Quaternary Volcanism in Myanmar: A Record of Indian Slab Tearing in a Transition Zone From Oceanic to Continental Subduction
Liyun ZhangWu FanLin DingMihai N. DuceaAlex PullenJiaxing LiYang SunYahui YueFulong CaiChao WangTouping PengKyaing Sein
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Abstract Magmatic processes that occur during the transition from oceanic to continental subduction and collision in orogens are critical and still poorly resolved. Oceanic slab detachment in particular is hypothesized to mark a fundamental change in magmatism and deformation within an orogen. Here, we report on two Quaternary volcanic centers of Myanmar that may help us better understand the process of slab detachment. The Monywa volcanic rocks are composed of low‐K tholeiitic, medium‐K calk‐alkaline, and high‐K to shoshonitic basalts with arc signatures, while the Singu volcanic rocks show geochemical characteristics similar to asthenosphere‐derived magmas. These volcanic rocks have low Os concentrations but extremely high 187 Os/ 186 Os i ratios (0.1498 to 0.3824) due to minor (<4%) crustal contamination. The Monywa arc‐like rocks were generated by small degrees of partial melting of subduction‐modified asthenospheric mantle at variable depths from the spinel to garnet stability fields. Distinct from the Monywa arc‐like rocks ( 87 Sr/ 86 Sr i = 0.7043 to 0.7047; ε Nd i = +2.3 to +4.7), the Singu OIB‐like rocks exhibit higher 87 Sr/ 86 Sr i (0.7056 to 0.7064) and lower ε Nd i (+0.8 to +1.6) values. These isotopic characteristics indicate a large contribution of an isotopically enriched asthenosphere layer beneath the Burmese microplate, which possibly flowed from SE Tibet. We interpret that this short‐lived, small‐scale, and low‐degree melting Quaternary volcanism in Myanmar was triggered by its position above a slab window resulting from the tearing of the oceanic lithosphere from buoyant continental lithosphere of the Indian plate.Keywords:
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At a convergent margin large amounts of structurally bound water are carried into
the Earth’s interior and - as the subducting plate descends and the temperature
rises - are driven off to some extent into the mantle wedge, where they are thought
to trigger intermediate-depth earthquakes in the Wadati-Benioff zone and melting
under volcanic arcs. However, a largely uncertain fraction outlasts sub-arc fluid
release and hence enters the deeper mantle, which leads to a connection between
the oceans and the Earth’s deep water cycle. Thus, a detailed knowledge of the water
budget of a subduction zone is not only important to understand arc volcanism, but
as well to comprehend the chemical development of the Earth’s mantle. For this
purpose, profound information about the amount of water that is subducted along
with the oceanic plate is indispensable.
The present thesis uses geophysical methods to determine the degree of hydration
of the Cocos Plate offshore Nicaragua, which is subducted beneath the Caribbean
Plate.
In general it was assumed that structured water is transported into the slab
in sediments and the upper crust only, though in recent years growing evidence
suggested that lower crust and upper mantle might contain capacious amounts of
fluids as well, since the bending of the incoming oceanic plate leads to a reactivation
or creation of normal faults (bend-faults), which are visible in batrymetric data
and have been inferred to cut deep enough into the plate to provide a pathway
for seawater to penetrate into the lithosphere, changing ”dry” peridotites to ”wet”
serpentinites, which contain up to 13% of water. Such a mechanism could transport
much more fluids into the earth’s interior than any other considered possibility.
However, the cutting depth of these bend-faults and hence the depth that seawater
could penetrate into the mantle was not well-defined, for one reason since focal depth
of earthquakes associated with the bend-faults were poorly known. Yet previous
studies assumed cutting depths such that serpentinization is firstly restricted by its
thermal limit of 600± C.
This study uses openly accessible, global broadband data of earthquakes offshore
Central America as well as an unique dataset from a local long-period earthquake
monitoring network offshore Nicaragua, to determine typical focal depths off earthquakes
at the trench-outer rise and further relates these focal depths to the cutting
depths of bend-faults. In addition, a full 3d-tomographic inversion that consistently
integrates seismic airgun blasts and local as well as regional seismicity, could show
reduced seismic mantle velocities at the outer rise and nearby the deep sea trench
with an evolutionary trend towards it. Best explained is this by a fractured and
ii
partly serpentinized lithosphere. The use of regional sources (i.e. earthquakes in
distances of ¸200 km from the seismic network) in the tomographic inversion process
made it possible, for the first time, to reflect the entire brittle lithosphere. In
a second approach, relative arrival times of large earthquakes that occurred during
the deployment of the seismic network were investigated. Again, it could be shown
that seismic mantle velocities decrease in accordance with the onset of bend-faults
in the bathymetry.
