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Controls on back‐arc basin formation
Learn more. The occurrence of an extensional stress regime adjacent to an area of major convergence, accretion and thrusting has often been considered counterintuitive and not easily explained by plate tectonic theory. Analogue experiments of the Mediterranean region [ Faccenna et al. Similarly, the analogue models of Schellart et al. Our plate model uses the new absolute reference frame of O'Neill et al.
The plate model also includes tighter constraints on the spreading histories between the major plates, including the motion between East and West Antarctica [ Cande et al. These subduction systems were chosen as they satisfied several criteria: 1 They border the major oceanic basins where tight constraints on the motion history of the downgoing plate are available.
The subduction systems within SE Asia and northeast of Australia e. The region is also riddled with continental and volcanic arc fragments. The Sandwich and Caribbean subduction systems do not have well constrained motion histories for the overriding plate.
The age of oceanic lithosphere subducting at the active trench is assumed to have a major influence over the style of subduction, the behavior of the subducted slab and the surface expression of this subduction on the overriding plate. Newly created oceanic lithosphere cools, subsides and thickens with increasing distance from the spreading axis due to thermal contraction.
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The density of the lithosphere increases as a function of its age while it cools and contracts and becomes less buoyant. Relationships between age and thickness [ Heestand and Crough , ; Stein and Stein , ], age and trench depth [ Grellet and Dubois , ], age and heat flow [ Sclater , ; Stein and Stein , ] and age and depth of oceanic lithosphere [ Parsons and Sclater , ; Stein and Stein , ] demonstrate the variations in character of the oceanic lithosphere depending upon its age.
As a result, the way in which oceanic crust of different ages is subducted at oceanic trenches and the processes that occur on the overriding plate will differ. The relationship between the age of subducting oceanic lithosphere and various subduction zone parameters, including convergence rate [ Jarrard , ], absolute motion of the overriding plate [ Carlson et al.
In their analysis only one measurement was obtained to represent an entire subduction zone for the present even though the lateral extent of these systems can be as much as km.
What Is The Back-arc Basin?
The youngest oceanic crust along the margin is located adjacent to the extinct spreading ridge. During its inception, the margin was dominated by the subduction of 40—80 m.
The South Fiji Basin formed between 25 and 33 Ma as two contemporaneous triple junctions [ Sdrolias et al. The initiation of the South Fiji Basin occurred when the subducting oceanic lithosphere ranged from 90 to 52 m.
However, in the far north, remnants of 60—90 m. The Wharton Basin ceased spreading at approximately 43 Ma [ Liu et al. When the Japan Sea was initiated at 30 Ma, the age of the subducting lithosphere was 60 m.
The age of subducted oceanic lithosphere at the Aleutian trench presently ranges from m.
Where Can Back-arc Basins Be Found?
In the earliest Cenozoic, the age of the crust being subducted was substantially younger than the present day, ranging from 40 to 50 m. The age of the subducting oceanic lithosphere along the Aleutian margin varies along strike and with age. The age of the crust ranges from 0 m. The crust at the Cascadia subduction zone remains relatively young during the entire Cenozoic and resulted in the eruption of adakitic andesites in the Mount Shasta and Lassen region of The Cascades [ Baker et al.
In the early Cenozoic, the crust reaches a maximum age of 40 m.
The eastern flank of the newly created Juan de Fuca crust is subsequently subducted along the margin. The Middle America subduction zone is characterized by the subduction of relatively young oceanic crust that does not exceed 60 m.
The Middle America subduction system is presently subducting the Cocos plate, the age of which varies from 0 m.
In the early Cenozoic, the age of the subducting oceanic lithosphere ranges from 25 to 60 m. The eruption of rocks with adakite genesis in the southern Middle America trench has been linked to the subduction of young oceanic lithosphere [ Johnston and Thorkelson , ]. The age of the subducting lithosphere along the Middle America trench never exceeded 60 m.
In the far north, the age of the subducting lithosphere is less than 20 m.
In the early Cenozoic, the age of the subducting lithosphere ranged from 30 to 60 m. These have significantly altered the shape, structure and dip of the subducting slab and the volcanism that has erupted on the overriding plate. The crust along the strike of the trench varies significantly through time, mainly due to its large lateral extent. However, the age of subducting lithosphere during the entire Cenozoic along this km boundary remained between 0 to 60 m.
As the convergence rate is a measure of the speed at which material is inserted into the mantle, it is expected that the convergence rate will have some influence on the shape, structure and behavior of the subducting slab.
Although no direct correlation was established using the convergence rate alone, studies combining the convergence rate with the age of subducting oceanic lithosphere yield important correlations between the length of the seismic zone [ Molnar et al.
The direction of convergence indicates the degree of convergence obliquity experienced at the margin and whether shear stresses convergence direction parallel to the margin or normal stresses convergence direction perpendicular to the margin are dominant.
In regions where convergence is orthogonal to the trench, such as Middle America, the margin is characterized by a prominent seismic dipping zone, steeper dipping slabs than normal and extensive arc volcanism Figure 8a.
How Is A Back-arc Basin Formed?
The convergence rate and direction for each subduction system was calculated at km intervals along the strike of the trench. The convergence rate between the Pacific and Australian plate increases through time and corresponds to an increase in the age of subducting oceanic lithosphere.
The rapid spreading rate in the South Fiji Basin may be analogous to the current high spreading rates experienced in the North Fiji Basin [ Lagabrielle et al. The overriding plate at all times was taken to be the Sumatran Block which is fixed to the Sundaland Block Figure 3a.
