**1. Introduction**

The term "subduction" was first used by Amstutz [1] to describe the original concept of the downward thrusting of oceanic lithosphere beneath a continental or oceanic upper plate bounded by a Wadati-Benioff zone of earthquake foci [2, 3]. This process is essential for maintaining Earth's surface constant as new oceanic lithosphere (crust and upper mantle) is formed at mid-ocean ridges, and older lithosphere is destroyed at convergent plate boundaries. With application to the various mechanisms of lithospheric convergence, foundering, and recycling, the term subduction has become broader over time and has lost some of its original meaning. It now refers to a series of processes in which material from the Earth's uppermost layer is

submerged into the asthenospheric mantle, and its chemical constituents are recycled back into the Earth's interior. Regardless of submerging Earth's lithosphere process into the convective asthenosphere, two categories of subduction zones on the modern Earth (ca, 1 Ga, [4]) can be distinguished based on the association of magmatism: (a) The subduction zones associated with volcanism and (b) subduction zones missing arc-volcanicity.

The subduction zones associated with giant volcanism are referred to as Beniofftype (or B-type or Pacific-type) subduction, which is characterized by the spontaneous initiation of giant oceanic lithosphere foundering, previously formed at a mid-ocean ridge into the convective upper mantle beneath oceanic or continental upper plates [5–8]. While at the ocean-ocean convergence boundary, the older and colder plate (i.e., the denser plate) often subducts the younger and warmer oceanic plate, at the ocean-continental convergence boundary, the oceanic plate subducts beneath the less dense continental plate. Although the density contrast between the oceanic lithosphere and the asthenosphere may be a possible driving force for the initiation of subduction, it is considered a second order of importance compared to convective currents in the asthenosphere that exert drag forces on the base of the lithosphere [9, 10]. However, the negative buoyancy of the sinking lithosphere (which is denser than the underlying asthenosphere) results in slab pulls, which are thought to be the dominant driving forces of plate motions [6, 11–13]. One or two planar zone(s) of seismicity in the downgoing slab (the Wadati-Benioff plane) [3, 14] can reach the mantle transition depth of ~660 km [15]. In addition, heating of the subducting crust releases a significant volume of water from the hydrated lithologies of the subducting material, leading to the fluid-flux melting within the overlying mantle wedge and the generation of hydrous, near-continuous arc magmatism [4, 8, 15–19]. The generated magma is lighter than the surrounding mantle material and rises through the mantle and overlying crust [20]. It creates a chain of volcanic islands on the ocean floor known as an island arc at oceanocean convergence margins, or it forms a mountain chain with many volcanoes known as a volcanic arc at ocean-continental convergence margins. The hydrous, calcareous magmatism with low FeO content "calc-alkaline" is predominant in the magmatic arcs [21–24]. A deep trench usually forms parallel to the convergence boundary as the crust sinks downward. More volcanic material and sedimentary rocks accumulate around the island arcs, eventually thrust into an accretionary wedge and onto the continental plate [15]. As a result, high-pressure, low-temperature metamorphic facies series are common at these convergent boundaries [25–27].

The term Ampferer-type subduction was first used to describe the "missing" continental crust in the Alpine orogen (European Alps) [28]. The term was recently revived considering a new understanding of hyper-extended basins and passive margins by [7, 8, 29], who interpreted the mechanism of lithospheric recycling in the Pyrenees and Western and Central Alps. The Ampferer-type (or A-type) subduction occurs when a continent or large island collides with another continent by subducting hyperextended continental basins that contain minor oceanic crust formed at rift margins [7, 8, 20]. These hyper-thinned basins and their mechanically weak serpentinized mantle beneath serve as focal points for initiating convergence and down-thrusting of these basins beneath passive margins [7, 20, 30–32]. Only the dry root lithosphere is subducting, while hydrated lithologies from the descending plate (hydrated mantle rocks and sedimentary deposits) are sequentially accreted into a nascent orogenic wedge, resulting in amagmatic closure associated with tremendous deformation of preexisting continental rocks, forcing the material upward, creating high mountains [7, 32–34].

*The Ampferer-Type Subduction: A Case of Missing Arc Magmatism DOI: http://dx.doi.org/10.5772/intechopen.109406*

Since the advent of plate tectonic theory in the 1960s, which assumes that subduction of the oceanic lithosphere primarily controls rigid plate motions [35, 36], alternative concepts for the lithosphere's foundering process have been neglected [7, 8, 20, 37]. Many convergent boundaries that exhibit Ampferer-type subduction features could have been considered Benioff-type subduction in the strict sense. Therefore, this chapter presents the well-known cases of Ampferer-type subduction zones and addresses the challenging questions of when, where, and how subduction initiation (SI) occurs at/around passive margins and the reasons for missing arc magmatism along these margins.
