Continental Convergence Zone Research Articles

A Magnetotelluric Survey of Ophiolites in the Neyriz area of southwestern Iran

A wide band magnetotelluric study of the ophiolitic zone of the Zagros orogenic belt was conducted in the Neyriz area of southwestern Iran. The purpose of the study was to image subsurface structures electrically and relocate the main Zagros thrust fault in the region. The thrust fault has a complex structure with obscure behavior and is believed to be located within a zone of ongoing continental plate convergence. The fault zone with a NW–SE geological trend is parallel to the Zagros orogenic belt and separates the Neyriz ophiolite assemblage from the adjacent Sanandaj-Sirjan metamorphic zone. Magnetotelluric data were collected along a SW–NE profile across the geologic strike; the study included 18 stations and modeling was performed using a 2-D inversion scheme. Analysis of both modes of magnetotelluric data (TE and TM) clarifies the signatures of large resistivity variation in the study area. Due to the presence of a high contrast in resistivity between the ophiolites and neighboring rocks, we are able to discern two sharp boundaries as faulting planes and borders of the ophiolite–radiolarite zone in the north-eastern and southwestern parts of the 2-D resistivity models, respectively.

Diabatic heating profiles over the continental convergence zone during the monsoon active spells

The present paper aims to bring out the robust common aspects of spatio-temporal evolution of diabatic heating during the monsoon intraseasonal active phases over the continental tropical convergence zone (CTCZ). The robustness of spatio-temporal features is determined by comparing the two state-of-the art reanalyses: NCEP Climate Forecast System reanalysis and Modern ERA Retrospective Analysis. The inter-comparison is based on a study period of 26 years (1984–2009). The study confirms the development of deep heating over the CTCZ region during the active phase and is consistent between the two datasets. However, the detailed temporal evolution of the vertical structure (e.g., vertical tilts) of heating differs at times. The most important common feature from both the datasets is the significant vertical redistribution of heating with the development of shallow (low level) heating and circulation over the CTCZ region 3–7 days after the peak active phase. The shallow circulation is found to be associated with increased vertical shear and relative vorticity over certain regions in the subcontinent. This increased vertical shear and relative vorticity in the lower levels could be crucial in the sustenance of rainfall after the peak active phase. Model experiments with linear dynamics affirm the role of shallow convection in increasing the lower level circulation as observed.

Paleomagnetic constraints on Cenozoic deformation along the northwest margin of the Pacific‐Australian plate boundary zone through New Zealand

New Zealand straddles the boundary between the Australian and Pacific plates, a zone of oblique continental convergence and transform motion. The actively deforming region offers a unique opportunity to study the dynamics of deformation, including vertical‐axis rotation of rigid blocks within a transcurrent plate boundary zone. We present and interpret paleomagnetic data from three new and three previously published sites from the NW part of the South Island (NW Nelson region), where sedimentary strata dated between 36 and 10 Ma overlie the crystalline Paleozoic basement assemblages of the Gondwana margin. Compared with reference directions from the Australian apparent polar wander path, none of the results provide evidence of post‐Eocene vertical‐axis rotation. This suggests that for the past 36 Myr NW Nelson has remained a strong, coherent block that has moved as a contiguous part of the Australian plate. This is in marked contrast to the strongly rotated nature of more outboard accreted terranes to the east. For example, the Hikurangi Margin in the North Island (NW of the Alpine Fault) and the Marlborough region in the NE of the South Island (SE of the Alpine Fault), have both undergone diverse clockwise rotations of up to 140° since the early Paleogene. The NW tip of the South Island seems to have acted as a rigid backstop relative to these more complex oroclinal deformations. We infer that, because of its relatively stiff bulk rheology, it has not been drawn into the distributed plate boundary rotational deformation associated with the New Zealand Orocline.

