ALBERT

Description: Here we present the results of local source tomographic inversion beneath central Java. The data set was collected by a temporary seismic network. More than 100 stations were operated for almost half a year. About 13,000 P and S arrival times from 292 events were used to obtain three-dimensional (3-D) Vp, Vs, and Vp/Vs models of the crust and the mantle wedge beneath central Java. Source location and determination of the 3-D velocity models were performed simultaneously based on a new iterative tomographic algorithm, LOTOS-06. Final event locations clearly image the shape of the subduction zone beneath central Java. The dipping angle of the slab increases gradually from almost horizontal to about 70°. A double seismic zone is observed in the slab between 80 and 150 km depth. The most striking feature of the resulting P and S models is a pronounced low-velocity anomaly in the crust, just north of the volcanic arc (Merapi-Lawu anomaly (MLA)). An algorithm for estimation of the amplitude value, which is presented in the paper, shows that the difference between the fore arc and MLA velocities at a depth of 10 km reaches 30% and 36% in P and S models, respectively. The value of the Vp/Vs ratio inside the MLA is more than 1.9. This shows a probable high content of fluids and partial melts within the crust. In the upper mantle we observe an inclined low-velocity anomaly which links the cluster of seismicity at 100 km depth with MLA. This anomaly might reflect ascending paths of fluids released from the slab. The reliability of all these patterns was tested thoroughly.

Description: The convergent Sunda margin off Indonesia displays all geological features characteristic of an accretion-dominated subduction zone. A combined interpretation of prestack depth-migrated seismic reflection data and velocity information gained from refraction studies is supplemented by high-resolution bathymetric data and for the first time allows the exact mapping of backstop regimes. Initially, the outer high evolved as material was pushed against a static rigid arc framework backstop underlying a forearc basin. Increasing material strength of the outer high due to lithification formed a dynamic backstop, which controls accretion today. An out-of-sequence thrust marks the transition from the recent active frontal accretionary prism to the outer high and may be traced in the seismic and bathymetric data over the whole extent of the study area. The existence of a static as well as a dynamic backstop controls the forearc geometry and is associated with the segmentation of the forearc, which is observed in regimes of frontal as well as of oblique subduction. Mass balance calculations, which account for porosity changes and metamorphism, indicate a subduction history dominated by accretionary processes since the late Eocene. Accretion is associated with the low values of basal friction inferred for the Sunda margin. Structural investigations of conjugate fault planes indicate a very weak basal detachment. Effective stress analyses reveal that intrinsically weak material causes the high strength ratio of the detachment to the overlying sediments, whereas overpressuring within the frontal accretionary prism is negligible.

Description: The submarine Istanbul-Silivri fault segment, within 15km of Istanbul, is the only portion of the North Anatolian Fault that has not ruptured in the last 250years. We report first results of a seafloor acoustic ranging experiment to quantify current horizontal deformation along this segment and assess whether the segment is creeping aseismically or accumulating stress to be released in a future event. Ten transponders were installed to monitor length variations along 15 baselines. A joint least squares inversion for across-fault baseline changes, accounting for sound speed drift at each transponder, precludes fault displacement rates larger than a few millimeters per year during the 6month observation period. Forward modeling shows that the data better fit a locked state or a very moderate surface creep-less than 6mm/yr compared to a far-field slip rate of over 20mm/yr-suggesting that the fault segment is currently accumulating stress.

Description: The O'Higgins Seamount Group is a cluster of volcanic domes located 120 km west of the central Chilean Trench on the crest of the Juan Fernández Ridge. This aseismic hot spot track is subducting under South America triggering a belt of intraslab earthquake hypocenters extending about 700 km inland. The Juan Fernández Ridge marks the southern boundary of a shallow subduction segment. Subduction of oceanic basement relief has been suggested as a cause for the “flat” slab segments characterizing the Andean trench system. The Juan Fernández Ridge, however, shows only moderate crustal thickening, inadequate to cause significant buoyancy. In 2001, wide-angle seismic data were collected along two perpendicular profiles crossing the O'Higgins Group. We present tomographic images of the volcanic edifices and adjacent outer rise-trench environment, which indicate a magmatic origin of the seamounts dominated by extrusive processes. High-resolution bathymetric data yield a detailed image of a network of syngenetic structures reactivated in the outer rise setting. A pervasive fault pattern restricted to the hot spot modified lithosphere coincides with anomalous low upper mantle velocities gained from a tomographic inversion of seismic mantle phases. Reduced uppermost mantle velocities are solely found underneath the Juan Fernández Ridge and may indicate mineral alterations. Enhanced buoyancy due to crustal and upper mantle hydration may contribute an additional mechanism for shallow subduction, which prevails to the north after the southward migration of the Juan Fernández Ridge.

