Oceanic core complexes and back-arc basin spreading

As usual, it has been a little while since I have posted. Today I thought I was switch it up a bit and provide a more scientifically written article. As you may know, I am involved in several ongoing research projects with Japanese colleagues. One of which is the continued exploration of the world's largest oceanic core complex, which happens to lie along an extinct back-arc spreading ridge, the Godzilla Megamullion.

When I first began my work on Godzilla as a lowly undergraduate, I felt the need to write up a nice summary of why working on Godzilla was important to me and the scientific community. Looking back on that paper now, I find that its relevance has not degraded and in fact may have grown with time. That being said, I felt that a few minor things needed to be updated. So I have taken the liberty in adding to that paper and presenting here on Fiery Low-Down Rock Talk as an informative post. Many may not know so much about back-arc basins and/or oceanic core complexes, and I am certain that many do not know much about the Godzilla Megamullion.

So for your enjoyment and enlightenment I present:

Oceanic core complexes and back-arc basin spreading

Considered as analogs to Mid-Ocean Ridges (MORs), back-arc spreading systems generate a large portion of the magmatic basement that forms the Earth’s oceanic crust that of which is doomed to be subducted. The study of back-arc spreading systems has a strong bearing on two very important aspects of the Earth’s evolution, as it relates intimately with both subduction zone and mid-oceanic ridge dynamics. However, this important tectonic setting is much less well understood than are arcs or mid-oceanic ridges (Ohara et al, 2003). In both MORs and back-arc spreading systems, new oceanic crust is created by the upwelling of asthenospheric mantle. The melting of the mantle, accumulation of melt in lithospheric magma chambers and extrusion in volcanic centers, forms the same or similar structures as true mid-ocean ridges. It is believed that an over-representation of these systems may be present in the geologic record, due to their proximity to arcs, which makes them more likely to be obducted alongside arc and forearc lithosphere during ocean-continent convergence and continent-continent collision. None of the many active or extinct back-arcs worldwide have actively produced oceanic lithosphere for more than a few million years. Continuous crustal accretion over millions of years is normal for MORs, but back arc rifts once born from rifting of the rear of an island arc volcanic system will spread for a few million years and die.  

Formation of Back-arcs

Since the inception of plate tectonics, geologists have been searching for the reasons behind why and how extensional basins open near the boundaries between convergent plates, and suddenly stop opening. There have been two basic concepts proposed for the formation of back-arc basins. In one case, the rate at which the subducting plate descends into the mantle can be greater than the rate of convergence with the overriding plate. In this case, the subducting slab will roll back from the overriding plate and a smaller portion of the “arc” or “forearc” plate might break off that will remain in contact with the trench, while the portion left behind of the overriding plate trails behind (Elsasser, 1971; Moberly, 1972). In the second proposed case, the overriding plate may pull away from the trench; this anchors the slab in the mantle, causing the trench to resist migration with the retreating overriding plate (Scholz and Campos, 1995). As in the first case, a small piece breaks off from the overriding plate and remains in contact with the subducting plate along the subduction hinge. The truth is, both of the cases tend to act simultaneously. ( See figure 1)

Figure 1. Modes of back-arc basin opening. The panels from top to bottom show a schematic time sequence of back-arc basin opening. Left-hand panels show the case of slab roll back (Elsasser, 1971; Moberly, 1972) in which the trailing plate is considered fixed (indicated by black dot) and the trench hinge moves relatively seaward (indicated by small right-pointing arrow), breaking off a section of the overriding plate that moves with the trench from a trailing plate that remains fixed. In the right-hand panels, the trench hinge is considered to resist motion (black dot) because of a slab “sea anchor” force (Scholz and Campos, 1995). (Martinez et al, 2007)

When arc systems rift, the breakup generally occurs in the vicinity of the arc volcanic front (Molnar and Atwater, 1978), defined usually as the maximum locus of volcanism in the arc (Tatsumi, 1986; Tatsumi and Eggins, 1995). The arc volcanic front is a region of thickened crust, melt emplacement, high heat flow, and large gravitational stresses, all of which tend to towards breaking apart if extensional stresses are enacting upon the area. Breakup is not always exactly centered on the volcanic front and may occur as far as ±50 km (Taylor and Karner, 1983) so that some leeway is seen as some systems may rift behind the arc while others will rift in the forearc. Local weaknesses in the overriding plate are not the only factors controlling breakup. Other effects, such as the motion and geometry of the subducting slab and traction with the mantle wedge during breakup, may also be important. After breakup, some basins, appear to have began rifting at a central area and then grown by typical MOR processes as well as widening of the rift, that is, the progressive extension of the rift along strike into the arc volcanic front (Stern et al., 1984). Other basins, such as the Havre Trough, appear to be opening more uniformly along their entire lengths (Delteil et al., 2002). These basins would seem to be better analogs to MORs.