Geodynamic evolution of Upper Cretaceous Zagros ophiolites: formation of oceanic lithosphere above a nascent subduction zone

AbstractThe Zagros fold-and-thrust belt of SW Iran is a young continental convergence zone, extending NW–SE from eastern Turkey through northern Iraq and the length of Iran to the Strait of Hormuz and into northern Oman. This belt reflects the shortening and off-scraping of thick sediments from the northern margin of the Arabian platform, essentially behaving as the accretionary prism for the Iranian convergent margin. Distribution of Upper Cretaceous ophiolites in the Zagros orogenic belt defines the northern limit of the evolving suture between Arabia and Eurasia and comprises two parallel belts: (1) Outer Zagros Ophiolitic Belt (OB) and (2) Inner Zagros Ophiolitic Belt (IB). These belts contain complete (if disrupted) ophiolites with well-preserved mantle and crustal sequences. Mantle sequences include tectonized harzburgite and rare ultramafic–mafic cumulates as well as isotropic gabbro lenses and isolated dykes within the harzburgite. Crustal sequences include rare gabbros (mostly in IB ophiolites), sheeted dyke complexes, pillowed lavas and felsic rocks. All Zagros ophiolites are overlain by Upper Cretaceous pelagic limestone. Limited radiometric dating indicates that the OB and IB formed at the same time during Late Cretaceous time. IB and OB components show strong suprasubduction zone affinities, from mantle harzburgite to lavas. This is shown by low whole-rock Al2O3and CaO contents and spinel and orthopyroxene compositions of mantle peridotites as well as by the abundance of felsic rocks and the trace element characteristics of the lavas. Similarly ages, suprasubduction zone affinities and fore-arc setting suggest that the IB and OB once defined a single tract of fore-arc lithosphere that was disrupted by exhumation of subducted Sanandaj–Sirjan Zone metamorphic rocks. Our data for the OB and IB along with better-studied ophiolites in Cyprus, Turkey and Oman compel the conclusion that a broad and continuous tract of fore-arc lithosphere was created during Late Cretaceous time as the magmatic expression of a newly formed subduction zone developed along the SW margin of Eurasia.

Tectonic Geomorphology of the Southernmost Sagaing Fault and Surface Rupture Associated with the May 1930 Pegu (Bago) Earthquake, Myanmar

Abstract The Sagaing fault is a continental transform fault between the India and Sunda plates that connects spreading centers in the Andaman Sea and the continental convergence zone along the Himalayan front. Several M >7 earthquakes occurred along the fault in the last century, and Global Positioning System campaigns revealed a right-lateral slip rate of 18 mm/yr, about half of the total India–Sunda displacement rate of 35 mm/yr. However, there are few fundamental geologic data on the Sagaing fault, such as detailed fault trace locations, information on late Quaternary slip rate, or spatial extent of surface ruptures during historical earthquakes. We conducted geologic field investigations along the southernmost 120 km long stretch of the fault zone that ruptured during the M 7.3 1930 Pegu (Bago) earthquake. We found well-defined tectonic geomorphic features across a deltaic lowland, including fault scarps, tectonic depressions, stream offsets, and pressure ridges. The sense of displacement is predominantly right-lateral strike-slip with vertical motion less than 1/5 of horizontal motion. Based on right-lateral offsets of stream channels, terrace risers, and property boundaries, we estimate coseismic displacement during the 1930 earthquake as ≥3.0 m. This in turn gives us a preliminary recurrence interval of surface-rupturing earthquakes on the Sagaing fault east of Yangon as ≥160 yr. Coulomb stress change induced by the May 1930 Pegu (Bago) earthquake may have triggered the December 1930 Pyu earthquake immediately to the north. This study demonstrates that detailed geomorphic investigations are key for a better assessment of seismic hazard for the Sagaing fault.

