Application of fluid mechanic principles to the study of geodynamic processes at trench-arc-back arc systems

Geological processes at trench-arc-back arc systems are some of the most complex tectonic processes in need of study. A large amount of data based on geological and geophysical observations has been accumulated. To synthesize these data, mathematical models have proven to be very useful. Because geological processes are directly related to the dynamical behavior of the solid earth, many of them can be investigated in terms of fluid flow models. Some applications of fluid flow principles in studying tectonic processes at convergent plate boundaries are discussed in this paper. When mantle processes are modeled as convective systems, they are found to have direct implications for the determination of slab dip angles. Additionally, they can also account for the high heat flows in the back arc basins, provide a mechanism for back arc opening, and resolve the question whether subducted oceanic crust can reach melting at shallow depth for island arc magma generation. Besides mantle processes, flow models can also be used to study surface processes. A simple one-parameter plane flow theory is used to model the evolution of trench geometries. This model is able to fit simulataneously the trench curvature and the differential paleomagnetic rotation between volcanic islands for the Mariana. Despite the simplicity of many of these models, their ability to synthesize geological and geophysical data at convergent plate boundaries is quite remarkable.     

Large earthquakes in the macquarie ridge complex: Transitional tectonics and subduction initiation

While most aspects of subduction have been extensively studied, the process of subduction initiation lacks an observational foundation. The Macquarie Ridge complex (MRC) forms the Pacific-Australia plate boundary between New Zealand to the north and the Pacific-Australia-Antarctica triple junction to the south. The MRC consists of alternating troughs and rises and is characterized by a transitional tectonic environment in which subduction initiation presently occurs. There is a high seismicity level with 15 large earthquakes (M>7) in this century. Our seismological investigation is centered on the largest event since 1943: the 25 MAY 1981 earthquake. Love, Rayleigh, andP waves are inverted to find: a faulting geometry of right-lateral strike-slip along the local trend of the Macquarie Ridge (N30°E); a seismic moment of 5×10²⁷ dyn cm (M _w=7.7) a double event rupture process with a fault length of less than 100km to the southwest of the epicenter and a fault depth of less than 20km. Three smaller thrust earthquakes occurred previous to the 1981 event along the 1981 rupture zone; their shallow-dipping thrust planes are virtually adjacent to the 1981 vertical fault plane. Oblique convergence in this region is thus accommodated by a dual rupture mode of several small thrust events and a large strike-slip event. Our study of other large MRC earthquakes, plus those of other investigators, produces focal mechanisms for 15 earthquakes distributed along the entire MRC; thrust and right-lateral strike-slip events are scattered throughout the MRC. Thus, all of the MRC is characterized by oblique convergence and the dual rupture mode. The true best-fit rotation pole for the Pacific-Australia motion is close to the Minster & Jordan RM2 pole for the Pacific-India motion. Southward migration of the rotation pole has caused the recent transition to oblique convergence in the northern MRC. We propose a subduction initiation process that is akin to crack propagation; the 1981 earthquake rupture area is identified as the crack-tip region that separates a disconnected mosaic of small thrust faults to the south from a horizontally continuous thrust interface to the north along the Puysegur trench. A different mechanism of subduction initiation occurs in the southernmost Hjort trench region at the triple junction. newly created oceanic lithosphere has been subducted just to the north of the triple junction. The entire MRC is a soft plate boundary that must accommodate the plate motion mismatch between two major spreading centers (Antarctica-Australia and Pacific-Antarctica). The persistence of spreading motion at the two major spreading centers and the consequent evolution of the three-plate system cause the present-day oblique convergence and subduction initiation in the Macquarie Ridge complex.     

