Rapid Microplate Rotations and Backarc Rifting at the Transition Between Collision and Subduction

Rapid microplate rotations and backarc rifting at the transition between collision and subduction

Laura M. Wallace Institute of Geological and Nuclear Science, Lower Hutt 6315, New Zealand Robert McCaffrey Department of Earth and Environmental Science, Rensselaer Polytechnic Institute, 110 8th Street, Troy, New York 12180, USA John Beavan   Institute of Geological and Nuclear Science, Lower Hutt 6315, New Zealand Susan Ellis 

ABSTRACT In all regions studied, we de®ne the geometry Using global positioning system velocities from convergent plate boundaries in Papua of block boundaries (faults) based on earth- New Guinea, New Zealand, Tonga, Vanuatu, and the Marianas, we note a spatial corre- quakes and geology. To place the regional lation between rapid tectonic block rotations and the transition from subduction to col- GPS velocities into larger plate kinematic con- lision. We present a mechanism for the block rotations, in which the change from collision texts, we use GPS velocities from sites on the of a buoyant indentor to normal subduction exerts a torque on the upper-plate microplate. bounding plates (e.g., Australian, Paci®c, and This work improves our understanding of the causes of rapid vertical axis rotations, often Philippine Sea plates) given in Beavan et al. observed in paleomagnetic studies. We also show how collision-induced rotations may lead (2002) and Sella et al. (2002). to backarc rifting. Papua New Guinea Keywords: GPS, global positioning system, collision, subduction, microplate rotation, backarc Papua New Guinea forms the boundary rifting, tectonics. zone between the obliquely and rapidly (ϳ110 mm/yr) converging Australian and Paci®c INTRODUCTION the interseismic period are hampered by the plates (Fig. 1). The southern boundary of the The fastest modern-day tectonic block ro- effects of elastic strain due to locking on faults South Bismarck microplate changes (from tations, as much as 9Њ/m.y., occur in the fore- (particularly the subduction zone). To deal west to east) from collision between the Fin- arcs of convergent plate margins (e.g., Cal- with this problem, we simultaneously invert isterre Range and the underthrusting New mant et al., 2003; Wallace et al., 2004a). It GPS velocities, earthquake slip vector azi- Guinea Highlands to subduction of oceanic has been suggested that the bending and ro- muths, transform fault orientations, and long- crust at the New Britain trench in the Solomon tation of subduction-zone forearcs can be term geological fault slip rates (including sea- Sea. The northern boundary of the South Bis- caused by collision of buoyant features with ¯oor spreading rates) for angular velocities of marck microplate is marked by the Manus and the subduction zone (e.g., Vogt et al., 1976; tectonic blocks and the degree of coupling on New Guinea backarc rifts, which are connect- McCabe, 1984). Earlier studies of forearc faults bounding the blocks (e.g., McCaffrey, ed by left-lateral transform faults along the block rotations were based on paleomagnetic 1995, 2002). Throughout this paper we refer Bismarck Sea seismic lineation (e.g., Taylor, declination anomalies that can provide the to the point where the angular velocity inter- 1979). GPS velocities (Wallace et al., 2004a) long-term vertical axis rotation rate, but can- sects Earth's surface as the ``pole of rotation.'' show that the South Bismarck microplate ro- not constrain the full angular velocity, so that We refer to the tectonic blocks or microplates tates clockwise relative to the New Guinea the orientation of the spin axis is unknown. (often coinciding with the forearc and/or arc) Highlands microplate (the lower plate of the Using published global positioning system as ``convergent plate boundary microblocks.'' collision) at ϳ9Њ/m.y. about