Arc Polarity Reversal Research Articles

From subduction initiation to arc–polarity reversal: Life cycle of an Archean subduction zone from the Zunhua ophiolitic mélange, North China Craton

Subduction initiation and arc–polarity reversal have rarely been recognized in the Archean rock record. We document Neoarchean subduction initiation, fore-arc magmatism, and an arc–polarity reversal event from the Zunhua ophiolitic mélange along the eastern margin of the Central Orogenic Belt (COB) of the North China Craton (NCC). The Zunhua ophiolitic mélange is a mappable unit characterized by blocks of metamorphosed harzburgite/lherzolite, podiform chromite–bearing dunite, pyroxenite, amphibolite, metabasites (basalt and diabase) with rare intermediate volcanics, chert, and tectonic lenses of banded iron formation in a strongly sheared metapelitic matrix. New geochronological and geochemical analyses of magmatic blocks within the ophiolitic mélange show that the crustal magmatic rocks were produced in a fore-arc region at 2.55–2.52 Ga from depleted harzburgitic–lherzolitic mantle tectonites. Chemical, petrological, and temporal links between the depleted mantle blocks, and the suite of magmatic blocks derived from partial melting and metasomatism of these depleted mantle blocks, unequivocally shows that they represent part of the same original Neoarchean fore-arc ophiolite suite. After formation and accretion in the oceanic realm, the mélange was emplaced on the continental margin of the Eastern Block between 2.52 and 2.50 Ga, and underwent two stages of metamorphism at ca. 2.48–2.46 Ga and 1.81 Ga. Metamorphosed intermediate–mafic volcanic blocks exhibit systematic successive geochemical variations, from MORB-like to volcanic arc-like, and the N-MORB-like meta-basalts show remarkable similarity with the subduction initiation-related Izu–Bonin–Mariana (IBM) fore-arc basalts. We suggest that the Zunhua fore-arc complex records continuous geodynamic processes from subduction initiation to arc magmatism. The Zunhua ophiolitic mélange is part of a ca. 2.5 Ga suture between an oceanic arc of the COB and Eastern Block of the NCC. After the arc–continent collision, an arc–polarity reversal event initiated a new eastward–dipping subduction zone on the western side of the COB. This arc–polarity reversal is traced for more than 1600 km along the length of the orogen, similar in scale, geometry, and duration between collision and polarity flip to the present-day arc–polarity reversal of the Sunda–Banda arc during its ongoing collision with the Australia continent. This indicates that a life cycle of an Archean subduction zone, including birth (subduction initiation), maturity (arc magmatism), death (arc-continent collision) and re-birth (arc–polarity reversal), is recorded in the Zunhua ophiolitic mélange, and the geodynamics of plate tectonics at the end of the Archean was similar to those of today.

Magmatic record of Neoarchean arc-polarity reversal from the Dengfeng segment of the Central Orogenic Belt, North China Craton

The Dengfeng segment of the Archean Central Orogenic Belt in the North China Craton (NCC) includes circa 2.66–2.5 Ga metavolcanics, metasedimentary and plutonic rocks. All units are intruded by ∼2.5–2.4 Ga mafic dykes and a suite of coeval circa 2.5–2.45 Ga potassic granites and leucogranitic veins. Mafic dykes are characterized by mixed alkaline and calc-alkaline affinities, enriched LREE and negative Nb and Zr anomalies, indicating a lithospheric mantle source region. Potassic granitic dykes exhibit highly fractionated LREE patterns, and negative HFSE anomalies and positive zircon εHf(t) values, which are similar to those of TTG gneisses in the area. Leucogranitic veins cutting across the diorites are characterized by high contents of SiO2 and K2O, LREE-enriched patterns, positive Eu and Sr anomalies, negative Nb and Ti anomalies. We suggest that the granitic dykes and leucogranitic veins were derived from partial melting of thickened older crusts. Combined with other reported 2.5 Ga mafic dykes and potassic granites from the NCC, we propose that they demonstrate an ancient example of arc-polarity reversal and provide a record of the formation of a new plate boundary in the Archean. The underplating of the mafic dyke source beneath the newly accreted Eastern Block may have generated the widespread 2.5–2.47 HT-LP metamorphism with anticlockwise P-T paths in the Eastern Block.

