Measuring the north–south coseismic displacement component with high‐resolution multi‐aperture InSAR

Earthquake source inversions based on space‐borne Synthetic Aperture Radar interferometry (InSAR) are used extensively. Typically, however, only the line‐of‐sight (LoS) surface displacement component is measured, which is mainly sensitive to the vertical and E–W deformations, although well‐established methods also exist to estimate the flight‐path component, which is highly sensitive to the N–S displacement. With high‐resolution sensors, these techniques are particularly appealing, because accuracies in the order of 3 cm can be achieved, while retaining spatial resolutions between 45 m and a few km, depending on the required level of filtering. We discuss the application to COSMO‐SkyMed SAR imagery of the Spectral Diversity or Multi Aperture Interferometry technique, presenting the first SAR flight‐path displacement field associated with the Mw 6.3, 2009 L'Aquila event (central Apennines). Finally, we observe and characterize a previously unknown misregistration pattern.     

Crustal structure under the Sydney Basin and Lachlan Fold Belt,determined from explosion seismic studies

The Lachlan Fold Belt has the velocity‐depth structure of continental crust, with a thickness exceeding 50 km under the region of highest topography in Australia, and in the range 41–44 km under the central Fold Belt and Sydney Basin. There is no evidence of high upper crustal velocities normally associated with marginal or back‐arc basin crustal rocks. The velocities in the lower crust are consistent with an overall increase in metamorphic grade and/or mafic mineral content with depth. Continuing tectonic development throughout the region and the negligible seismicity at depths greater than 30 km indicate that the lower crust is undergoing ductile deformation.

The upper crustal velocities below the Sydney Basin are in the range 5.75–5.9 km/s to about 8 km, increasing to 6.35–6.5 km/s at about 15–17 km depth, where there is a high‐velocity (7.0 km/s) zone for about 9 km evident in results from one direction. The lower crust is characterised by a velocity gradient from about 6.7 km/s at 25 km, to 7.7 km/s at 40–42 km, and a transition to an upper mantle velocity of 8.03–8.12 km/s at 41.5–43.5 km depth.

Across the central Lachlan Fold Belt, velocities generally increase from 5.6 km/s at the surface to 6.0 km/s at 14.5 km depth, with a higher‐velocity zone (5.95 km/s) in the depth range 2.5–7.0 km. In the lower crust, velocities increase from 6.3 km/s at 16 km depth to 7.2 km/s at 40 km depth, then increase to 7.95 km/s at 43 km. A steeper gradient is evident at 26.5–28 km depth, where the velocity is about 6.6—6.8 km/s. Under part of the area an upper mantle low‐velocity zone in the depth range 50–64 km is interpreted from strong events recorded at distances greater than 320 km.

There is no substantial difference in the Moho depth across the boundary between the Sydney Basin and the Lachlan Fold Belt, consistent with the Basin overlying part of the Fold Belt. Pre‐Ordovician rocks within the crust suggest fragmented continental‐type crust existed E of the Precambrian craton and that these contribute to the thick crustal section in SE Australia.     

A first interpretation of crustal structure in the Adelaide Geosyncline in South Australia using quarry blasts

Explosions at two large open cut mines (Leigh Creek and Iron Baron) were used as sources of seismic energy to record along two linear profiles, parallel and approximately transverse to the axis of the Adelaide geosyncline in South Australia. Records at approximately 120 sites were obtained out to distances of the order of 350 km with Kinemetrics PS‐1A portable seismographs, using smoked paper and a recording speed of 4 mm/s. Times of blasting were determined from records at some of the permanent stations of the University of Adelaide seismograph network. Station spacing was normally 5 km, but at large distances from the source this increased to the order of 10 km.