a pole where the (GPS) velocities from ®ve convergent margins around the world, we estimate the angular velocities of forearc blocks to reveal an apparent Figure 1. Tectonic setting spatial relationship between the rotation of up- of Papua New Guinea and per-plate microplates and the collisions of vectors showing rotation buoyant crustal masses (e.g., oceanic plateaus, of South Bismarck plate seamount chains, or continental fragments) (SBS) relative to lower plate (see Wallace et al., with the subduction zones. We suggest that the 2004a, for details of mod- change from collision to subduction exerts a eling results). Pole of ro- torque on the upper-plate microplate, causing tation for South Bismarck the microplate to rotate rapidly (relative to the microplate relative to New Guinea Highlands subducting plate) about a point near where the (NGH) is shown by semi- collision occurs. These rotations often result circular arrow, clockwise. in backarc rifting. Our observations have im- Rates of rifting in Manus plications for the knowledge of what drives Basin (white arrows) are rapid crustal block rotations, and the causes of given in mm/yr. BSSLÐ Bismarck Sea seismic lin- backarc rifting. eation. In best-®tting ؍ ␹2 model, we obtain n KINEMATIC MODELING OF BLOCK 1.99, using 333 earth- ROTATIONS AT CONVERGENT quake slip vectors and 66 PLATE BOUNDARIES global positioning system velocities to estimate Estimates of tectonic block rotations at sub- 44 parameters. duction margins from GPS velocities during

᭧ 2005 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. Geology; November 2005; v. 33; no. 11; p. 857±860; doi: 10.1130/G21834.1; 5 ®gures. 857 Figure 3. Tectonic setting of Tonga (Tong) and Va- nuatu (Van), global positioning system (GPS) velocities (observed, black; model, red), and rotation poles of Vanuatu and Tonga tectonic blocks relative to subducting plate. GPS velocity vectors shown relative to Austra- lian (Aus) plate. White vectors with rates next to them show relative motion in backarc (rates are in mm/yr, predicted from block model). For best ؍ ␹2 model, we obtain n 2.13, using 952 earthquake slip vectors, 4 spreading rates, and 83 GPS velocities to estimate 42 free parameters. NHTÐNew Hebrides Trench; EFTÐErromango-Futuna Trough; WTPÐWest Torres Plateau; DERÐD'Entrecasteaux Ridge; VANÐVanuatu; WNFBÐwestern North Fiji Basin; HHRÐHazel Holme Ridge; CSRÐcentral spreading ridge.

velocities show that several crustal blocks in wise rotation rate of Tonga estimated from pa- the eastern North Island rotate clockwise at leomagnetic studies (Sager et al., 1994). 3.8Њ±5.0Њ/m.y. relative to the subducting Pa- ci®c plate (Wallace et al., 2004b). Paleomag- Vanuatu Figure 2. Rotation of New Zealand tectonic netic declinations and geological strain esti- Vanuatu straddles the Australia±Paci®c blocks relative to Paci®c plate and tectonic mates also suggest clockwise rotation of the plate boundary zone between the Tonga-Fiji setting (see Wallace et al., 2004b, for details eastern North Island (e.g., Walcott, 1984; region and New Guinea (Fig. 3). The major of modeling results). Tectonic blocks and Beanland and Haines, 1998). Backarc rifting tectonic features in this region are the New their respective poles of rotation are labeled (block names: WAIRÐWairarapa, RAUKÐ in the Taupo volcanic zone (central North Is- Hebrides Trench, where Australia subducts be- Raukumara, CHKGÐCentral Hikurangi, land) is related to eastern North Island rota- neath Vanuatu, and a complex series of rifts AXIRÐAxial Ranges). White vectors are lation. The poles of rotation of the eastern North and transforms in the North Fiji Basin. Sub- beled with extension rates in Taupo volcanic Island tectonic blocks relative to the subduct- duction of various buoyant features at the New zone (TVZ) and convergence rates at Hiku- ing Paci®c plate are located adjacent to where Hebrides Trench (e.g., D'Entrecasteaux Ridge; rangi Trough (rates in mm/yr). Note: arrows in TVZ are not to scale. In our best-®tting the buoyant Chatham Rise impinges upon the West Torres Plateau) has in¯uenced the recent ؍ ␹2 model, we obtain n 1.32 from 367 GPS margin (Fig. 2). deformation of the Vanuatu island chain (e.g., velocities, 10 earthquake slip vectors, and Pelletier et al., 1998; Calmant et al., 2003). 