Geochemistry of Neoarchean mafic volcanic rocks and late mafic dikes in the Zanhuang Complex, Central Orogenic Belt, North China Craton: Implications for geodynamic setting

An Archean mélange belt has been recognized in the Zanhuang Complex, Central Orogenic Belt (COB), North China Craton (NCC). All units of the mélange belt are intruded by a ca. 2.5Ga granite pluton and cross-cut by undeformed 2.5Ga pegmatites, which constrains the Archean formation age of the mélange. Some exotic mafic–ultramafic blocks are tectonically dispersed in the metasedimentary matrix to the mélange and mainly consist of metabasaltic rocks, deformed pillow lavas with epidosite cores, gabbros, and ultramafic rocks. Both metabasalts and epidosites are characterized by enrichment in light rare earth elements (LREE) and pronounced negative Nb and Zr anomalies on chondrite- and primitive mantle- normalized diagrams. These geochemical characteristics are consistent with supra-subduction zone geochemical signatures. Based on the pillow structures and seafloor hydrothermal alteration, combined with the geochemical characteristics, an intra-oceanic arc–forearc setting is proposed for the formation of these exotic blocks. Furthermore, a west-dipping intra-oceanic subduction zone is proposed to have formed in an ocean between the Eastern Block and an arc terrane prior to 2.5Ga. The collision between the arc and the Eastern Block of the NCC happened at ca. 2.5Ga, resulting in the formation of the Archean mélange belt in the Zanhuang Complex. All of the units of the mélange belt were intruded by late mafic dikes, which are present as boudinaged or lenticular shapes in the mélange and surrounding gneisses and are intruded by younger ca. 2.5Ga pegmatites. Geochemical characteristics of the late mafic dikes are consistent with an arc-related mantle source region, rather than an OIB-like source. Accordingly, pending determination of their precise age, an arc-polarity reversal event is proposed to have occurred following the arc–continent collision, resulting in east-dipping subduction beneath the newly accreted arc and Eastern Block, forming the swarm of arc-affinity mafic dikes. The new subduction zone further resulted in ponded magma below the accreted arc/Block, which induced the partial melting of thickened crust and the intrusion of ca. 2.5Ga granitic and pegmatitic melts into the mélange and the mafic dikes. The mafic dikes were then deformed and metamorphosed when the remaining open part of the ocean between the arc-collision modified margin of the Eastern Block collided with the Western Block of the NCC.

Thematic Section: Geological anatomy of East and South Asia: Paleogeography and paleoenvironments in Eastern Tethys (IGCP 516) Part 1

Thematic Section: Geological anatomy of East and South Asia: Paleogeography and paleoenvironments in Eastern Tethys (IGCP 516) Part 1

Baguio Mineral District: An oceanic arc witness to the geological evolution of northern Luzon, Philippines

AbstractThe Baguio Mineral District exposes rock formations that evince the geological and tectonic evolution of this district from a subduction‐related marginal basin to an island arc setting. Available onshore and offshore data are consistent with an Early (onset phase) to Middle (developed phase) Miocene arc polarity reversal from the east (termination of subduction along the proto‐East Luzon Trough) to the west (initiation of subduction along the Manila Trench). Geophysical modeling and geochemical data calculation showed a 30 ± 5 km crustal thickness for the mineral district. Subduction‐related multiple arc magmatism and ophiolite accretion contributed to crustal thickening. Recent information on the Oligo–Miocene Zigzag and Klondyke formations in the mineral district reveal that the marginal basin, where these rocks were deposited, has received eroded materials from adjacent terrains characterized by siliceous lithologies. Furthermore, adakitic rocks, high permeable zones and extensional zones which are exploration markers applied to identify possible mineralization targets, are prevalent in the mineral district. The geological evolution that the district had undergone mimics the evolution of island arcs worldwide in general and northern Luzon in particular.