Conditions for Formation

Recent published findings by Sdrollias and Muller have placed a limit to backarc spreading usually occuring when oceanic crust that is older than 55 million years is subducted. The older, colder slab sinks into the mantle at a high intermediate dip angle of always 30° or greater to backarc spreading, causing slab rollback which exerts a large amount of tension on the overriding plate. Back-arc basin formation is usually precluded by a pull of the overriding plate away from the subduction hinge. This creates accommodation space between the overriding and subducting plates, which allows for both spreading and subducting to continue to occur. Once back-arc extension is established, it continues regardless of the motion of the overriding plate. This indicates that back-arc spreading is not a simple consequence of overriding plate behavior (Sdrollias and Muller 2005). Tectonic extension in the arc and forearc is usually shown by volcanism in the areas, but in the backarc, active mantle upwelling is induced in the mantle wedge by the traction of the subducting plate on the mantle wedge in the overlying arc. The upwelling causes mantle melting which feeds the production of oceanic crust in the backarc. The zone of upwelling will push any back arc spreading that initiates directly above it landward due to accretion of crust to both sides of the rift. Once back-arc extension is established, subduction hinge rollback appears to be the main force responsible for continued creation of accommodation space. As the system migrates away from the arc with time, the backarc spreading system will take on the appearance of being more like a MOR. The accretion of material derived from a fertile mantle source over the course of the spreading systems era of productivity will gradually change into a thickened crust fed by less-mixed depleted melt. Overall, the driving mechanisms for back-arc extension have been shown to be a combination of surface kinematics, properties of the subducting slab, the effect of mantle flow on the slab, and mantle wedge dynamics. (Sdrollias and Muller 2005).   

Oceanic Core Complexes in Back-arc Spreading Systems

Oceanic core complexes (OCCs) are morphological features that were first noted and characterized along the Mid-Atlantic Ridge (e.g., Karson, 1990; Tucholke and Lin, 1994; Cann et al., 1997; Blackman et al., 1998; Tucholke et al., 1998). OCCs usually occur near the intersection of transform faults and the axis of a spreading ridge (i.e., a ridge-transform intersection), and are characterized by a domal/curviplanar surface, corrugations oriented parallel to the spreading direction (referred to as megamullion surfaces), and exposed lower crust and mantle material (Figure 2) (e.g., Cann et al., 1997; Blackman et al., 1998; Tucholke et al., 1998; Karson et al., 2006). These features are general found along regions of a spreading ridge where tectonic extension dominates over magmatic processes. Such a tectonic environment leads to asymmetric plate spreading and the formation of OCCs (Blackman et al., 1998; Tucholke et al., 1998; Smith et al., 2006, 2008).Compared with continental metamorphic core complexes, OCCs along a spreading axis are likened to represent the exhumed footwalls of oceanic detachment faults (e.g., Karson, 1990; Dick et al., 1991; Cann et al., 1997; Blackman et al., 1998; Tucholke et al., 1998; Karson, 1999; Dick et al., 2000; MacLeod et al., 2002; Escartin et al., 2003; Ildefonse et al., 2007). Oceanic detachment faults are long-lived, low-angle, large offset extensional faults that efficiently accommodate a significant portion of plate separation (up to 60-100% extension) on-axis during asymmetric spreading (Buck, 1988; Tucholke and Lin, 1994; Tucholke et al., 1998; Buck et al., 2005; deMartin et al., 2007; Grimes et al., 2008; Smith et al., 2008; Morris et al., 2009). It is generally agreed that these detachment faults do not initiate at low angles, but instead initiate as steep normal faults at depth and shallow into a low angle normal fault through footwall rotation (Morris et al., 2009). Many questions still remain regarding where and how the detachment fault roots and ultimately why they form and cease in the first place (Tucholke et al., 2008; MacLeod et al., 2009; Olive et al., 2010; Escartin and Canales, 2011).