138 free parameters. Tonga Schellart et al. (2002) suggested that the North The Paci®c plate subducts westward be- Fiji Basin is produced by asymmetric opening Finisterre collision is most advanced (Fig. 1). neath the Australian plate (Fig. 3) at the Tonga about a hinge point located near 11ЊS, 165ЊE, South Bismarck microplate rotation results in Trench east of the Tonga Islands. West of Ton- and that this asymmetric opening is produced rapid rifting in the Manus and New Guinea ga, some of the highest documented rates of by variation in the degree of rollback of the Basins. Paleomagnetic evidence indicates that backarc rifting in the world are in the Lau Ba- subducting Australian plate. the South Bismarck microplate has been ro- sin (e.g., Bevis et al., 1995; Taylor et al., Following Calmant et al. (2003), we divide tating clockwise at an average rate of 8Њ/m.y. 1996). The southern limit of rifting in the Lau Vanuatu into central and southern tectonic since the Finisterre collision initiated (Weiler Basin is near 26ЊS, adjacent to where the Lou- blocks and include the western North Fiji Ba- and Coe, 2000), consistent with the geodetic isville Seamount chain subducts, and rifting sin as a separate block (Fig. 3). The eastern rates. propagates south at ϳ100 mm/yr (Parson and boundaries of the southern and central blocks Wright, 1996; Taylor et al., 1996) (Fig. 3). coincide with the zone of backarc thrusting New Zealand Much of the rifting history of the eastern Lau and extension just to the east of the island The plate boundary in the North Island, Basin is linked to the 128 mm/yr southward chain. We invert GPS velocities (Calmant et New Zealand, accommodates subduction of migration of the Louisville Seamount chain± al., 2003), spreading rates (Pelletier et al., the Paci®c plate beneath the North Island at Tonga Trench intersection (Pelletier et al., 1998), and earthquake slip vectors for angular the Hikurangi Trough (Fig. 2). Northeast of 1998; Ruellan et al., 2003). velocities of the blocks, and coupling on the the North Island, oceanic crust is subducted at GPS velocities from Tonga (Bevis et al., subduction zone. The poles of rotation for the Kermadec Trench (the northeastern contin- 1995) and 431 earthquake slip vectors on the central and southern Vanuatu relative to the uation of the Hikurangi Trough). The sub- Tonga Trench indicate that Tonga rotates Australian plate cluster near 165ЊE, 11ЊS, just ducting oceanic Paci®c plate changes south- clockwise (ϳ9.3ЊϮ0.3Њ/m.y.) relative to the to the north of D'Entrecasteaux Ridge and ward to the Hikurangi Plateau (an oceanic subducting Paci®c plate about a pole just to West Torres Plateau subduction, with a clock- plateau), and eventually the Chatham Rise (a the south of where the Louisville Seamount wise rotation rate of 6.0Њ±7.5Њ/m.y. (Fig. 3). continental fragment) collides with the Hiku- chain subducts beneath the margin (Fig. 3). These rates are comparable to paleomagnetic rangi margin where subduction ceases. GPS This is comparable to the 6Њ±8Њ/m.y. clock- estimates of 28Њ of rotation (clockwise) of

858 GEOLOGY, November 2005 teau collides with the subduction margin (Fig. 4). The GPS-derived kinematics of Marianas backarc rifting are largely consistent with ma- rine geophysical studies, which suggest that opening rates increase from the northern end of the Marianas rift (20ЊN, ϳ10 mm/yr) to the southern end (30±45 mm/yr from 18ЊNto 14ЊN) (see Kato et al., 2003, for a review).