The simplest model of the crust consistent with the observed travel times comprises two essentially homogeneous layers overlying the mantle. The average P wave velocities in the upper and lower crustal layers are 5.94 km/s and 6.46 km/s, with the boundary between the layers at approximately 18 km and possibly 8 km below Eyre Peninsula. Although such a division has been found in other parts of Australia, none of the earlier studies in S.A. found evidence for such a discontinuity or velocity gradient. The P wave velocity in the upper mantle is 7.97 km/s and the mean thickness of the crust is 39 km. Both the intermediate and Moho “discontinuities” may vary by up to 5 km from their mean depths. Shear waves have velocities of 3.43 and 4.45 km/s in the upper crustal layer and the upper mantle, respectively.     

Exhumation of high-pressure rocks beneath the Solund Basin, Western Gneiss Region of Norway

The Solund–Hyllestad–Lavik area affords an excellent opportunity to understand the ultrahigh‐pressure Scandian orogeny because it contains a near‐complete record of ophiolite emplacement, high‐pressure metamorphism and large‐scale extension. In this area, the Upper Allochthon was intruded by thec. 434 Ma Sogneskollen granodiorite and thrust eastward over the Middle/Lower Allochthon, probably in the Wenlockian. The Middle/Lower Allochthon was subducted to c. 50 km depth and the structurally lower Western Gneiss Complex was subducted to eclogite facies conditions at c. 80 km depth by c. 410–400 Ma. Within < 5–10 Myr, all these units were exhumed by the Nordfjord–Sogn detachment zone, producing shear strains > 100. Exhumation to upper crustal levels was complete by c. 403 Ma. The Solund fault produced the last few km of tectonic exhumation, bringing the near‐ultrahigh‐pressure rocks to within c. 3 km vertical distance from the low‐grade Solund Conglomerate.     

Crustal structure of the southern Dabie ultrahigh-pressure orogen and Yangtze foreland from deep seismic reflection profiling

A new 140‐km‐long seismic reflection profile provides a high‐resolution crustal‐scale image of the southern Dabieshan high‐pressure (HP) metamorphic belt and the Yangtze foreland fold‐and‐thrust belt. The seismic image of the stacked section shows that the southern Dabieshan metamorphic terrane and Yangtze foreland belt are separated by a large north‐dipping fault. In the foreland the upper crust is dominated by a series of folds and thrusts formed during the collisional stage in the mid‐Triassic; it was reworked by crustal extension resulting in the formation of a late Jurassic and Cretaceous red‐bed basin. The southern Dabieshan profile shows stacked crustal slabs developed along the margin of the collisional orogenic belt. The Moho reflectors at 10–11 s (～30–33 km) are seismically prominent and segmented by a number of south‐verging thrusts that were probably developed by foreland‐directed thrusting of the deeply subducted continental crust during exhumation. The seismic reflection profile suggests that structures related to the Triassic–Jurassic subduction and exhumation of the Yangtze plate are preserved despite the severe crustal extension superimposed during the late Mesozoic and Cenozoic.     

Geophysical differences in the lithosphere between phanerozoic and precambrian Australia