Oligocene to Recent tectonic history of the Central Solomon intra-arc basin as determined from marine seismic reflection data and compilation of onland geology

Systematic analysis of a grid of 3450 km of multichannel seismic reflection lines from the Solomon Islands constrains the late Tertiary sedimentary and tectonic history of the Solomon Island arc and its convergent interaction with the Cretaceous Ontong Java oceanic plateau (OJP). The OJP, the largest oceanic plateau on Earth, subducted beneath the northern edge of the Solomon arc in the late Neogene, but the timing and consequences of this obliquely convergent event and its role in the subduction polarity reversal process remain poorly constrained. The Central Solomon intra-arc basin (CSB), which developed in Oligocene to Recent time above the Solomon arc, provides a valuable record of the tectonic environment prior to and accompanying the OJP convergent event and the subsequent arc polarity reversal. Recognition of regionally extensive stratigraphic sequences—whose ages can be inferred from marine sedimentary sections exposed onland in the Solomon Islands—indicate four distinct tectonic phases affecting the Solomon Island arc. Phase 1: Late Oligocene–Late Miocene rifting of the northeast-facing Solomon Island arc produced basal, normal-fault-controlled, asymmetrical sequences of the CSB; the proto-North Solomon trench was probably much closer to the CSB and is inferred to coincide with the trace of the present-day Kia-Kaipito-Korigole (KKK) fault zone; this protracted period of intra-arc extension shows no evidence for interruption by an early Miocene period of convergent “soft docking” of the Ontong Java Plateau as proposed by previous workers. Phase 2: Late Miocene–Pliocene oblique convergence of the Ontong Java Plateau at the proto-North Solomon trench (KKK fault zone) and folding of the CSB and formation of the Malaita accretionary prism (MAP); the highly oblique and diachronous convergence between the Ontong Java plateau and the Solomon arc terminates intra-arc extension first in the southeast (Russell subbasin of the CSB) during the Late Miocene and later during the Pliocene in the northwest (Shortland subbasin of the CSB); folds in the CSB form by inversion of normal faults formed during Phase 1; Phinney et al. [Sequence stratigraphy, structural style, and age of deformation of the Malaita accretionary prism (Solomon arc-Ontong Java Plateau convergent zone)] show a coeval pattern of southeast to northwest younging in folding and faulting of the MAP. Phase 3: Late Pliocene–early Pleistocene arc polarity reversal and subduction initiation at the San Cristobal trench. Effects of this event in the CSB include the formation of a chain of volcanoes above the subducting Australia plate at the San Cristobal trench, the formation of the broad synclinal structure of the CSB with evidence for truncation at the uplifted flanks, and widespread occurrence of slides and “seismites” (deposits formed by seismic shaking). Phase 4: Pleistocene to Recent continued shortening and synclinal subsidence of the CSB. Continued Australia-Pacific oblique plate convergence has led to deepening of the submarine, elongate basin axis of the synclinal CSB and uplift of the dual chain of the islands on its flanks.

The Cambrian-Silurian tectonic evolution of the northern Appalachians and British Caledonides: history of a complex, west and southwest Pacific-type segment of Iapetus