WHY DO CONVERGENT PLATE BOUNDARY MICROBLOCKS ROTATE RAPIDLY AT SUBDUCTION MARGINS WHERE A COLLISION OCCURS? In the ®ve areas discussed here, the poles of rotation for the convergent plate boundary microblocks relative to the subducting plate occur near where a buoyant crustal mass col- Figure 4. Tectonic setting of Marianas (Mari). lides with the subduction zone (Figs. 1±4). Global positioning system (GPS) velocities Wallace et al. (2004a) suggested that in Papua (observed, black; model, red), and rotation New Guinea, the change from collision to sub- Figure 5. Schematic of our model for pole of Marianas arc (all relative to Paci®c duction explains the rapid rotation of the collision-induced rotation. CPBMÐconver- [Paci] plate). White arrows with extension gent plate boundary microblock. rates next to them (in mm/yr) are predicted South Bismarck microplate and subsequent relative motion in backarc. For this model, rifting in the Manus Basin. This inference is ؍ ␹2 n 2.28, estimating 24 free parameters us- based on three lines of evidence: (1) the South ing 39 local and regional GPS velocities and torque on the convergent plate boundary mi- 23 earthquake slip vectors. Bismarck microplate rotates relative to the un- croblock, causing it to rotate relative to the derthrusting New Guinea Highlands about a lower plate about a pole near where the buoy- pole where collisional resistance forces are ex- ant indentor enters the subduction zone (Fig. much of Vanuatu since ca. 6 Ma (Falvey, pected to be highest (Fig. 1); (2) the Finisterre 5). If slab rollback occurs, it will enhance the 1978). The clockwise rotation of Vanuatu collision initiated at 3.5±4 Ma (Abbott et al., resulting rotation, although our mechanism leads to rifting in the Erromango-Futuna 1994), while rifting in the Manus Basin began does not require rollback of the slab. We must Troughs (30±60 mm/yr) and shortening in the simultaneously or soon afterward, ca. 3.5 Ma point out that there are some locations where backarc region (north of 18ЊS) just to the east (Taylor, 1979); and (3) paleomagnetic studies a transition from collision to subduction oc- of Vanuatu (Fig. 3). indicate that the South Bismarck microplate curs, yet rapid rotations are not apparent in ϳ Њ has been rotating at 8 /m.y. (consistent with modern geodetic data (e.g., Nazca Ridge sub- Marianas the modern geodetic estimate) since Finisterre duction in South America, and Cocos Ridge The Marianas Islands are located between collision initiation (Weiler and Coe, 2000). subduction offshore Costa Rica). the active Marianas backarc rift and the Ma- Similar (but less complete) arguments can be rianas Trench, where the Paci®c plate subducts constructed for the other regions addressed in CORRELATION OF COLLISION- westward beneath the Philippine Sea plate this paper. INDUCED ROTATIONS WITH (Fig. 4). The Marianas backarc began rifting We suggest the following mechanism to ex- BACKARC RIFTING in the late Miocene, separating the Mariana plain the observed rapid microplate rotations In all our case study areas, there is a clear arc from the West Mariana Ridge (e.g., Karig at convergent margins (Fig. 5). Where a buoy- link between convergent plate boundary mi- et al., 1978), and propagated northward with ant indentor enters a subduction zone, con- croblock rotation and extension in the backarc time; the current rift tip is near 24ЊN (Stern et vergence is inhibited due to high collisional region (Figs. 1±5). Does the torque exerted on al., 1984). North of the Marianas, the Ogasa- resistance forces, and the convergent plate the microplate by an along-strike change in wara Plateau collides with the trench, and to boundary microblock is pushed into the upper subducting plate buoyancy merely modify the the south, the Caroline Plateau impinges upon plate as shortening is transferred from the rate of pre-existing backarc opening, or can it the margin (Fig. 4). The collision point of the trench into the backarc region. Where subduc- actually trigger localized extension in the Ogasawara Plateau migrates northward at ϳ12 tion of more negatively buoyant oceanic lith- backarc? Based on the Papua New Guinea mm/yr, based on Paci®c and Philippine Sea osphere occurs, the subducting slab may either case, we suggest that the latter is possible. plate relative motion. McCabe and Uyeda roll back (e.g., Molnar and Atwater, 1978), or Any viable