Cannikin atomic bomb recordings indicate that there are differences in travel-times from the Aleutian Islands test site to Phanerozoic and Precambrian provinces in Australia of up to 1.1 s. Explosion seismic studies in central and southeastern Australia enable travel-time corrections for crustal and upper mantle structure to be made to recordings of such teleseismic events. Structure in the upper 60 km can account for, at most, about 0.2 s of the residual difference, but attempts to constrain the remaining residual time to the region above the Lehmann discontinuity at about 200 km depth are difficult to reconcile with explosion seismic models. Regional differences in seismic velocity structure between Phanerozoic and Precambrian Australia therefore appear to exist at depths greater than 200 km.Electrical conductivities within the mantle have been investigated using two methods. Long-period electromagnetic depth sounding using magnetometer arrays demonstrates that conductivities increase at about 200 km under Phanerozoic Australia but not until about 500 km depth under Precambrian Australia. Shorter period magnetotelluric measurements can only resolve shallower structures; these too indicate a similar trend but with sub-crustal conductivities increasing at less than 100 km under Phanerozoic Australia. Magma at these depths and shallower may be the source for Cainozoic volcanism in eastern Australia. Under Precambrian central and northern Australia magnetotelluric investigations indicate that pronounced conductivity increases do not occur until depths of 150–200 km are reached.Oceanic magnetic observations indicate that the Australian lithospheric plate as a whole is separating from Antarctica at a rate of about 7 cm/yr. The seismic and conductivity structures under the continental region of this plate indicate that lateral inhomogeneities possibly extend to depths as great as 500 km and are probably caused by the passage of eastern Australia over a hot spot. Hawaiian studies indicate that hot spots are not local features but result from large scale disturbances in the mantle. Conductivity increases commencing in the depth range 100–250 km may give an indication of uppermost zones within which the Palaeozoic lithospherc has been substantially modified resulting in elevated surface heat flow, volcanism and seismic travel-time anomalies.     

Crustal structure of the Paleozoic Kunlun orogeny from an active-source seismic profile between Moba and Guide in East Tibet, China

We herein present a new seismic refraction/wide-angle reflection profile that crosses the Songpan–Ganzi terrane, the Animaqing suture zone and the eastern Kunlun mountains (comprised of the South Kunlun and Middle Kunlun blocks separated by the Middle Kunlun fault). The profile is 380 km long and extends from Moba to Guide in eastern Tibet. The crustal thickness is about 62 km under the Songpan–Ganzi terrane, 62–64 km under the South Kunlun, and 60 km under the Middle Kunlun block. The Songpan–Ganzi flysch seems to be present up to a depth of 15 km south of the Animaqing suture zone, and up to a depth of 10 km in the Middle Kunlun block, with thicknesses elsewhere that depend on assumptions about the likely lithologies. The profile exhibits clear lateral variations both in the upper and lower crust, which are indicative of different crustal blocks juxtaposed by the Kunlun fault system. Whether or not the Songpan–Ganzi flysch was originally deposited on oceanic crust, at the longitude of our profile (100°E) it is now underlain by continental crust, and the presence of continental crust beneath the Songpan–Ganzi terrane and of a continental arc under the South Kunlun block suggest Paleozoic continent–continent arc collision in the eastern Kunlun Mountains. Comparison of crustal velocity columns from all wide-angle seismic profiles across the eastern Kunlun mountains indicates a remarkable west-to-east change in the Moho topography across the Kunlun fault system (15–20 km Moho step at 95°E, but only 2–5 km along our profile at 100°E). Lower-crustal thickness of the Kunlun terranes is rather uniform, about 35 km, from 80°–95°E, which suggests that similar thrust-thickening processes have played a role where the Qaidam Basin abuts the Kunlun fault, but thins to 20–25 km at 100°E, east of the Qaidam Basin. The increased crustal thickness from 93° to 98°E compared to that at 100°E may be due to the differences in the thickness of the crust of the two plates before their collision, and/or largely achieved by thickening of the lower crust, perhaps indicating a crustal flow mechanism operating more strongly in the western region.     

A strike‐slip core complex from the Najd fault system,Arabian shield

Metamorphic core complexes are usually thought to be associated with regional crustal extension and crustal thinning, where deep crustal material is exhumed along gently dipping normal shear zones oblique to the regional extension direction. We present a new mechanism whereby metamorphic core complexes can be exhumed along crustal‐scale strike‐slip fault systems that accommodated crustal shortening. The Qazaz metamorphic dome in Saudi Arabia was exhumed along a gently dipping jog in a crustal‐scale vertical strike‐slip fault zone that caused more than 25 km of exhumation of lower crustal rocks by 30 km of lateral motion. Subsequently, the complex was transected by a branch of the strike‐slip fault zone, and the segments were separated by another 30 km of lateral motion. Strike‐slip core complexes like the Qazaz Dome may be common and may have an important local effect on crustal strength.     