Figure 2: From Schoolmeesters et al. (2012) which modified after Grimes et al. (2008) and Escartin and Canales (2011). “Cartoon of a slow-spreading ridge showing asymmetric plate spreading and associated formation of an oceanic core complex. Image highlights main attributes, including the breakaway – location where the oceanic detachment fault originally surfaced on the seafloor; spreading-parallel corrugations on the surface of the core complex; gabbro plutons within upper mantle peridotite denuded by the fault (red indicates recently intruded melt rich plutons, orange indicates partially crystallized plutons, and blue indicates fully solidified plutons); and the termination where the oceanic detachment fault dips below the axial valley. The detachment fault forms via a rolling hinge model, initially with a moderate-to steep dip beneath the rift valley and flattens as it emerges to the seafloor at the rift valley wall.” (Schoolmeesters et al., 2012)

 The largest OCC currently being studied in the world, as reported by Ohara et al (2001) and Ohara et al (2003a), is the Godzilla Megamullion (Figure 3), which is 10 times larger than OCCs commonly found along the Mid-Atlantic Ridge. The significance of Godzilla Megamullion is that it is an OCC that formed on a back-arc spreading ridge (i.e. the Parece Vela Basin). Compared to typical OCC-forming spreading rates [ultraslow- (Southwest Indian ridge full rate: ~1.2–2 cm/y; Dick et al., 2003) or slow-spreading ridges (Mid-Atlantic Ridge full rate: 1–4 cm/y)] the Godzilla Megamullion developed at a spreading ridge that records seemingly intermediate spreading rates (full rate: 7.0 cm/y; Ohara et al., 2001). Recent U-Pb dating of zircons from gabbroic and leucocratic rocks from Godzilla Megamullion (Tani et al., 2011) constrain the duration of fault-induced spreading contemporaneous with magmatic accretion at the ridge axis to ~4 Ma and determined that the spreading rate gradually decreased until cessation of the PVR at ~7-9 Ma or later. It is estimated that the average half-spreading rate for the Godzilla Megamullion is 2.54 cm/yr. Although magnetic lineations in the Parece Vela Basin (PVB) are weak due to spreading occurring when the basin was near the equator (Okino et al., 1998, 1999), estimations based on poorly constrained data set the full-rate of spreading in the central basin at ~7.0 cm/yr (Ohara et al., 2001). The presence of well-ordered abyssal hills southwest of the breakaway for the Godzilla Megamullion suggests that prior to the formation of the detachment fault the western PVB experienced robust magmatism and little variation in its spreading rate. The Godzilla Megamullion is unusual in that it extends along the full length of the spreading segment (Ohara et al., 2001). (See Figure3)

Figure 3: Left: Major bathymetric features of the Western Pacific Ocean (after Ohara et al., 2001). The rectangle shows the location of the study area at the site of the Godzilla Megamullion, Parece Vela Basin. Right: Bathymetric map of the Godzilla Megamullion (outlined by dashed line), showing the location of dredge site D (Harigane et al 2008)

The presence of a huge megamullion structure in the PVB is a distinct morpho-tectonic characteristic that indicates a spreading dominated by tectonic extension and small melt volumes, and gives us an insight into the formation and life of back-arc spreading ridges. Peridotites have been recovered in abundance over the megamullion surface, as well as at a segment termination. Very notable is the small scale (i.e. single dredge haul) emplacement of fertile peridotite and depleted peridotite (dunite and plagioclase- peridotite). The Fertile (F-type) peridotite has been interpreted as being the residue of a small degree of melting (~4% near-fractional melting of a MORB-type mantle), whereas Dunite (D-type) and plagioclase-bearing peridotite (P-type) are products of melt-mantle interaction (Ohara et al 2003; Snow et al., 2009; Loocke et al., 2009). Godzilla Megamullion has provided a window not only into the inner-workings of our planet, but provides evidence which supports the hypotheses regarding the lifecycles of back-arc spreading ridges (Snow et al., submitted). 

In short, oceanic core complexes represent a significant feature that is synonymous between mid ocean ridges and back-arc spreading ridges. The continued study of the two allows for in depth examination of the inner workings of our earth and provides a better understanding of magmatic processes and mantle dynamics at mid ocean spreading ridges and subduction zones.  The magmatic histories of these structures, though short-lived, are relatively complex. It is with a better understanding of these histories, that we can interpret the formation of these structures, their significance in the creation of new oceanic crust, and their overall lifespan more precisely.

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Illustrations

Harigane, Y., Michibayashi, K., Ohara, Y. 2008. Shearing within lower crust during progressive retrogression: Structural analysis of gabbroic rocks from the Godzilla Mullion, an oceanic core complex in the Parece Vela backarc basin. Tectonophysics. 457, 183–196

Martinez, F. , Okino, K., Ohara, Y.,  Reysenbach, A., Goffredi, S. Back-Arc Basins. Oceanography. 20, 116- 127.

Schoolmeesters, N., Cheadle, M.J., John, B.E., Reiners, P.W., Gee, J., Grimes, C.B., 2012. The cooling history and the depth of detachment faulting at the Atlantis Massif oceanic core complex. Geochemistry Geophysics Geosystems, 13, Q0AG12, doi:10.1029/2012GC004314