GEOLOGICAL JOURNAL Geol. J. (2009) Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/gj.1144 The processes of underthrusting and underplating in the geologic record: structural diversity between the Franciscan Complex (California), the Kodiak Complex (Alaska) and the Internal Ligurian Units (Italy) F. MENEGHINI 1,2*, M. MARRONI 1,3, J.C. MOORE 2, L. PANDOLFI 1,3 and C.D. ROWE 4 1Dipartimento di Scienze della Terra, Universita` di Pisa, Pisa, Italy 2Earth and Planetary Sciences Department, University of California at Santa Cruz, CA., USA 3C.N.R., Istituto di Geoscienze e Georisorse, Pisa, Italy 4Department of Geological Sciences, University of Cape Town, South Africa Existing studies on active subduction margins have documented the wide diversity in structural style between accretionary prisms, both in space and time. Together with physical boundary conditions of the margins, the thickness of sedimentary successions carried by the lower plate seems to play a key role in controlling the deformation and fluid flow during accretion. We have tested the influence of the subducting sedimentary section by comparing the structural style and fluid-related structures of four units from three fossil accretionary complexes characterized by similar physical conditions but different subducting sediment thicknesses: (1) the Franciscan Complex of California, (2) the Internal Ligurian Units of Italy and (3) the Kodiak Complex, Alaska. Subducting plates bearing a thick sedimentary cover generally result in coherent accretion through polyphase deformation represented by folding and thin thrusting events, while underplating of sediment-starved oceanic sections results in diffuse deformation and me´lange formation. These two structural styles can alternate through time in a single complex with a long record of accretion such as Kodiak. The parallel analysis of the selected analogues show that although the volume of sediments carried by the lower plate determines different structural styles, deformation is strongly controlled by injection of overpressured fluids during under- thrusting and accretion. Transient hydrofracturing occurs through the development of a system of dilatant fractures grossly parallel to the de´collement zone. Copyright # 2009 John Wiley & Sons, Ltd. Received 30 January 2008; accepted 30 October 2008 KEY WORDS subduction; underplating; me´lange; sediment thickness; hydrofracture; Franciscan Complex; Ligurian Units; Kodiak Complex 1. INTRODUCTION The study of active convergent margins reveals that accretionary prisms are continuously shaped by various tectono-metamorphic processes, in a complex circulation of masses and fluids comprising accretion, the growing of the prism by transfer of material from the downgoing to the upper plate; tectonic erosion, through which material previously accreted is removed from the prism base and underthrust at depth and, finally, exhumation that completes the cycle transferring the material in the prism interior to the surface. Since the advent of plate tectonic theory, different models have been proposed to explain the deformation processes active during accretion. Seismic reflection imaging and ocean drilling data from modern margins, as well * Correspondence to: F. Meneghini, Dipartimento di Scienze della Terra, Universita` di Pisa, via S. Maria, 53, 56126 Pisa, ITALY. E-mail: [email protected] Copyright # 2009 John Wiley & Sons, Ltd. f. meneghini ET AL. as field studies on ancient accreted complexes suggest an imbricate-thrust model for shallow accretion and horizontal prism growing (Karig and Sharman 1975 and references therein) and, at increasing depths, a vertical thickening of the prisms through underplating of internally coherent duplexes (Silver et al. 1985; Sample and Fisher 1986; Moore and Sample 1986; Kusky et al. 1997; Hashimoto and Kimura 1999) or by formation of a me´lange (Hsu 1968; Cowan 1974, 1985; Cloos 1982; Moore and Sample 1986; Moore and Byrne 1987; Kusky et al. 1997). The data available for modern margins have demonstrated that there is a wide structural diversity along the same margin both in space and time, both along and across the deformation front (von Huene 1984; von Huene and Scholl 1991; Le Pichon et al. 1992; Shipley et al. 1995; Maltman et al. 1997; Moore et al. 1998; Clift and Vannucchi 2004). A single margin can show accretionary and non-accretionary features on different transects across it (i.e. Middle America Trench, Aleutian margin), or can show different accretionary styles and different prism dimensions, depending on the distance from sedimentary sources (i.e. Lesser Antilles margin). Both types of variations can be observed in the same margin when changing the scale of observation (von Huene 1984) or through time. Other than