High-T, low-P metamorphism in the Palaeoproterozoic Halls Creek Orogen, northern Australia: the middle crustal response to a mantle-related transient thermal pulse

High‐T, low‐P metamorphic rocks of the Palaeoproterozoic central Halls Creek Orogen in northern Australia are characterised by low radiogenic heat production, high upper crustal thermal gradients (locally exceeding 40 °C km^?1) sustained for over 30 Myr, and a large number of layered mafic‐ultramafic intrusions with mantle‐related geochemical signatures. In order to account for this combination of geological and thermal characteristics, we model the middle crustal response to a transient mantle‐related heat pulse resulting from a temporary reduction in the thickness of the mantle lithosphere. This mechanism has the potential to raise mid‐crustal temperatures by 150–400 °C within 10–20 Myr following initiation of the mantle temperature anomaly, via conductive dissipation through the crust. The magnitude and timing of maximum temperatures attained depend strongly on the proximity, duration and lateral extent of the thermal anomaly in the mantle lithosphere, and decrease sharply in response to anomalies that are seated deeper than 50–60 km, maintained for <5 Myr in duration and/or have half‐widths <100 km. Maximum temperatures are also intimately linked to the thermal properties of the model crust, primarily due to their influence on the steady‐state (background) thermal gradient. The amplitudes of temperature increases in the crust are principally a function of depth, and are broadly independent of crustal thermal parameters. Mid‐crustal felsic and mafic plutonism is a predictable consequence of perturbed thermal regimes in the mantle and the lowermost crust, and the advection of voluminous magmas has the potential to raise temperatures in the middle crust very quickly. Although pluton‐related thermal signatures significantly dissipate within <10 Myr (even for very large, high‐temperature intrusive bodies), the interaction of pluton‐ and mantle‐related thermal effects has the potential to maintain host rock temperatures in excess of 400–450 °C for up to 30 Myr in some parts of the mid‐crust. The numerical models presented here support the notion that transient mantle‐related heat sources have the capacity to contribute significantly to the thermal budget of metamorphism in high‐T, low‐P metamorphic belts, especially in those characterised by low surface heat flow, very high peak metamorphic geothermal gradients and abundant mafic intrusions.     

The northern Walker Lane refraction experiment: Pn arrivals and the northern Sierra Nevada root

In May 2002, we collected a new crustal refraction profile from Battle Mountain, Nevada across western Nevada, the Reno area, Lake Tahoe, and the northern Sierra Nevada Mountains to Auburn, CA. Mine blasts and earthquakes were recorded by 199 Texan instruments extending across this more than 450-km-long transect. The use of large mine blasts and the ultra-portable Texan recorders kept the field costs of this profile to less than US$10,000. The seismic sources at the eastern end were mining blasts at Barrick's GoldStrike mine. The GoldStrike mine produced several ripple-fired blasts using 8000–44,000 kg of ANFO each, a daily occurrence. First arrivals from the larger GoldStrike blasts are obvious to distances of 300 km in the raw records. First arrivals from a quarry blast west of the survey near Watsonville, CA, located by the Northern California Seismic Network with a magnitude of 2.2, can be picked across the recording array to distances of 600 km. The Watsonville blast provides a western source, nearly reversing the GoldStrike blasts. A small earthquake near Bridgeport, CA. also produced pickable P-wave arrivals across the transect, providing fan-shot data. Arrivals from M5 events in the Mariana and Kuril Islands also appear in the records. This refraction survey observes an unexpectedly deep crustal root under the northern Sierra Nevada range, over 50 km in thickness and possibly centered west of the topographic crest. Pn delays of 4–6 s support this interpretation. At Battle Mountain, Nevada, we observe anomalously thin crust over a limited region perhaps only 150 km wide, with a Moho depth of 19–23 km. Pn crossover distances of less than 80 km support this anomaly, which is surrounded by observations of more normal, 30-km-thick crust. A 10-km-thick and high-velocity lower-crustal “pillow” is an alternative hypothesis, but unlikely due to the lack of volcanics west of Battle Mountain. Large mine and quarry blasts prove very effective crustal refraction sources when recorded with a dense receiver array, even over distances exceeding 600 km. New elastic synthetic seismogram modeling suggests that Pn can be strong as a first arrival, easing the modeling and interpretation of crustal refraction data. Fast eikonal computations of first-arrival time can match pickable Pn arrival times.     