This paper is included in the Special Publication entitled 'Lyell: the past is the key to the present', edited by D.J. Blundell and A.C. Scott. This paper presents new ideas on the Early Palaeozoic geography and tectonic history of the Iapetus Ocean involved in the formation of the northern Appalachian-British Caledonide Orogen. Based on an extensive compilation of data along the length of the orogen, particularly using well-preserved relationships in Newfoundland as a template, we show that this orogen may have experienced a very complicated tectonic evolution that resembles parts of the present west and southwest Pacific Ocean in its tectonic complexities. Closure of the west and southwest Pacific Ocean by forward modelling of the oblique collision between Australia and Asia shows that transpressional flattening and non-coaxial strain during terminal collision may impose a deceptively simple linearity and zonation to the resultant orogen and, hence, may produce a linear orogen like the Appalachian-Caledonian Belt. Oceanic elements may preserve along-strike coherency for up to several thousands of kilometres, but excision and strike-slip duplication, as a result of oblique convergence and terminal collisional processes, is expected to obscure elucidation of the intricacies of their accretion and collisional processes. Applying these lessons to the northern Appalachian-Caledonian belt, we rely principally on critical relationships preserved in different parts of the orogen to constrain tectonic models of kinematically-related rock assemblages. The rift-drift transition, and opening of the Iapetus Ocean took place between c.590-550 Ma. Opening of Iapetus was temporally and spatially related to final closure of the Brazilide Ocean and amalgamation of Gondwanaland. During the Early Ordovician, the Laurentian margin experienced obduction of young, supra-subduction-zone oceanic lithosphere along the length of the northern Appalachian-British Caledonian Belt. Remnants of this lithosphere are best preserved in western Newfoundland and are referred to as the Baie Verte Oceanic Tract. Convergence between Laurentia and the Baie Verte Oceanic Tract was probably dextrally oblique. Slab break-off and a subsequent subduction polarity reversal produced a continental magmatic arc, the Notre Dame Arc, on the edge of the composite Laurentian margin. The Notre Dame Arc was mainly active during the late Tremadoc-Caradoc interval and was flanked by a southeast- or south-facing accretionary complex, the Annieopsquotch Accretionary Tract. Southerly drift of Laurentia to intermediate latitudes of c.20-25°S was associated with the compressive (Andean) nature of the arc and the accompanying backthrusting of the already-accreted Baie Verte Oceanic Tract further onto the Laurentian foreland. Equivalents of the Notre Dame Arc and its forearc elements in the British Isles have been preserved as independent slices in the Midland Valley and possibly the Northern Belt of the Southern Uplands. During the late Tremadoc (c.485 Ma), the passive margin on the eastern side of Iapetus also experienced obduction of primitive oceanic arc lithosphere. This arc is referred to as the Penobscot Arc. The eastern passive margin was built upon a Gondwanan fragment (Ganderia) that rifted off Amazonia during the Early Ordovician and probably travelled together with the Avalonian terranes as one microcontinent. The departure of Ganderia and Avalonia from Gondwana opened the Rheic Ocean. Equivalents of the Penobscot Arc may be preserved in New Brunswick and Maine, Leinster in eastern Ireland, and Anglesey in Wales. An arc-polarity reversal along the Ganderian margin after the soft Penobscot collision produced a new arc: the west-facing Popelogan-Victoria Arc, which probably formed a continuous arc system with the Bronson Hill Arc in New England. The Popelogan-Victoria Arc transgressed from a continental to an oceanic substrate from southern to northeastern Newfoundland. Rapid roll-back rifted the Popelogan-Victoria Arc away from Ganderia during the late Arenig (c.473 Ma) and opened a wide back-arc basin; the Tetagouche-Exploits back-arc basin. The Popelogan-Victoria Arc was accreted sinistrally oblique to the Notre Dame Arc and, by implication, Laurentia during the Late Ordovician. After accretion, the northwestward-dipping subduction zone stepped eastwards into the Tetagouche-Exploits back-arc basin.

Thrust emplacement of the Hispaniola peridotite belt: Orogenic expression of the mid-Cretaceous Caribbean arc polarity reversal?

Articulo sobre el emplazamiento fallero en la zona de peridotitas de la Hispaniola (Republica Dominicana), estudiando si es la expresion orogenica de la polaridad reversa en la zona del Caribe en el cretacico medio.