Neoproterozoic nappes and superposed folding of the Itremo Group, west-central Madagascar

The Precambrian geology of west-central Madagascar is reviewed and re-interpreted in light of new field observations, Landsat Thematic Mapper image analysis, and U–Pb geochronology. The bedrock of the area consists of: (1) late Archean (to Paleoproterozoic) migmatite gneiss and schist; (2) Mesoproterozoic stratified rocks (Itremo, Amborompotsy, and Malakialina Groups) perhaps deposited unconformably on the older metamorphic rocks (1, above); (3) Proterozoic (∼ 1000 Ma–720 Ma) plutonic rocks emplaced into both units above (1 and 2), and; (4) latest Neoproterozoic to middle Cambrian (∼ 570–520 Ma) granitoids emplaced as regionally discordant and weakly foliated plutons throughout the regions. The effects of Neoproterozoic orogenic processes are widespread throughout the region and our observations and isotopic measurements provide important constraints on the tectonic history of the region: (i) Archean gneisses and Mesoproterozoic stratified rocks are the crystalline basement and platformal sedimentary cover, respectively, of a continental fragment of undetermined tectonic affinity (East or West Gondwanan, or neither). (ii) This continental fragment (both basement and cover) was extensively invaded by subduction-related plutons in the period from ∼ 1000 Ma to ∼ 720 Ma that were emplaced prior to the onset of regional metamorphism and deformation. (iii) Continental collision related to Gondwana's amalgamation began after ∼ 720 Ma and before ∼ 570 Ma. Collision related deformation and metamorphism continued throughout the rest of the Neoproterozoic with thermal effects that lasted until ∼ 520 Ma. The oldest structures produced during continental collision were km-scale fold- and thrust-nappes with east or southeast-directed vergence (present-day direction). They resulted in the inversion and repetition of Archean and Proterozoic rocks throughout the region. During this early phase of convergence warm rocks were thrust over cool rocks thereby producing the present distribution of regional metamorphic isograds. The vergence of the nappes and the distribution of metamorphic rocks are consistent with their formation within a zone of west or northwest-dipping continental convergence (present-day direction). (iv) Later upright folding of the nappes (and related folds and thrusts) produced km-scale interference fold patterns. The geometry and orientation of these younger upright folds is consistent with E–W horizontal shortening (present-day direction) within a sinistral transpressive regime. We relate this final phase of deformation to motion along the Ranotsara and related shear zones of south Madagascar, and to the initial phases of lower crustal exhumation and extensional tectonics within greater Gondwana.

An updated digital model of plate boundaries

A global set of present plate boundaries on the Earth is presented in digital form. Most come from sources in the literature. A few boundaries are newly interpreted from topography, volcanism, and/or seismicity, taking into account relative plate velocities from magnetic anomalies, moment tensor solutions, and/or geodesy. In addition to the 14 large plates whose motion was described by the NUVEL‐1A poles (Africa, Antarctica, Arabia, Australia, Caribbean, Cocos, Eurasia, India, Juan de Fuca, Nazca, North America, Pacific, Philippine Sea, South America), model PB2002 includes 38 small plates (Okhotsk, Amur, Yangtze, Okinawa, Sunda, Burma, Molucca Sea, Banda Sea, Timor, Birds Head, Maoke, Caroline, Mariana, North Bismarck, Manus, South Bismarck, Solomon Sea, Woodlark, New Hebrides, Conway Reef, Balmoral Reef, Futuna, Niuafo'ou, Tonga, Kermadec, Rivera, Galapagos, Easter, Juan Fernandez, Panama, North Andes, Altiplano, Shetland, Scotia, Sandwich, Aegean Sea, Anatolia, Somalia), for a total of 52 plates. No attempt is made to divide the Alps‐Persia‐Tibet mountain belt, the Philippine Islands, the Peruvian Andes, the Sierras Pampeanas, or the California‐Nevada zone of dextral transtension into plates; instead, they are designated as “orogens” in which this plate model is not expected to be accurate. The cumulative‐number/area distribution for this model follows a power law for plates with areas between 0.002 and 1 steradian. Departure from this scaling at the small‐plate end suggests that future work is very likely to define more very small plates within the orogens. The model is presented in four digital files: a set of plate boundary segments; a set of plate outlines; a set of outlines of the orogens; and a table of characteristics of each digitization step along plate boundaries, including estimated relative velocity vector and classification into one of 7 types (continental convergence zone, continental transform fault, continental rift, oceanic spreading ridge, oceanic transform fault, oceanic convergent boundary, subduction zone). Total length, mean velocity, and total rate of area production/destruction are computed for each class; the global rate of area production and destruction is 0.108 m2/s, which is higher than in previous models because of the incorporation of back‐arc spreading.