Estimation of strong ground motions from hypothetical earthquakes on the Cascadia subduction zone,Pacific Northwest

Strong ground motions are estimated for the Pacific Northwest assuming that large shallow earthquakes, similar to those experienced in southern Chile, southwestern Japan, and Colombia, may also occur on the Cascadia subduction zone. Fifty-six strong motion recordings for twenty-five subduction earthquakes ofM _s7.0 are used to estimate the response spectra that may result from earthquakesM _w<81/4. Large variations in observed ground motion levels are noted for a given site distance and earthquake magnitude. When compared with motions that have been observed in the western United States, large subduction zone earthquakes produce relatively large ground motions at surprisingly large distances. An earthquake similar to the 22 May 1960 Chilean earthquake (M _w 9.5) is the largest event that is considered to be plausible for the Cascadia subduction zone. This event has a moment which is two orders of magnitude larger than the largest earthquake for which we have strong motion records. The empirical Green's function technique is used to synthesize strong ground motions for such giant earthquakes. Observed teleseismicP-waveforms from giant earthquakes are also modeled using the empirical Green's function technique in order to constrain model parameters. The teleseismic modeling in the period range of 1.0 to 50 sec strongly suggests that fewer Green's functions should be randomly summed than is required to match the long-period moments of giant earthquakes. It appears that a large portion of the moment associated with giant earthquakes occurs at very long periods that are outside the frequency band of interest for strong ground motions. Nevertheless, the occurrence of a giant earthquake in the Pacific Northwest may produce quite strong shaking over a very large region.     

Large earthquake doublets and fault plane heterogeneity in the northern Solomon Islands subduction zone

In the Solomon Islands and New Britain subduction zones, the largest earthquakes commonly occur as pairs with small separation in time, space and magnitude. This doublet behavior has been attributed to a pattern of fault plane heterogeneity consisting of closely spaced asperities such that the failure of one asperity triggers slip in adjacent asperities. We analyzed body waves of the January 31, 1974,M _w =7.3, February 1, 1974,M _w =7.4, July 20, 1975 (1437)M _w =7.6 and July 20, 1975 (1945),M _w =7.3 doublet events using an iterative, multiple station inversion technique to determine the spatio-temporal distribution of seismic moment release associated with these events. Although the 1974 doublet has smaller body wave moments than the 1975 events, their source histories are more complicated, lasting over 40 seconds and consisting of several subevents located near the epicentral regions. The second 1975 event is well modeled by a simple point source initiating at a depth of 15 km and rupturing an approximate 20 km region about the epicenter. The source history of the first 1975 event reveals a westerly propagating rupture, extending about 50 km from its hypocenter at a depth of 25 km. The asperities of the 1975 events are of comparable size and do not overlap one another, consistent with the asperity triggering hypothesis. The relatively large source areas and small seismic moments of the 1974 doublet events indicate failure of weaker portions of the fault plane in their epicentral regions. Variations in the roughness of the bathymetry of the subducting plate, accompanying subduction of the Woodlark Rise, may be responsible for changes in the mechanical properties of the plate interface.To understand how variations in fault plane coupling and strength affect the interplate seismicity pattern, we relocated 85 underthrusting earthquakes in the northern Solomon Islands Are since 1964. Relatively few smaller magnitude underthrusting events overlap the Solomon Islands doublet asperity regions, where fault coupling and strength are inferred to be the greatest. However, these asperity regions have been the sites of several previous earthquakes withM _s 7.0. The source regions of the 1974 doublet events, which we infer to be mechanically weak, contain many smaller magnitude events but have not generated any otherM _s 7.0 earthquakes in the historic past. The central portion of the northern Solomon Islands Arc between the two largest doublet events in 1971 (studied in detail bySchwartz et al., 1989a) and 1975 contains the greatest number of smaller magnitude underthrusting earthquakes. The location of this small region sandwiched between two strongly coupled portions of the plate interface suggest that it may be the site of the next large northern Solomon Islands earthquake. However, this region has experienced no known earthquakes withM _s 7.0 and may represent a relatively aseismic portion of the subduction zone.     