Otway Continental Margin Transect: Crustal architecture from wide‐angle seismic profiling across Australia's southern margin

The Otway Basin in southeastern Australia formed on a triangular‐shaped area of extended continental lithosphere during two extensional episodes in Cretaceous to Miocene times. The extent of the offshore continental margin is highlighted by Seasat/Geosat satellite altimeter data. The crustal architecture and structural features across this southeast Australian margin have been interpreted from offshore‐onshore wide‐angle seismic profiling data along the Otway Continental Margin Transect extending from the onshore Lake Condah High, through the town of Portland, to the deep Southern Ocean. Along the Otway Continental Margin Transect, the onshore half‐graben geometry of Early Cretaceous deposition gives way offshore to a 5 km‐thick slope basin (P‐wave velocity 2.2–4.6 km/s) to at least 60 km from the shoreline. At 120 km from the nearest shore in a water depth of 4220 m, sonobuoy data indicate a 4–5 km sedimentary sequence overlying a 7 km thick basement above the Moho at 15 km depth. Major fault zones affect the thickness of basin sequences in the onshore area (Tartwaup Fault Zone and its southeast continuation) and at the seaward edge of the Mussel Platform (Mussel Fault). Upper crustal basement is interpreted to be attenuated and thinned Palaeozoic rocks of the Delamerian and Lachlan Orogens (intruded with Jurassic volcanics) that thin from 16 km onshore to about 3.5 km at 120 km from the nearest shore. Basement rocks comprise a 3 km section with velocity 5.5–5.7 km/s overlying a deeper basement unit with velocity 6.15–6.35 km/s. The Moho shallows from a depth of 30 km onshore to 15 km depth at 120 km from the nearest shore, and then to about 12 km in the deep ocean at the limits of the transect (water depth 5200 m). The continent‐ocean boundary is interpreted to be at a prominent topographic inflection point 170 km from shore at the bottom of the continental slope in 4800 m of water. P‐wave velocities in the lower crust are 6.4–6.8 km/s, overlying a thin transition zone to an upper mantle velocity of 8.05 km/s beneath the Moho. Outstandingly clear Moho reflections seen in deep‐marine profiling data at about 10.3 s two‐way time under the slope basin and continent‐ocean boundary place further strong controls on crustal thickness. There is no evidence of massive high velocity (>7 km/s) intrusives/underplate material in the lower crust nor any synrift or early post‐rift subaerial volcanics, indicating that the Otway continental margin can be considered a non‐volcanic margin, similar in many respects to some parts of the Atlantic Ocean margins e.g. the Nova Scotia ‐ Newfoundland margin off Canada and the Galicia Bank off the Iberian Peninsula. Using this analogue, the prominent gravity feature trending northwest‐southeast at the continent‐ocean boundary may indicate the presence of highly serpentinised mantle material beneath a thin crust, but this has yet to be tested by detailed work.     