working hypothesis regarding (1983) suggested that the collision of the Oga- the position of the slab will remain stationary the origin of backarc rifting must explain the sawara and Caroline Plateaus with the Mari- with respect to the surrounding mantle (e.g., episodic ephemeral nature of most backarc rift anas Trench explains the curvature and rota- Uyeda and Kanamori, 1979). The slab suction systems, the lack of rifting behind most sub- tion of the Marianas arc. effect (caused by ¯ow patterns induced in the duction margins, and the rapid rotation of con- We estimate the angular velocity of the Ma- mantle by the subduction process) causes the vergent plate boundary microblocks associat- rianas arc microplate by inverting GPS veloc- large upper plate to move toward the subduc- ed with many backarc rifts. Hence, it is likely ities from sites in the Marianas Islands (Kato tion zone, and requires the forearc to stay in that a special set of circumstances is required et al., 2003) and earthquake slip vectors on contact with the subducting plate (e.g., Conrad to initiate and maintain backarc rifting. Most the Marianas Trench. The Marianas arc rotates and Lithgow-Bertelloni, 2004). These two proposed models for backarc rifting are two- counterclockwise (2.5ЊϮ0.8Њ/m.y.) relative to competing effects (i.e., a landward push on the dimensional, and involve extension as either the subducting (Paci®c) plate about a pole of forearc at the collision point, and a trenchward actively driven by asthenospheric processes rotation adjacent to where the Ogasawara Pla- pull where normal subduction occurs) exert a (e.g., corner ¯ow and mantle diapir hypothe-

GEOLOGY, November 2005 859 ses), or passively responding to the kinematic Beanland, S., and Haines, J., 1998, The kinematics (SW Paci®c): Locking/unlocking induced by boundary conditions (e.g., slab anchor and of active deformation in the North Island, New seamount chain subduction: Geochemistry, Zealand, determined from geological strain Geophysics, Geosystems, v. 4, doi: 10.1029/ rollback hypotheses) (see Taylor and Karner, rates: New Zealand Journal of Geology and 2001GC000261. 1983, for a review). None of these can explain Geophysics, v. 41, p. 311±323. Sager, W.W., MacLeod, C.J., and Abrahamsen, N., the lack of backarc rifting at most subduction Beavan, J., Tregoning, P., Bevis, M., Kato, T., and 1994, Paleomagnetic constraints on Tonga arc zones, or its typically short duration (ϳ10 Meertens, C., 2002, Motion and rigidity of the tectonic rotation from sediments drilled at Paci®c plate and implications for plate bound- Sites 840 and 841, in Hawkins, J., Parson, L., m.y. or less). Our model of collision-induced ary deformation: Journal of Geophysical Re- Allan, J., et al., Proceedings of the Ocean Dril- rotations and backarc rifting incorporates search, v. 107, no. B10, p. 2261, doi: 10.1029/ ling Program, Scienti®c results, Volume 135: some of the former mechanisms, but differs in 2001JB000282. College Station, Texas, Ocean Drilling Pro- that along-strike variations in the processes Bevis, M., Taylor, F.W., Schutz, B.E., Recy, J., Is- gram, p. 763±783. acks, B.L., Helu, S., Singh, R., Kendrick, E., Schellart, W.P., Lister, G.S., and Jessell, M.W., 2002, are paramount. Invoking collisions as the Stowell, J., Taylor, B., and Calmant, S., 1995, Analogue modeling of arc and backarc defor- causative agent in the along-strike variations Geodetic observations of very rapid conver- mation in the New Hebrides and North Fiji also satis®es the ephemeral nature of the back- gence and back-arc extension at the Tonga arc: Basin: Geology, v. 30, p. 311±314, doi: Ͻ arc rifting. This mechanism may also in¯u- Nature, v. 374, p. 249±251, doi: 10.1038/ 10.1130/0091-7613(2002)030 0311: 374249a0. AMOAABϾ2.0.CO;2. ence backarc rifting in the Okinawa trough Calmant, S., Pelletier, B., Lebellegard, P., Bevis, M., Sella, G.F., Dixon, T.H., and Mao, A.L., 2002, REV- and southwest Japan (McCabe, 1984), and the Taylor, F.W., and Phillips, D.A., 2003, New in- EL: A model for recent plate velocities from Mediterranean. It is unlikely that our mecha- sights on the tectonics along the New Hebri- space geodesy: Journal of Geophysical Re- nism can explain rifting in the Scotia Basin or des subduction zone based on GPS results: search, v. 107, no. B4, doi: 10.1029/ 2000JB000033. Havre Trough, although kinematic data in