convergence rate, that alone could not explain variations across a single margin, some other factors might interplay to cause structural diversity along subduction boundaries (von Huene 1984; Clift and Vannucchi 2004): (1) lower plate topography (subduction of positive or negative sea-floor relief); (2) changes in dip and configuration of the Benioff zone; (3) type and volume of incoming sediment pile on the subducting plate; (4) depth of accretion and (5) volume of fluids migrating into the prism and related pore-pressure conditions. It has been shown that while prisms can grow preferentially through accretion of both coherent units and me´langes, most of the natural exhumed examples display both types at various times in their history (Fisher and Byrne 1987; Kusky et al. 1997; Hashimoto and Kimura 1999). What controls the accretion of thick coherent terranes and me´lange terranes is still poorly understood. Some authors have tried to solve this problem hypothesizing an important control of the volume rate of sedimentary input (a function of convergence rate and rate of deposition) on the coherent or diffusive style of accretion (Moore and Sample 1986; Kusky et al. 1997; Sample and Reid 2003), suggesting that subduction of slabs with a thin veneer of sedimentary cover generally lead to the formation of me´langes, while subduction of thickly-sedimented slabs result in large-scale accretion of relatively coherent packages, separated by sharp zones of shearing or type I me´lange (sensu Cowan 1985). Supporting this hypothesis are numerous observations and models of active and exhumed accretionary prisms (Sample and Fisher 1986; Sample and Moore 1987; Moore et al. 1988; Kimura and Mukai 1991; Taira and Ashi 1993; Kimura 1994; Plafker et al. 1994; Kusky et al. 1997; Ujiie 2002), which testify that rapid growth of accretionary wedges is often associated with input in the subduction system of a large amount of sediments. Clift and Vannucchi (2004) presented a quantitative comparison of mass flux for 30 active convergent plate boundaries, in an attempt to relate the accretionary (or erosive) character of a margin with various boundary conditions. We have tried to extend these observations at underplating depths by studying and comparing the structural style of ancient exhumed prism complexes. In particular, we have investigated the possible role of the thickness of the subducting sedimentary section in controlling structural style and fluid flow by comparing the accretion-related structural features of different units from three fossil accretionary complexes characterized by similar physical conditions but with different sedimentary input. The selected complexes (Figure 1) span from the Franciscan Complex of California, the source of one of the first definition of me´lange (Hsu 1968; Cowan 1974), to the Ligurian Units of the Northern Apennines, where the concepts of underplating in an accretionary prism were first applied in the Italian belt (Principi and Treves 1984; Marroni and Pandolfi 1996), through the Kodiak Complex of Alaska, one of the most studied prisms, famous for the wide exposure of both me´lange and coherent formations (Connelly 1978; Sample and Moore 1987; Fisher and Byrne 1987). In each complex the units have been selected on the basis of: - good exposure and documented geological background, - age control on sediments involved in underplating, in order to check the different nature of the sedimentary input to the three systems at the time of subduction, - comparable metamorphic conditions during accretion from units of different complexes. All the selected examples span between the zeolite and the prehnite-pumpellyite facies, representative of depth of accretion ranging between 8 and 13 km (Figure 1) Copyright # 2009 John Wiley & Sons, Ltd. Geol. J. (2009) DOI: 10.1002/gj processes of underthrusting and underplating Figure 1. (a) Location of analysed complexes: FC Franciscan Complex; ILU Internal Ligurian Units; KC Kodiak Complex and (b) P/T conditions¼ of metamorphic climax of analysed¼ complexes. ¼ - weak post-accretion deformation or, at least good cross-cutting relationships between different phases. Although a quantitative determination of thickness of sediments is intrinsically difficult in the fact of studying exhumed remnants of accreted subducting plates, the detailed geological mapping of the three complexes available allows, especially for the coherent units, a reconstruction of the stratigraphy of the subducting sequence, making possible assumptions on the relative thickness of sediments carried to the system in the three analysed complexes. In the paper, we also characterize the fluid flow regimes, through a detailed description of the vein systems, aiming to examine the control of sediment