Geochemical Constraints on Possible Subduction Components in Lavas of Mayon and Taal Volcanoes, Southern Luzon, Philippines

Mayon is the most active volcano along the east margin of southernLuzon, Philippines. Petrographic and major element data indicatethat Mayon has produced a basaltic to andesitic lava seriesby fractional crystallization and magma mixing. Trace elementdata indicate that the parental basalts came from a heterogeneousmantle source. The unmodified composition of the mantle wedgeis similar to that beneath the Indian Ocean. To this mantlewas added a subduction component consisting of melt from subductedpelagic sediment and aqueous fluid dehydrated from the subductedbasaltic crust. Lavas from the highly active Taal Volcano onthe west margin of southern Luzon are compositionally more variablethan Mayon lavas. Taal lavas also originated from a mantle wedgemetasomatized by aqueous fluid dehydrated from the subductedbasaltic crust and melt plus fluid derived from the subductedterrigenous sediment. More sediment is involved in the generationof Taal lavas. Lead isotopes argue against crustal contamination.Some heterogeneity of the unmodified mantle wedge and differencesin whether the sediment signature is transferred into the lavasource through an aqueous fluid or melt phase are needed toexplain the regional compositional variation of Philippine arclavas. KEY WORDS: Mayon Volcano; Philippines; sediment melt; subduction component; Taal Volcano     

Thermal consequences of the formation of a slab window beneath the Mid-Cretaceous southwest Japan arc: A 2-D numerical analysis

Abstract   The development of voluminous granitic magmatism and widespread high-grade metamorphism in Mid-Cretaceous southwest Japan have been explained by the subduction of a spreading ridge (Kula–Pacific or Farallon–Izanagi plate boundaries) beneath the Eurasian continent and the formation of a slab window. In the present study, the thermal consequences of the formation of a slab window beneath a continental margin are evaluated through a 2-D numerical simulation. The model results are evaluated by comparison with the Mid-Cretaceous geology of southwest Japan. Of particular interest are the absence of an amphibolite- to granulite-facies metamorphic belt near the Wadati–Benioff plane, and significant melting of the lower crustal-mafic rocks sufficient to form a large amount of granitic magma. Because none of the model results simultaneously satisfied these two geological interpretations, it is suggested that subduction of plate boundaries in Mid-Cretaceous southwest Japan was not associated with the opening of a slab window. According to previous studies, and the results of the present study, two different tectonic scenarios could reasonably explain the geological interpretations for Mid-Cretaceous southwest Japan: (i) The spreading ridge did not subduct beneath the Eurasian continent, but was located off the continental margin, implying the continuous subduction of very young oceanic lithosphere; (ii) ridge subduction beneath the continental margin occurred after active spreading had ceased. Consequently, in both tectonic scenarios, the subduction of plate boundaries at the Mid-Cretaceous southwest Japan was not associated with a slab window, but very young (hot) oceanic lithosphere.     

Joint bulk-sound and shear tomography for Western Pacific subduction zones

Detailed regional body wave tomographic inversion of the Western Pacific region has been performed using P and S travel times from common sources and receivers, with a joint inversion in terms of bulk-sound and shear wave-speed variations in the mantle. This technique allows the separation of the influence of bulk and shear moduli, and hence a more direct comparison with mineral physics information. The study region is parameterized with cells of side 0.5° to 2° and 19 layers to a depth of 1500 km, while the rest of the mantle was parameterized with 5×5° cells with 16 layers between the surface and the core–mantle boundary. A simultaneous inversion is made for regional and global structures to minimize the influence of surrounding structures on the regional image. A nested iterative inversion scheme is employed with local linearization and three-dimensional ray tracing through the successive model updates. The results of the regional tomographic inversion reveal the penetration of a subducted slab below the 660 km discontinuity at the Kurile–Kamchatka trench, while flattening of slabs above this depth is observed in the Japan and Izu–Bonin subduction zones on both the bulk-sound and shear wave-speed images. The penetration of a subducted slab down to a depth of at least 1200 km is seen below the southern part of the Bonin trench, Mariana, Philippine, and Java subduction zones. Fast shear wave-speed perturbations associated with the subducted slabs, down to the 410 km transition zone, are larger than the comparable bulk-sound perturbations for all these subduction zones except the Philippines. The bulk-sound signature for the subducted slab is more pronounced than for shear in the Philippines, Talaud, New Guinea, Solomon, and Tonga subduction zones, where penetration of the slab into the middle mantle is observed. Variation in the amplitude ratio between bulk-sound and shear wave-speed anomalies correlates well with the subduction parameters of the descending slab. Slabs younger than 90 Ma at the trench show bulk-sound dominance in the upper mantle, while older slabs have a stronger shear wave-speed signature. Spreading of the fast shear wave-speed zone between 800 and 1000 km is observed in the areas of deep subducted slab penetration, but has no comparable expression in the bulk-sound images. This high-velocity feature may reflect physical or chemical disequilibria introduced to the lower mantle by subducted slabs.     

