Chapter 1 - Plate Tectonics
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meet. Such junctions are a necessary consequence of rigid plates on a sphere, since this is the common way a plate bound- ary can end. There are sixteen possible combinations of ridge, trench, and transform-fault triple junctions (McKenzie and Morgan, 1969), of which only six are common. Triple junctions are classified as stable or unstable, depending on whether they preserve their geometry as they evolve. The geometric conditions for stability are described with vector velocity triangles in Figure 1.7 and only RRR triple junctions are stable for all orientations of plate boundaries. It is important to understand evolutionary changes in triple junctions, be- cause changes in their configuration can produce changes that superficially resemble changes in plate motions. Triple junction evolution is controlled by the lengths of transform faults, spreading velocities, and the availabil- ity of magma. Convergent boundaries (subduction zones) Convergent plate boundaries are defined by earthquake hypocentres that lie in an approximate plane and dip beneath arc systems. This plane, known as the seismic zone or Benioff zone, dips at moderate to steep angles and extends in some instances to the 66()-km seismic discontinuity. The seismic zone is interpreted as a brittle region in the upper 10-20 km of descending lithospheric slabs. Modem seismic zones vary significantly in hypo- centre distribution and in dip (Figure 1.8). Some, such as the seismic zone beneath the Aleutian arc, extend to depths < 300 km while others extend to the 660-km discontinuity (Figure 1.8, a and b respectively). In gen- eral, seismic zones are curved surfaces with radii of curvature of several hundred kilometres and with irregu- larities on scales of < 100 km. Approximately planar seismic zones are exceptional. Seismic gaps in some zones (e.g., c and e) suggest, although do not prove, fragmentation of the descending slab. Dips range from 30 ° to 90 °, averaging about 45 \ Considerable varia- tion may occur along strike in a given subduction zone, as exemplified by the Izu-Bonin arc system in the West- ern Pacific. Hypocentres are linear and rather continu- ous on the northern end of this arc system, (d), becoming progressively more discontinuous toward the south. Near the southern end of the arc, the seismic zone exhibits a pronounced gap between 150 and 400 km depth, (c). A large gap in hypocentres in descending slabs, such as that observed in the New Hebrides arc, (e), may indicate that the tip of the slab broke off and settled into the mantle. Because some slabs appear to penetrate the 660- km discontinuity (Chapter 4), the lack of earthquakes below 700 km probably reflects the depth of the brittle- ductile transition in descending slabs. An excellent cor- Plate tectonics RRR TTT TTF FFR FFT Geometry XV Velocity triangle RTF Stability All orientations stable Stable if ab,ac form a straight l ine, or if be is parallel to the slip vector CA Stable if ac,bc form a straight l ine, or if C lies on ab Stable if C lies on ab,or if ac, be form a straight line Stable if ab, be form a straight line, or if ae,be do so Stable if ab goes through C, or if ac, be form a straight line Example East Pacific Rise and Galapagos Rift zone. Central Japan. Intersection of the Peru-Chile Trench and the West Chile Rise. Owen fracture zone and the Carlsberg Ridge West Chile Rise and the East Pacific Rise. San Andreas Fault and Men- docino Fracture Zone. Mouth of the Gulf of California. Figure 1.7 Geometry and stability requirements of six common triple junctions. Dashed lines ab, be, and ac in the velocity triangles join points, the vector sum of which leave the geometry of AB, BC, and AC, respectively, unchanged. The junctions are stable only if ab, be, and ac meet at a point. Key: track symbol, trench; double line, ocean ridge; single line, transform fault. relation exists between the length of seismic zones and the product of plate convergence rate and age of the downgoing slab. First-motion studies of earthquakes in subduction zones indicate variation in movement both with lateral dis- tance along descending slabs and with slab depth. Sea- ward from the trench in the upper part of the lithosphere where the plate begins to bend, shallow extensional mechanisms predominate. Because of their low strength, sediments in oceanic trenches cannot transmit stresses, and hence are usually flat-lying and undeformed. Seis- mic reflection profiles indicate, however, that rocks on the landward side of trenches are intensely folded and faulted. Thrusting mechanisms dominate at shallow depths in subduction zones (20-100 km). At depths < 25 km, descending slabs are characterized by low seismicity. Large-magnitude earthquakes are generally thrust-types and occur at depths > 30 km (Shimamoto, 1985). Dur- ing large earthquakes, ruptures branch off the slab and extend upwards forming thrusts that dip away from the trench axis. Calculations of stress distributions at < 300 km depth show that compressional stresses generally dominate in the upper parts of descending slabs, whereas tensional stresses are more important in the central and lower parts (Figure 1.9). At 300-350 km depth in many slabs compressional stresses are very small, whereas at depths > 400 km a region of compressional stress may be bounded both below and above by tensional stress regions. The seismicity in descending slabs is strongly cor- related with the degree of coupling between the slab and the overriding plate (Shimamoto, 1985). The low seismicity in descending slabs at depths < 25 km may reflect relatively high water contents and the low strength of subducted hydrous minerals, both of which lead to decoupling of the plates and largely ductile deformation. At greater depths, diminishing water and hydrous min- eral contents (due to slab devolatilization) result in greater 10 Plate Tectonics and Crustal Evolution 0 100 ^ 200 E i 300 H 400 § 500 600 7nr> T V t * Aleutian Islands - (I74'W ITT^W) - a 1 1 1 1 1 T V • * - Northern ** • Mariana Arc .:. . b _ L _l 1 i» T V ! » " • 1 1 1 I 1 1 '^W^, 1 Vl*. 1 Southern :''..*•*•-•• . - • I . Izu-Bonin Arc J c 1 . J 1 1 1 1 1 1 0 100 200 J 300 X 4 0 0 h- Q. UJ 500 6 0 0 7 0 0 100 200 ^ i . M I 300 D V 0 100 S T A 200 300 N C E T ...-.y. Northern Izu-Bonin Arc 0 100 200 ( knn ) V 300 400 500 600 t->:- New Hebrides Arc 100 200 300 400 500 600 0 D I S T A N C 100 200 300 400 500 600 700 E (km) Figure 1.8 Vertical cross-sections of hypocentre distributions beneath modem arc-trench systems. Each diagram shows earthquakes for 7-10 year periods between 1954 and 1969. T = trench axis; V = recently active volcanic chain. Distance is measured horizontally from each trench axis in kilometres. Hypocentre data from many sources, principally from National Earthquake Information Center, US Coast and Geodetic Survey. 0 200 E ^ ^ 400 X h- Q. LJ Q 600 800 V T A • 1 1 \ ^ ^ 1 /&jtm^^ j^/S^^ 1 J^j^m/y /Kr^O^ / y/W^ //^f0^ /^t/wW^ 1 /j(^J^W^ '^^^m^ / y^^r / /^^^ ^ ^ 1 V n i l Tension O Connpression J \ \ \ 1 300 200 100 0 DISTANCE (km) 100 200 Figure 1.9 Calculated distribution of stresses in a descending slab. Subduction rate, 8 cm/y and dip = 45 °. After Goto et al. (1985). V, volcanic front; T, trench. coupling of overriding and descending plates, and thus to the onset of major earthquakes.