This can be attributed to the commencement of clockwise rotation of the Philippine Sea plate at 20—21 Ma [ Sdrolias et al. The rotational history of the Philippine Sea plate [ Hall et al. An increase in the convergence rate occurs between 40 and 45 Ma along the entire margin and may be related to the change in spreading direction in the Pacific. The convergence rate and direction values for the Aleutian subduction system were calculated by taking the Kula plate as the downgoing plate from 30 to 60 Ma and the Pacific plate from 0 to 30 Ma.
The orientation of the Aleutian subduction system varies greatly along strike and thus significantly affects the convergence history along the margin. The convergence rate and direction data for the Cascadia subduction system were calculated by taking the Farallon plate as the downgoing plate between 20 and 60 Ma and the Juan de Fuca plate from 0 to 20 Ma.
The overriding plate is taken to be the North American plate during the entire Cenozoic. The convergence data for the Middle America subduction system were calculated based upon the Farallon plate as the downgoing plate between 23 and 60 Ma and the Cocos plate from 0 to 23 Ma.
The overriding plate was taken to be the Caribbean plate which we have fixed to the North American plate during the Cenozoic Figure The southern Middle America trench experienced a sharp increase in convergence rate at 20 Ma and may be related to the breakup of the Farallon plate into the Cocos and Nazca plates.
In the northern Middle America trench, a sharp decrease in the convergence rate was experienced at 10 Ma. The overriding plate was taken to be the South American plate during the entire Cenozoic Figure There is a sharp increase in the convergence rate between 50 and 55 Ma and this lasts until 45 Ma, after which there is a decrease in convergence rate. The absolute motion of a plate is a measure of the velocity of an individual plate relative to the spin axis of the Earth. The absolute reference frame which best approximates the motion of individual plates is a controversial issue but has previously been based, most commonly, on the fixed hot spot reference frame [e.
A relationship between the age of the subducting lithosphere and the absolute rate of motion of the subducting plate was established by Carlson et al. Thereafter, the absolute motion of the overriding plate changed such that the Australian plate moved slightly away from the trench until 20 Ma in the far north and 10 Ma in the center.
The motion of the overriding plate away from the trench continued throughout the history of South Fiji Basin spreading and until 20 Ma in the far north of the margin.
The overriding plate subsequently moved toward the trench between 10 and 20 Ma. The Australian plate has been moving away landward from the trench from 0 to 45 Ma in the southern portion of the margin Figure The fastest velocity of the downgoing plate toward the trench occurred in the early Cenozoic. Thereafter, the overriding plate was moving away from the trench at a similar velocity. The Andaman Sea formed oceanic crust during the Miocene to the present, at the time when the overriding plate was moving away form the trench in this location.
In the southeastern portion of the margin, the overriding plate moves toward the trench throughout most of the Cenozoic. Paleomagnetic studies have been used to determine the rotation of the Philippine Sea plate as its motion through time cannot be linked to the global plate circuit due to the Philippine Sea plate being entirely surrounded by subduction zones.
In the south, the absolute motion of the overriding plate changes from toward the trench to away form the trench at 35 Ma Figure The overriding plate continues to migrate away from the trench until 20—25 Ma, corresponding to the onset of clockwise rotation of the Philippine Sea plate. The overriding plate moves away from the trench during the opening of the Mariana Trough due to the initiation of rotation of the Philippine Sea plate at 5 Ma [ Hall et al.
The velocities of the downgoing plate are high compared with the velocity values for the overriding plate.
The increase in velocity of the Pacific plate southward is a function of the location of the pole of rotation of the Pacific plate with respect to the position of the trench. The overriding plate travels toward the trench between 40 and 60 Ma and then migrates slowly away from the trench between 25 and 40 Ma.
After 25 Ma, the overriding plate again travels toward the trench but with a greater velocity. The initiation of spreading in the Japan Sea occurred at 30 Ma [ Jolivet et al. In the western Aleutians, the absolute motion of both the downgoing and overriding plate is quite small Figure The absolute motion of the overriding plate is slow in comparison, migrating in the direction of the trench throughout its entire Cenozoic history.
The absolute motion of the downgoing and overriding plates along the Cascadia subduction zone is toward the trench with a reasonably high rate of motion for all times in the Cenozoic.
The overriding North American plate is moving in the direction of the trench throughout the Cenozoic at a smaller velocity than the downgoing Juan de Fuca and Farallon plates. The North American plate is the overriding plate at all times in the Cenozoic and its motion is always toward the trench, with minor changes in the magnitude of velocity over time.
The absolute motion of the downgoing and overriding plates along the Andes trench is characterized by the fast velocity of the downgoing plate toward the trench and the constant trenchward motion of the overriding South American plate Figure A similar pattern, but smaller in magnitude, is observed at the southern end of the Andes system. The peaks in velocity of the downgoing plate coincide with the increase in the age of material being subducted along the trench and the break up of the Farallon plate in the latter case.
The fast motion of the overriding South American plate toward the trench causes extensive compression and subduction erosion along the Andean margin [ Lamb and Davis , ].
The absolute motion of the overriding plate is forcing the rollback of the Andes subduction hinge. The absolute direction of motion of the overriding plate along the entire eastern Pacific margin is oriented toward the trench, with the fastest rates experienced adjacent to the South American plate along the Andes.
A relatively fast motion of the overriding plate toward the trench results in a compressional strain regime on the overriding plate adjacent to the subduction boundary and extensive subduction erosion and compression is observed along the margin.
Volcanic Arcs and Subduction
In contrast, the absolute motion of the overriding plate along the western Pacific changes through time from being toward the trench where we expect a compressional stress regime to away from the trench resulting in an extensional stress regime.
The separation of subduction zones based on the strain regime on the overriding plate by Jarrard  from highly compressional to highly extensional was found to be strongly related to the absolute motion of the overriding plate and the age of the subducting oceanic lithosphere.