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) |
 |
| 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
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