Description: We use seismic reflection and refraction data to determine crustal structure, to map a fore-arc basin containing 12 km of sediment, and to image the subduction thrust at 35 km depth. Seismic reflection megasequences within the basin are correlated with onshore geology: megasequence X, Late Cretaceous and Paleogene marine passive margin sediments; megasequence Y, a similar to 10,000 km(3) submarine landslide emplaced during subduction initiation at 22 Ma; and megasequence Z, a Neogene subduction margin megasequence. The Moho lies at 17 km beneath the basin center and at 35 km at the southern margin. Beneath the western basin margin, we interpret reflective units as deformed Gondwana fore-arc sediment that was thrust in Cretaceous time over oceanic crust 7 km thick. Raukumara Basin has normal faults at its western margin and is uplifted along its eastern and southern margins. Raukumara Basin represents a rigid fore-arc block > 150 km long, which contrasts with widespread faulting and large Neogene vertical axis rotations farther south. Taper of the western edge of allochthonous unit Y and westward thickening and downlap of immediately overlying strata suggest westward or northwestward paleoslope and emplacement direction rather than southwestward, as proposed for the correlative onshore allochthon. Spatial correlation between rock uplift of the eastern and southern basin margins with the intersection between Moho and subduction thrust leads us to suggest that crustal underplating is modulated by fore-arc crustal thickness. The trench slope has many small extensional faults and lacks coherent internal reflections, suggesting collapse of indurated rock, rather than accretion of > 1 km of sediment from the downgoing plate. The lack of volcanic intrusion east of the active arc, and stratigraphic evidence for the broadening of East Cape Ridge with time, suggests net fore-arc accretion since 22 Ma. We propose a cyclical fore-arc kinematic: rock moves down a subduction channel to near the base of the crust, where underplating drives rock uplift, oversteepens the trench slope, and causes collapse toward the trench and subduction channel. Cyclical rock particle paths led to persistent trench slope subsidence during net accretion. Existing global estimates of fore-arc loss are systematically too high because they assume vertical particle paths. Citation: Sutherland, R., et al. (2009), Reactivation of tectonics, crustal underplating, and uplift after 60 Myr of passive subsidence, Raukumara Basin, Hikurangi-Kermadec fore arc, New Zealand: Implications for global growth and recycling of continents, Tectonics, 28, TC5017, doi: 10.1029/2008TC002356.

Description: We present a detailed 3-D P-wave velocity model obtained by first-arrival travel-time tomography with seismic refraction data in the segment boundary of the Sumatra subduction zone across Simeulue Island, and an image of the top of the subducted oceanic crust extracted from depth-migrated multi-channel seismic reflection profiles. We have picked P-wave first arrivals of the air-gun source seismic data recorded by local networks of ocean-bottom seismometers, and inverted the travel-times for a 3-D velocity model of the subduction zone. This velocity model shows an anomalous zone of intermediate velocities between those of oceanic crust and mantle that is associated with raised topography on the top of the oceanic crust. We interpret this feature as a thickened crustal zone in the subducting plate with compositional and topographic variations, providing a primary control on the upper plate structure and on the segmentation of the 2004 and 2005 earthquake ruptures.