Structure and kinematics of oblique continental convergence in northern Fiordland, New Zealand

Southeast of the Alpine Fault in Fiordland, New Zealand, Late Tertiary faults envelope exposures of Early Cretaceous high- P (12–16 kbar) granulite facies orthogneisses. These exposures allowed us to examine how pre-Cenozoic structures influenced variations in the degree and style of kinematic partitioning within a zone of oblique continental convergence. Fault-slip data and kinematic modelling show that oblique-slip and reverse displacements preferentially were partitioned east of the Alpine Fault onto curved NE-striking surfaces and moderately dipping (30–52°), N- and NW-striking surfaces, respectively. Approximately 3.0–5.5 km of exhumation and 6.5–7.0 km of oblique-dextral displacement occurred on faults that display palm tree-style geometries and subhorizontal detachments in profile. This heterogeneous style contrasts with the near horizontal slip on subvertical surfaces that define the Alpine Fault north of Milford Sound. The principal axes of instantaneous strain we determined using faults are compatible with principal stress axes derived from earthquakes. Our data indicate that mechanical anisotropies created by inherited structures controlled the locations and highly variable geometries of faulting since 7–9 Ma. The effects of these pre-existing structures and a focusing of contractional deformation along the northeast margin of Fiordland resulted in an unusually high degree of strike-slip partitioning across the southernmost onshore segment of the Alpine Fault.

Mechanisms limiting the southward extent of the South American Summer Monsoon

An intermediate atmospheric model coupled with a simple land‐surface model and a mixed‐layer ocean model is used to examine effects that determine the southward extension of summer precipitation over South America. The extent of the continental convergence zone is mainly determined by two mechanisms, which we term ventilation and the “interactive Rodwell‐Hoskins mechanism”. Ventilation refers to the import into South America of low moist static energy air from the cooler ocean, primarily the Pacific. In the interactive Rodwell‐Hoskins mechanism, Rossby‐wave‐induced subsidence to the west of the diabatic heating interacts with the convection zone. Because of the shape of South America, the interactive Rodwell‐Hoskins mechanism is of comparable importance to ventilation. Soil moisture feedback also helps limit poleward movement of the continental convergence zone, but its effect is relatively weak compared to the above two effects. The characteristic northwest‐southeast tilt of the continental convergence zone appears to be due to a combination of ventilation and the interactive Rodwell‐Hoskins mechanism.

