mass balance calculations on world active margins (von Huene & Scholl, 1991) suggest that half of the total volume of oceanic deposits entering the systems is transferred to the prism (either by frontal accretion or underplating), while the other 50% bypass the prism and is subducted to depth where he reach the mantle modifying its composition and rheology. This percentage can change if we are referring to non-accretionary (e.g. Middle America Trench, Northern Chile) or accretionary margins with small (e.g. Northern Japan, Peru and Aleutians) and big wedges (Barbados). From these studies on active margins comes also the complexity of quantifying precisely accretion and subduction, due essentially to the difficulty of discerning frontal accreted from underplated material, as well as to separate material underplated at depth from that bypassing completely the prism. This complexity comes from the high number of factors controlling and influencing mass flux and dynamic equilibrium of convergent margins (MARGINS, 2004 and reference therein):

- convergence vector controls the rate at which subduction processes operates, through controlling sediment and fluid flux into the system. Convergence vector also influences distribution and features of shear zones, the energy released as heat and seismicity and deformation partitioning;

- age and temperature of subducting crust. Old lithospheres are thick and cold, leading to development of an entirely different thermal structure then young, hot lithospheres and influencing metamorphism and composition and migration of fluids;

- upper plate thickness has a direct control on thermal gradient and overburden;

- type and quantity of material convoyed to the trench by the lower plate determine mass distribution and composition on the upper plate (Moore & Sample, 1986). The behavior of material through the upper 40 km of the subduction zone, the one we expect to investigate in this project, is intimately linked to the nature of incoming sediment and rock sequence, its compaction dewatering diagenesis and cementation, fore-arc deformation and the nature of the seismogenic zone;

- topographical and structural features of the lower plate (presence of negative/positive structures) can cause gravitative instabilities on prism and lead, finally, to tectonic erosion (Fisher et al., 1998; Fisher et al. 2004);

- the nature of fluid entering the prism controls heat and mass transfer, sediment deformation, mineral transformation, mantle

- nature of sediments and fluid overpressure influence also plate decoupling, and, consequently the state of stress along the décollement (Moore et al., 1987).

The interaction between all the different factor determine the complex evolution of the margins, so that, for example, although a low convergence rate and a high input of sediments usually lead to big accretionary prisms, the same big input of sediments cause the décollement to migrate into the sedimentary sequence, suggesting that big wedges are also associated to a high volume of subducted sediments. The complexity of mass flux in different convergent margins is accompanied by a profound structural diversity along a single margin (von Huene, 1984 and reference there in). In fact, although the same margin is presumably characterized by the same boundary conditions, most of the known margins show variation in space and time of tectonic and accretionary processes, magmatic activity and associated products, as well as a different topographic and geomorphologic expression of the margin.

The complex evolution of material in convergent margins is completed by all the processes completing the cycle, that is those leading to upward migration and exhumation of material from the various depth of accretion up to the surface. During their upward migration, the rocks, already deformed and metamorphosed during underthrusting and underplating, are involved into different episodes of deformation and suffer a metamorphic re-equilibration to the different conditions. Timing, way and structural mechanisms of mass transfer during exhumation are some of the least known aspects of the evolution of accretionary prisms: only recently radiometric data have highlighted how rapid the exhumation processes can be (Amato et al., 1999; Rubatto & Hermann, 2001), with exhumation rates in the order of some cm/a. Although the comprehension of these phenomena is necessary to define type and rate of continental growth, data and precise modelling of the upward migration are still lacking, and we still need to identify what kind of investigation we need to characterize the nature and the paths of the exhumation processes into the prism. Some attempts of mass balance calculation in active margins have been proposed by the American, NSF-funded project, known as

"Subduction Factory" (MARGINS, 2004). The assumption is that mass flux is accompanied by chemical elements flux, so element fluxes can be used to determine mass movement. If one knows the starting and final compositions and the partition coefficient of some geochemical tracers, the mass fraction can be calculated. In fossil prisms, while peak metamorphic conditions testify the maximum depth of subduction and the geothermal gradient, the relationships between metamorphic assemblages and deformations are the most used instruments used to unravel the metamorphic-structural evolution of the exhumed units. However, the conservation of the early metamorphic assemblages and of the coeval deformative fabrics strongly depends on the way they are exhumed, influencing strongly the error of estimation we can made. The main factor controlling the upward migration of material and the conservation of data is rate of exhumation: fast exhumation allows the metastable conservation of HP-LT assemblages and of related early structures, whereas slow exhumation enhances the development of retrogressive metamorphism and related deformation, that overprint and obliterate the early structures and HP-assemblages. Once the exhumation rate is established, we still need to address the question "what determine exhumation rate?".

Deformation rate and typology of associated deformation processes also influence metamorphic re-equilibration: grainsize reduction and mylonitic texture development increase the reaction surface, whereas the development of schistosity favours the circulation of fluids (another kinetic factor) in the rock volumes. The strain distribution in the exhuming rock volumes is important too: the concentration of strain in thin shear zones can preserve undeformed rock volumes, with no metamorphic retrogression, in which the early stages of the tectono-metamorphic evolution can be recognized. Another important factor is the reactivity of chemical systems, which react with different rates following the variations of pressure and temperature conditions.

Summarizing, here are some of the fundamental questions that still need an answer:

- what are the chemical, physical and tectonic factors controlling transfer, circulation and exhumation of material in accretionary wedges? How do those factors act?

- what causes the time and space alternation of accretion and tectonic erosion episodes, and what are the consequences on wedge dynamic equilibrium?

- what causes the spatial migration of the décollement zone, that, during time, seems to cut the subduction plate stratigraphy at different levels?

The project aims to address most of these questions through the analysis of fossil prisms. The results will allow a direct comparison with those coming from a parallel study conducted on active margins by the American researchers participating to the NSF-funded "Subduction Factory" project. The project is analyzing all the processes responsible to mass and elements circulation on selected margins (Central America and western Pacific/Izu-Bonin-Mariana system). Although active margins allows a precise estimate of boundary conditions and the monitoring of continuous activity, the investigation of fossil margins is able to provide the identification of physical properties variation and the tectonic responses, and offer the possibility of a direct analysis of involved rocks. More over, in the fossil margins we have access to the deepest portions of the system, so we can make a more complete mass balance, because we can quantify the percentage of underplating.

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