Description: Geophysical investigations of the northern Hikurangi subduction zone northeast of New Zealand, image fore‐arc and surrounding upper lithospheric structures. A seismic velocity (Vp) field is determined from seismic wide‐angle data, and our structural interpretation is supported by multichannel seismic reflection stratigraphy and gravity and magnetic modeling. We found that the subducting Hikurangi Plateau carries about 2 km of sediments above a 2 km mixed layer of volcaniclastics, limestone, and chert. The upper plateau crust is characterized by Vp = 4.9–6.7 km/s overlying the lower crust with Vp 〉 7.1 km/s. Gravity modeling yields a plateau thickness around 10 km. The reactivated Raukumara fore‐arc basin is 〉10 km deep, deposited on 5–10 km thick Australian crust. The fore‐arc mantle of Vp 〉 8 km/s appears unaffected by subduction hydration processes. The East Cape Ridge fore‐arc high is underlain by a 3.5 km deep strongly magnetic (3.3 A/m) high‐velocity zone, interpreted as part of the onshore Matakaoa volcanic allochthon and/or uplifted Raukumara Basin basement of probable oceanic crustal origin. Beneath the trench slope, we interpret low‐seismic‐velocity, high‐attenuation, low‐density fore‐arc material as accreted and recycled, suggesting that underplating and uplift destabilizes East Cape Ridge, triggering two‐sided mass wasting. Mass balance calculations indicate that the proposed accreted and recycled material represents 25–100% of all incoming sediment, and any remainder could be accounted for through erosion of older accreted material into surrounding basins. We suggest that continental mass flux into the mantle at subduction zones may be significantly overestimated because crustal underplating beneath fore‐arc highs have not properly been accounted for.

Description: The Sunda‐Banda arc transition at the eastern termination of the Sunda margin (Indonesia) represents a unique natural laboratory to study the effects of lower plate variability on upper plate deformational segmentation. Neighboring margin segments display a high degree of structural diversity of the incoming plate (transition from an oceanic to a continental lower plate, presence/absence of an oceanic plateau, variability of subducting seafloor morphology) as well as a wide range of corresponding fore‐arc structures, including a large sedimentary basin and an accretionary prism/outer arc high of variable size and shape. Here, we present results of a combined analysis of seismic wide‐angle refraction, multichannel streamer and gravity data recorded in two trench normal corridors located offshore the islands of Lombok (116°E) and Sumba (119°E). On the incoming plate, the results reveal a 8.6–9.0 km thick oceanic crust, which is progressively faulted and altered when approaching the trench, where upper mantle velocities are reduced to ∼7.5 km/s. The outer arc high, located between the trench and the fore‐arc basin, is characterized by sedimentary‐type velocities (Vp 〈 5.5 km/s) down to the top of the subducting slab (∼13 km depth). The oceanic slab can be traced over 70–100 km distance beneath the fore arc. A shallow serpentinized mantle wedge at ∼16 km depth offshore Lombok is absent offshore Sumba, where our models reveal the transition to the collisional regime farther to the east and to the Sumba block in the north. Our results allow a detailed view into the complex structure of both the deeper and shallower portions of the eastern Sunda margin.

Description: Three active-source seismic refraction profiles are integrated with morphological and potential field data to place the first regional constraints on the structure of the Kermadec subduction zone. These observations are used to test contrasting tectonic models for an along-strike transition in margin structure previously known as the 32°S boundary. We use residual bathymetry to constrain the geometry of this boundary and propose the name Central Kermadec Discontinuity (CKD). North of the CKD, the buried Tonga Ridge occupies the forearc with VP 6.5–7.3 km s-1 and residual free-air gravity anomalies constrain its latitudinal extent (north of 30.5°S), width (110 ± 20 km) and strike (~005° south of 25°S). South of the CKD the forearc is structurally homogeneous down-dip with VP 5.7–7.3 km s-1. In the Havre Trough backarc, crustal thickness south of the CKD is 8-9 km, which is up-to 4 km thinner than the northern Havre Trough and at least 1 km thinner than the southern Havre Trough. We suggest that the Eocene arc did not extend along the current length of the Tonga-Kermadec trench. The Eocene arc was originally connected to the Three Kings Ridge and the CKD was likely formed during separation and easterly translation of an Eocene arc substrate during the early Oligocene. We suggest that the first-order crustal thickness variations along the Kermadec arc were inherited from before the Neogene and reflect Mesozoic crustal structure, the Cenozoic evolution of the Tonga-Kermadec-Hikurangi margin and along-strike variations in the duration of arc volcanism.