But not only seismic velocities decrease nearby the trench, the average moment
magnitude of outer rise earthquakes does as well, though the number of events
increases significantly. We explain this a weakened lithosphere and hence a reduced
yield strain, which again suggests an occurrence of serpentinite.
However, tomographic images suggest that the area of reduced seismic velocities
and in turn possible serpentinization does not reach the cutting depth of bend-faults
nor the depth of the 600± C isotherm. Focal mechanisms of several earthquakes were
determined via moment tensor inversion and forward modelling respectively and it
could be shown that where seismic velocities are reduced only tensional ruptures
occur, which allow for water infiltration, meanwhile the area beneath is dominated by
compressional rupture behaviour, which presents a barrier for seawater. This result
does not only confirm and enlarge flexure models of subducting plates [Chapple
and Forsyth, 1979; Christensen and Ruff, 1988], but also establishes a coherent
connection between stress distribution in the incoming plate and penetration depth
of seawater and is the first study in this vein.
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Some of the fundamental features of plate tectonics are interpreted in connection with the behavior of oceanic crust. It is shown to be likely that the oceanic crust which is produced at the mid-ocean ridge by chemical differentiation may be removed from the downgoing slab by melting at the depth of asthenosphere behind the deep-sea trench. The melting of crustal material after the subduction is made possible by an efficient supply of heat through the well-developed asthenosphere with a low-velocity and high-attenuation of seismic waves. The removal of subducted oceanic crust from the slab is consistent with the positive gravity anomaly behind trenches and the double Benioff zone recently discovered. We propose new type of driving forces of plate motion, which arises from the density contrast between the crust and mantle when the oceanic crust is either created or destructed. The proposed driving mechanism is consistent with the non-uniform size and shape of individual plates, the migration of mid-ocean ridges and compressional intraplate stress, while these facts are difficult to understand in the framework of conventional models. A continuous accumulation of basaltic magma beneath the trench-arc system results in a catastrophic overflow of material, which corresponds to back-arc spreading. The picture presented in this paper explains the evolution of marginal basins that is characterized by the presence of remnant arcs, the changes in stress field and the dip angle of the slab, and the anomalous depth-age relationships.
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We use numerical models to investigate the dynamics of two interacting slabs with parallel trenches. Cases considered are: a single slab reference, outward dipping slabs (out-dip), inward dipping slabs (in-dip) and slabs dipping in the same direction (same-dip). Where trenches converge over time (same-dip and out-dip systems), large positive dynamic pressures in the asthenosphere are generated beneath the middle plate and large trench-normal extensional forces are transmitted through the middle plate. This results in slabs that dip away from the middle plate at depth, independent of trench geometry. The single slab, the front slab in the same-dip case and both out-dip slabs undergo trench retreat and exhibit stable subduction. However, slabs within the other double subduction systems tend to completely overturn at the base of the upper mantle, and exhibit either trench advance (rear slab in same-dip), or near-stationary trenches (in-dip). For all slabs, the net slab-normal dynamic pressure at 330 km depth is nearly equal to the slab-normal force induced by slab buoyancy. For double subduction, the net outward force on the slabs due to dynamic pressure from the asthenosphere is effectively counterbalanced by the net extensional force transmitted through the middle plate. Thus, dynamic pressure at depth, interplate coupling and lithospheric stresses are closely linked and their effects cannot be isolated. Our results provide insights into both the temporal evolution of double slab systems on Earth and, more generally, how the various components of subduction systems, from mantle flow/pressure to interplate coupling, are dynamically linked.
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The current data set from USArray provides an unprecedented opportunity to investigate mantle transition zone structures beneath the western United States. We have made transition zone images with the Common Converted Point (CCP) stacking method. More than 9600 high quality receiver functions were stacked with reference to two different three‐dimensional tomography models and a one‐dimensional velocity model. Where the Gorda plate passes through the transition zone, the 410 discontinuity has been elevated ∼25 km and the 660 discontinuity has been depressed ∼35 km. We interpret the transition zone topography in terms of mineral physics results in several different ways, noting in particular that recent measurements on the Clapeyron slope for the ringwoodite‐to‐perovskite phase transition under dry conditions give a phase boundary slope of ∼−1.3 to −0.4 MPa/K. The ∼35 km deflection of the 660 discontinuity observed in the receiver functions seems to be the evidence that the subducted slab can carry abundant water from the surface to the transition zone, and in the transition zone the water in the slab may be fully saturated (e.g. the water content is ∼2.0 wt%). Analyses of the velocity perturbations in the tomography models and the transition zone thickness indicate that the deep water is likely well confined within the subducted slab. We infer that the presence of water in the subducted Gorda slab might have contributed ∼15 km and the thermal anomaly in the slab might have contributed ∼20 km to the depression of the 660 discontinuity.
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