Abstract The development of theories that link structure, material properties, and fluid flow to the secular variations of the seismic cycle is in its infancy. The importance of this linkage provided the motivation for a March 2012 GSA Penrose conference “Fluid Flow, Material Transfer and Deformation in the Forearcs of Convergent Margins” (Lucca, Italy, in the Northern Apennines). This volume, created for that conference, includes two field trips. One describes a two-day trip in the Ligurides and the other describes a one-day trip in the Alpi Apuane. Together, these field trips address convergent margin processes at a variety of depths within the subduction system.

Abstract This guide provides background information and an itinerary for a two-day field trip in the Northern Apennines leaving from Modena and ending at Riolunato (Modena). The proposed field trip route leads through the Po Valley side of the Northern Apennines, in the Emilia region. The field trip provides opportunities to examine the exposed geological features related to a phase of early-middle Miocene convergence between the European and Adria plates. In particular, outcrops have been selected that exhibit features characterizing deformation in an exhumed plate boundary shear zone, interpreted as an erosive plate boundary shear zone. The sedimentary evolution and deformation of the upper and lower plates are also highlighted.

Abstract This guide provides background information and an itinerary for a one-day field trip leaving Barga Garfagnana (Lucca) and crossing the Alpi Apuane toward Versilia. This field-trip route provides the opportunity to examine structures and strain features that record the underplating and exhumation of the Apuane metamorphic units. Special emphasis will given to the structures produced at the different scales in the Carrara marble, known throughout the world as a highly desirable building stone. The field trip will touch on the role of fluids, fluid-rock interaction, and deformation during underplating and subsequent exhumation of the region along major normal faults.

Abstract Subduction zones are both the source of most new continental crust and the locations where crustal material is returned to the upper mantle. Globally the total amount of continental crust and sediment subducted below forearcs currently lies close to 3.0 Armstrong Units (1 AU=1 km 3 a −1 ), of which 1.65 AU comprises subducted sediments and 1.33 AU tectonically eroded forearc crust, compared with an average of c . 0.4 AU lost during subduction of passive margins during Cenozoic continental collision. Margins may retreat in a wholesale, steady-state mode, or in a slower way involving the trenchward erosion of the forearc coupled with landward underplating, such as seen in the central and northern Andean margins. Tephra records of magmatism evolution from Central America indicate pulses of recycling through the roots of the arc. While this arc is in a state of long-term mass loss this is achieved in a discontinuous fashion via periods of slow tectonic erosion and even sediment accretion interrupted by catastrophic erosion events, probably caused by seamount subduction. Crustal losses into subduction zones must be balanced by arc magmatism and we estimate global average melt production rates to be 96 and 64 km 3 Ma −1 km −1 in oceanic and continental arc, respectively. Critical to maintaining the volume of the continental crust is the accretion of oceanic arcs to continental passive margins. Mass balancing across the Taiwan collision zones suggests that almost 90% of the colliding Luzon Arc crust is accreted to the margin of Asia in that region. Rates of exhumation and sediment recycling indicate that the complete accretion process spans only 6–8 Ma. Subduction of sediment in both erosive and inefficient accretionary margins provides a mechanism for returning continental crust to the upper mantle. Sea level governs rates of continental erosion and thus sediment delivery to trenches, which in turn controls crustal thicknesses over 10 7 –10 9 years. Tectonically thickened crust is reduced to normal values (35–38 km) over time scales of 100–200 Ma.

Abstract The mega-faults between actively converging plates have recently been penetrated by the Ocean Drilling Program at three plate margins: Barbados, Costa Rica and Nankai. Cores, downhole instrumentation and detailed seismic imagery provide data which may be helpful in interpreting ancient examples of shear zones. The mega-faults, developed in poorly lithified sediments, separate major lithospheric plates yet are merely tens of metres in thickness. They respond to ongoing strain by intensifying inwards rather than propagating outward splays and can grow thinner because of continuing compaction. Surprisingly, lithological influence on the localization of fault propagation seems slight, but lithology determines the deformation style within the faults. The resulting structures show asymmetric distributions within the zones but, in these flat-lying structures, tend to show a downward increase in strain. Upper margins are typically gradational whereas lower boundaries can be strikingly abrupt. The fluid-transport behaviours are complex. In some situations the horizontal flux is very diffuse but centred around the fault. Some faults can efficiently channelize fluids — for distances of tens of kilometres — while at the same curbing flow across them. The fluid transport is clearly episodic and heterogeneous. Fingers of pressured fluid migrate within the fault zone, in patterns that constantly change through time.