Tectonic stress in the plates

The state of stress in the lithosphere provides strong constraints on the forces acting on the plates. The directions of principal stresses in the plates as indicated by midplate earthquake mechanisms, in situ stress measurements, and stress‐sensitive geological features are used to test plate tectonic driving force models, under the premises that enough data exist in selected areas to define a regionally consistent stress field and that for most such areas the dominant forces producing such stresses are plate tectonic in origin. Force models include buoyancy forces at ridges, subduction zones, and continental convergence zones and variously parameterized viscous shear between the lithosphere and the asthenosphere. A linear finite element method, based on the wave front solution technique, is used to predict the intraplate stress for each force model. Several long‐wavelength patterns for the orientation of horizontal principal deviatoric stresses are observable in the stress data. Maximum compressive stresses trend E‐W to NE‐SW for much of stable North America and E‐W to NW‐SE for continental South America. In western Europe the maximum compressive stresses trend NW‐SE, while in Asia the trend is more nearly N‐S, especially near the Himalayan front. In the Indian plate the trend varies from nearly N‐S in continental India to more nearly E‐W in Australia. Horizontal stresses are variable in Africa but tend to indicate a NW‐SE trend for the maximum compressive stress in west Africa and an E‐W trend for the minimum compressive stress in east Africa. Oceanic lithosphere away from plate boundaries is generally in a state of deviatoric compression, although few focal mechanisms can be constrained to define the orientation of the principal stresses.Comparison of stress orientations predicted for a wide range of driving force models to these regional stress observations provides a powerful test of the models. Ridge pushing forces are required in all models that match the stress orientation field. The net pulling force of subducted lithosphere, if such a force acts approximately symmetrically about the plate boundary, is at most a few times larger than other forces acting on the plates. Resistive forces associated with trench thrust faults and motion of the slab with respect to the mantle must therefore nearly balance the large gravitational potential of the slab. The upper limit on the ratio of net slab to ridge forces may be increased by less than a factor of 2 if net slab forces are reduced for the fastest moving plates by assuming that the resistance to subduction increases with convergence rate. Forces acting to resist further continental convergence along the Alpine‐Himalayan belt are important for models of the intraplate stress field in Europe, Asia, and the Indian plate. Inclusion of continental convergence zone forces, however, does not affect the upper bound on the ratio of net slab to ridge forces. A variety of possible lithosphere‐asthenosphere interactions have been tested. Resistive viscous drag forces acting on the base of the lithosphere improve the fit between calculated and observed stresses in the Nazca and South American plates as long as the drag coefficient is nonzero beneath oceanic lithosphere. The calculated intraplate stress field is not very sensitive to an increased drag coefficient beneath old oceanic lithosphere compared to young oceanic or continental lithosphere. Increasing the drag coefficient beneath continents compared to oceanic lithosphere by a factor of 5 or 10 has little effect on the overall fit of calculated stresses to observed stresses. Models in which viscous drag forces drive, rather than resist, plate motions are in poor agreement with intraplate stress data, although this lack of fit may depend on the oversimplified model of mantle flow patterns that has been assumed. A model of the driving mechanism in which slab forces act only on the plate with subducted lithosphere and where viscous drag forces are assumed to balance the torque on each plate produced by ridge and trench forces predicts stresses in good agreement with the data for most continental regions. The fit between calculated and observed stresses for this model is relatively poor for most oceanic regions. This model suggests, however, that it is probably an oversimplification to assume that the net force exerted by the slab acts symmetrically about the plate boundary, though some pulling force on the overthrust plate appears necessary. While the role of drag forces in the driving mechanism remains poorly constrained, the finite element technique that has been developed may be applied in the future to any specific and realizable flow pattern that is proposed for the mantle.

Detailed Seismicity of the Alpine Fault Zone and Fiordland Region, New Zealand

A study of the Alpine fault zone and the Fiordland region of the South Island of New Zealand from February through April 1972 indicates high but diffuse microearthquake activity. Composite focal mechanism solutions show that a regional northwest-southeast compression dominates the tectonic pattern. This direction is nearly normal to the Alpine fault, indicating that the Alpine fault is now undergoing a large component of thrust faulting. This agrees with geologic data for uplift of the Southern Alps along the Alpine fault beginning in mid-Miocene time and accelerating in the Pliocene, the time of the Kaikoura orogeny. Before the Kaikoura orogeny, the Alpine fault apparently was a transcurrent fault. This major change in the New Zealand tectonic pattern could have been produced by a relatively minor migration of the nearby Indian-Pacific pole of rotation. Incipient underthrusting of the Tasman Sea appears to be occurring off the Fiordland coast, terminating at the point where the Lord Howe Rise intersects the coast. To the north is a zone of oblique continental convergence, with the Southern Alps being rapidly uplifted along the Alpine fault. North of the Alps, much of the motion is transferred to several faults that have more easterly strike; these formed in the Kaikoura orogeny and constitute a new transform fault system.