Journal of Geophysical Research, v. 108, no. B6, doi: 10.1029/2001JB000644. Stern, R.J., Smoot, N.C., and Rubin, M., 1984, Un- these locations are too sparse to test this Conrad, C.P., and Lithgow-Bertelloni, C., 2004, The zipping of the volcano arc, Japan: Tectono- possibility. temporal evolution of plate driving forces: Im- physics, v. 102, p. 153±174, doi: 10.1016/ Subduction zones are complex systems, and portance of ``slab suction'' versus ``slab pull'' 0040-1951(84)90012-X. Taylor, B., 1979, Bismarck Sea: Evolution of a it is likely that feedbacks between the various during the Cenozoic: Journal of Geophysical Research, v. 109, no. B10, doi: 10.1029/ back-arc basin: Geology, v. 7, p. 171±174, doi: processes also contribute to the observed na- 2004JB002991. 10.1130/0091-7613(1979)7Ͻ171:BSEOABϾ ture of rifting. For example, rifting initially Falvey, D.A., 1978, Analysis of paleomagnetic data 2.0.CO;2. induced by collision and rotation may in¯u- from the New Hebrides: Australian Society of Taylor, B., and Karner, G., 1983, On the evolution of marginal basins: Reviews of Geophysics, ence ¯ow patterns in the mantle wedge, and Exploration Geophysical Bulletin, v. 9, p. 117±123. v. 21, p. 1727±1741. facilitate upwelling of molten material, adding Karig, D.E., Anderson, R.N., and Bibee, L.D., 1978, Taylor, B., Zellmer, K., Martinez, F., and Goodliffe, to the total extension budget (a positive feed- Characteristics of backarc spreading in the A., 1996, Sea¯oor spreading in the Lau back- back). Alternatively, the mantle ¯ow may re- Mariana Trough: Journal of Geophysical Re- arc basin: Earth and Planetary Science Letters, v. 144, p. 35±40, doi: 10.1016/0012- quire increasingly large stress, which may search, v. 83, p. 1213±1226. Kato, T., Beavan, J., Matsushima, T., Kotake, Y., 821X(96)00148-3. limit the deformation rates, or changes in the Camacho, J.T., and Nakao, S., 2003, Geodetic Uyeda, S., and Kanamori, H., 1979, Back-arc open- curvature of the subduction zone or fault ori- evidence of back-arc spreading in the Mariana ing and mode of subduction: Journal of Geo- entations resulting from block rotation could Trough: Geophysical Research Letters, v. 30, physical Research, v. 84, p. 1049±1061. Vogt, P.R., Lowrie, A., Bracey, D.R., and Hey, R.N., affect the force balance of the system. no. 12, doi: 10.1029/2002GL016757. McCabe, R., 1984, Implications of paleomagnetic 1976, Subduction of aseismic oceanic ridges: data on the collision-related bending of island Effects on shape, seismicity, and other char- CONCLUSIONS arcs: Tectonics, v. 4, p. 409±428. acteristics of consuming plate boundaries: We document a correlation between rapid McCabe, R.J., and Uyeda, S., 1983, Hypothetical Geological Society of America Special Paper 172, 59 p. convergent plate boundary microblock rota- model for the bending of the Mariana arc, in Hayes, D.E., ed., The tectonic and geological Walcott, R.I., 1984, The kinematics of the plate tions and the collision of buoyant features evolution of Southeast Asian seas and islands, boundary zone through New Zealand: A com- with a subduction margin. To explain this phe- Part 2: American Geophysical Union Geo- parison of short- and long-term deformations: Royal Astronomical Society Geophysical nomenon, we suggest a mechanism in which physical Monograph 27, p. 281±293. Journal, v. 79, p. 613±633. McCaffrey, R., 1995, DEF-NODE users guide: the collision modi®es the along-strike gradient Wallace, L., Stevens, C., Silver, E., McCaffrey, R., Troy, New York, Rensselaer Polytechnic In- in the convergence rate, which in turn causes Loratung, W., Hasiata, S., Stanaway, R., Cur- stitute, 19 p. ley, R., Rosa, R., and Taugaloidi, J., 2004a, the rapid microplate rotations. In some cases, McCaffrey, R., 2002, Crustal block rotations and GPS and seismological constraints on active these tectonic block rotations may lead to plate coupling, in Stein, S., and Freymueller, tectonics and arc-continent collision in Papua J., eds., Plate boundary zones: American Geo- backarc rifting. New Guinea: Implications for mechanics of physical Union Geodynamics Series 30, microplate rotations in a plate boundary zone: p. 100±122. ACKNOWLEDGMENTS Journal of Geophysical Research, v. 109, Molnar, P., and Atwater, T., 1978, Interarc spreading no. B5, doi: 10.1029/2003JB002481. 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860 GEOLOGY, November 2005