Geodetic measurements of convergence at the New Hebrides island arc indicate arc fragmentation caused by an impinging aseismic ridge

Global positioning system (GPS) measurements in 1990 and 1992 from two sites on the southern New Hebrides island arc give convergence rates with the Australian plate of 103 ± 5 mm/yr and 118 ± 10 mm/yr. In contrast, GPS measurements in the central New Hebrides indicate anomalously low convergence rates of ≈42 mm/yr. On geologic time scales, the mean central New Hebrides convergence rate has been 85–132 mm/yr. Elastic fault models with a locked interplate thrust zone indicate that maximum possible rates of horizontal elastic strain are insufficient to account for the anomalously slow convergence. Therefore, we propose that the central New Hebrides segment is moving eastward relative to adjacent arc segments at a rate of ≈36–83 mm/yr. This displacement is accommodated by crustal shortening at the eastern margin of the arc and strike-slip faults crosscutting the arc. Resistance to subduction of the aseismic D9Entrecasteaux Ridge system is the likely cause for horizontal forces sufficient to shove a large segment eastward and fragment the arc. This process demonstrates that subducting bathymetric features can impose fundamental structural modifications on an arc that may represent the initial stages of arc polarity reversal.

Deep structure of an arc‐continent collision: Earthquake relocation and inversion for upper mantle P and S wave velocities beneath Papua New Guinea

We examine mechanisms of lithospheric growth and modes accommodating subcrustal convergence following the cessation of subduction, by analyzing the seismicity and upper‐mantle velocities in the New Guinea arc‐continent collision. Earthquakes in the Papua New Guinea (PNG) region from 1967 to 1984 have been relocated in a joint inversion with seismic velocities, using P and S wave arrival times recorded by seismographs in PNG. A total of 957 well‐recorded earthquakes were chosen for use in three‐dimensional velocity inversions from an initial catalog of 12,960 events, based on the stability of initial hypocenter relocations. Constant velocity blocks were defined as irregular polyhedra to give fine detail in heavily sampled areas without introducing a large number of poorly controlled parameters, and to exploit a priori knowledge of the geometry of velocity anomalies. Spherical geometry was used to accurately calculate long ray paths (up to 1500 km in length). Most of the ∼700 best located hypocenters from the PNG data set are in the upper mantle beneath the Finisterre‐Huon (FH) ranges (the newly accreted island arc terrane), beneath the Papuan Peninsula, and in the New Britain seismic zone (an oceanic subduction system). These hypocenters show a well‐defined seismic zone dipping vertically or steeply to the north beneath the northern FH ranges from 125‐ to 250‐km depth, continuous along strike with the New Britain seismic zone to the east (which shows earthquakes to 600 km depth). The steeply dipping intermediate‐depth seismicity flattens near 100 km depth beneath the FH ranges and forms a subhorizontal seismic zone beneath the FH ranges. By contrast, seismicity continues upward to the surface beneath New Britain where the arc has not yet collided with New Guinea. The 100‐km‐deep flat zone does not continue south of the FH ranges but appears to be truncated below the eastern and southern boundaries of the island arc terrane, and no clear evidence for a south‐dipping seismic zone could be found. The relationship of the flat part of the slab 100 km beneath the FH ranges and surface faulting is difficult to discern; the apparent surface suture bounding the south side of the FH ranges is 100 km directly above the southern termination of intermediate‐depth seismicity. Nowhere in PNG could clear evidence for arc polarity reversal be found from seismicity. All of the well‐located subcrustal earthquakes beneath the Papuan peninsula lie in a narrow horizontal line between 125‐ and 175‐km depth that follows the center of the peninsula, and are distinctly separated from the FH seismic zone to the north. These events do not require a southward subducting plate beneath the Papuan peninsula. An alternate and reasonable explanation for these events is that they are a result of unstable thickening of the lithosphere. Information on lateral heterogeneity in the mantle is limited because large variations in crustal velocities dominate the observed pattern of travel time residuals. Allowing for only crustal heterogeneity reduces travel time residual variances by 40% from a simple one‐dimensional structure, while allowing for additional mantle heterogeneity results in ≤ 10% further reduction for the various parameterization s attempted here. Inversions with several different block geometries show low velocities at 35‐ to 171‐km km depth beneath the Bismarck Sea back‐arc basin and high velocities just south of the intermediate depth FH seismic zone. The pattern of lateral heterogeneity supports the inference from seismicity of north‐dipping slabs beneath both northeast New Guinea and New Britain.