Silicic arc volcanism in Central Luzon, the Philippines: Characterization of its space, time and geochemical relationship

Abstract   The silicic volcanic rocks in Central Luzon show a temporal and spatial relationship with its geochemistry. Volcanic centers dated to approximately 5 Ma are silicic in geochemical composition whereas those between <5–1 Ma expose basaltic to andesitic rocks. Volcanic centers dated <1 Ma are characterized by a wide range of geochemistry encompassing basaltic through andesitic to dacitic signatures. Aside from changes in geochemistry through time, the areas (i.e. fore-arc to back-arc region) where the volcanic centers are formed also vary. The shift in the location of the volcanic centers in Central Luzon is attributed to changes in the dip of subduction of the South China Sea crust along the Manila Trench. Flat subduction resulted from the subduction of the Scarborough Seamount Chain, an oceanic bathymetric high along the Manila Trench west of northern Luzon. However, collision of Luzon with Taiwan in the north and Palawan in the south resulted in steepening of the subduction angle. The silicic volcanic centers in the forearc (Ce/Yb = 20–140) and back-arc (Ce/Yb = 20–60) regions are generally characterized by higher Ce/Yb compared to the basaltic-andesitic volcanic rocks in the main volcanic arc (Ce/Yb = 20) and back-arc (Ce/Yb = 20–30) regions. This across-arc geochemical variation highlights the contributions from the slab, mantle and crust coupled with the effects of geochemical processes that include partial melting, fractionation, magma mixing and mantle–melt interaction.     

Insights into operation of the subduction factory from the oxygen isotopic values of the southern Izu–Bonin–Mariana Arc

Abstract Oxygen is the most abundant element in the earth, and isotopic analysis of this element in island arc lavas potentially provides sensitive constraints on the proportion of oxygen recycled from subducted material, relative to that extracted from the mantle. Here we report on 225 new oxygen isotopic analyses of whole‐rock and glass samples, and clinopyroxene separates, from lavas collected from the southernmost 1500 km of the Izu–Bonin–Mariana (IBM) convergent margin. Whole‐rock samples clustered around a mean of 6.11 ± 0.47‰, whereas Mariana Trough glasses and mafic melts, calculated to be in equilibrium with mafic phenocrysts, clustered narrowly around a mean of 5.7‰. These data demonstrate that unequivocal identification of magmatic oxygen requires analysis of fresh glass or mafic minerals, and that the source of southern IBM Arc melts is entirely, or almost entirely, in equilibrium with normal mantle oxygen. If the elemental enrichments characteristic of the subduction component originate in subducted materials, these oxygen isotopic data are most consistent with the interaction of a small amount of sediment melt (<4%; mostly less than 1%) with mantle peridotite to yield the hybrid mantle that melts to form IBM Arc magmas.     

青海热水-日月山断裂带的新活动特征

热水 -日月山断裂带是发育在青藏高原东北缘柴达木 -祁连活动地块内部的 1条重要的NNW向逆 -右旋走滑活动断裂带 ,长约 183km。断裂由 4条不连续的次级断裂段右阶羽列而成 ,阶距 2～ 3km左右 ,在不连续部位形成拉分区。主断裂两端则形成帚状分叉。断裂活动形成了一系列山脊、冲沟和阶地等右旋断错微地貌 ,其中Ⅰ级阶地右旋断错约 8～ 11m ,Ⅱ级阶地右旋断错约 35m。同时沿断裂带还形成了许多断层陡坎 ,Ⅰ级阶地或洪积台地上断层陡坎高约 0 .5～ 1m ,最高达 2 .8m ,Ⅱ级阶地或台地上的断层陡坎高约 2 .5～ 3m ,最高达 4～ 5m。根据相应的阶地年代 ,计算得到断裂带全新世以来的平均水平滑动速率为 3 16mm/a ,垂直滑动速率为 0 .83mm/a