Regional Offshore Geology of Central and Western Solomon Islands and Bougainville, Papua New Guinea: ABSTRACT

The central and western Solomon Islands and the Bougainville regions are parts of a complex island-arc system that includes an intra-arc basin and remnants of both forearc and back-arc depositional wedges. These features formed in response to episodic Cenozoic tectonism along the convergent boundary between the Pacific and Australia-India plates. Presumed early Tertiary southwest-directed subduction of the Pacific plate and associated arc magmatism were curtailed by impingement of the leading edge of the Ontong Java Plateau. Aprons of back-arc and forearc sediment were derived from highstanding parts of the arc during the late Oligocene and early Miocene. Late Tertiary arc-polarity reversal and northeastward-directed subduction of the Woodlark spreading system caused a renewal of island-arc magmatism that completed the construction of the Central Solomons Trough as an enclosed intra-arc basin. Interpretations of multichannel profiles from 1982 and 1984 CCOP/SOPAC Tripartite Cruises of the research vessel R/V S.P. Lee indicate that the Central Solomons Trough is a composite intra-arc basin containing as much as 5.5 km of late Oligocene(.) and younger sedimentary rocks. As many as five lenticular seismic-stratigraphic units can be identified on the basis of unconformities and abrupt velocity changes. Late Miocene and younger folds and faults deform the northeastmore » and southwest flanks of the basin. Profiles across the Kilinailau Trench show Ontong Java Plateau rocks covered by 2-4 km of trench sediment. The inner trench wall consists of folded, upfaulted, and rotated blocks of trench and forearc strata. The deep-water basin northwest of Bougainville is a southeastward extension of the New Ireland forearc basin, the southern margin of which is formed by a subsided part of the early Cenozoic arc. There, Oligocene(.) and younger basin strata, as much as 7 km thick, are deformed by pre-Pliocene faults and folds.« less

Polarity reversal in the Solomon Islands arc

McKenzie1 proposed that the direction of subduction beneath an island arc will reverse following arc–continent collision. Dewey and Bird2 suggested that such an arc polarity reversal occurred in northern New Guinea; Johnson and Molnar3, however, proposed two alternative models of plate convergence that could explain the observed seismicity. We present here a spatial seismicity study of the Solomon Islands region which has revealed the existence of two juxtaposed Wadati–Benioff (W–B) zones of opposite polarity. Shallow and intermediate foci define a north-east-dipping W–B zone associated with active subduction along the New Britain and San Cristobal trenches and a south-west-dipping W–B zone associated with the inactive North Solomon trench (NST). These data provide the first direct seismic evidence of a reversal in subduction polarity at an island arc.

Plate convergence between the Philippines and the Ryukyu islands

The region between the Philippine and Ryukyu Island arc systems, including the island of Taiwan, is one of the most complex parts of the consuming Philippine-Asian plate boundary. The marine geology of the area east and north of Luzon indicates that the Luzon arc extends northward beyond the south end of Taiwan and has collided with the latter in Pliocene time. Collision later occurred between the northern end of the Luzon arc system and the Ryukyu system. These collisions caused trench and trench slope rocks of the Luzon arc system to be uplifted as the Coastal Range of Taiwan and the arc system to be shortened in a north-south direction by strike-slip faulting. Continued plate consumption was partly absorbed by shallow east-dipping thrust faults in central and western Taiwan. Seismological and marine geophysical evidence suggest that arc polarity reversal, probably in response to collision, is now occurring along the Luzon arc system. A new trench is forming along the east side of Luzon but compressional features have not yet developed along the entire east flank of the Luzon system. Before collision, the Ryukyu and Luzon arc systems were separated by several hundred kilometers and joined by a trench-trench transform fault, which is now destroyed. In Early Tertiary time, there was probably an east-facing arc system along the Luzon trend, which may have been joined by another transform fault to the south end of Taiwan. Behind this Late Cretaceous-Early Tertiary east-facing arc system, the China Basin is thought to have opened as an inter-arc basin.