Seismic velocities from explosions off the central coast of New South Wales

Travel times from explosions fired on the continental shelf off the central coast of New South Wales were observed at permanent stations and spreads of seismic exploration instruments, and combined with existing results to give a seismic crustal profile across part of southeastern Australia. An intermediate layer, dipping to the southwest, underlies the surface rocks and has a P velocity of about 6–52 km./sec. Beneath Sydney, its top may either be in contact with the basin sediments at a depth of about 5 km., or separated from them by a wedge of a few kilometres of 6 km./sec. material. The Mohorovi?i? discontinuity (M) is at a depth of 25 km., dips to the southwest at about 4 degrees, and the velocity under it is about 7.86 km./sec. The depth to the top of the intermediate layer under the Snowy Mountains is about 20 km., and the revised depth to M is about 42 km. M dips at about 2° to the southwest in this region, and the velocity at the top of the mantle is 8.1 km./sec.     

Geophysical constraints of shear zones and geometry of the Hiltaba Suite granites in the western Gawler Craton,Australia

The emplacement of the ca 1590–1575 Ma Hiltaba Suite granites records a large magmatic event throughout the Gawler Craton, South Australia. The Hiltaba Suite granites intrude the highly deformed Archaean‐Palaeoproterozoic rocks throughout the craton nuclei. Geophysical interpretation of the poorly exposed central western Gawler Craton suggests that the region can be divided into several distinct domains that are bounded by major shear zones, exhibiting a sequence of overprinting relationships. The north‐trending Yarlbrinda Shear Zone merges into the east‐trending Yerda Shear Zone that, in turn, merges into the northeast‐trending Coorabie Shear Zone. Several poorly exposed Hiltaba Suite granite plutons occur within a wide zone of crustal shearing that is bounded to the north by the Yerda Shear Zone and to the south by the Oolabinnia Shear Zone. This wide zone of crustal shearing is interpreted as a major zone of synmagmatic dextral strike‐slip movement that facilitated the ascent of Hiltaba Suite granite intrusions to the upper crust. The aeromagnetic and gravity data reveal that the intrusions are ～15–25 km in diameter. Forward modelling of the geophysical data shows that the intrusions have a tabular geometry and are less than 6 km deep.     

Seismic imaging and crustal architecture across the Lachlan Transverse Zone,a possible early cross‐cutting feature of eastern Australia

A 2‐D crustal velocity model has been derived from a 1997 364 km north‐south wide‐angle seismic profile that passed from Ordovician volcanic and volcaniclastic rocks (Molong Volcanic Belt of the Macquarie Arc) in the north, across the Lachlan Transverse Zone into Ordovician turbidites and Early Devonian intrusive granitoids in the south. The Lachlan Transverse Zone is a proposed west‐northwest to east‐southeast structural feature in the Eastern Lachlan Orogen and is considered to be a possible early lithospheric feature controlling structural evolution in eastern Australia; its true nature, however, is still contentious. The velocity model highlights significant north to south lateral variations in subsurface crustal architecture in the upper and middle crust. In particular, a higher P‐wave velocity (6.24–6.32 km/s) layer identified as metamorphosed arc rocks (sensu lato) in the upper crust under the arc at 5–15 km depth is juxtaposed against Ordovician craton‐derived turbidites by an inferred south‐dipping fault that marks the southern boundary of the Lachlan Transverse Zone. Near‐surface P‐wave velocities in the Lachlan Transverse Zone are markedly less than those along other parts of the profile and some of these may be attributed to mid‐Miocene volcanic centres. In the middle and lower crust there are poorly defined velocity features that we infer to be related to the Lachlan Transverse Zone. The Moho depth increases from 37 km in the north to 47 km in the south, above an underlying upper mantle with a P‐wave velocity of 8.19 km/s. Comparison with velocity layers in the Proterozoic Broken Hill Block supports the inferred presence of Cambrian oceanic mafic volcanics (or an accreted mafic volcanic terrane) as substrate to this part of the Eastern Lachlan Orogen. Overall, the seismic data indicate significant differences in crustal architecture between the northern and southern parts of the profile. The crustal‐scale P‐wave velocity differences are attributed to the different early crustal evolution processes north and south of the Lachlan Transverse Zone.     

Upper mantle structure from the Trans‐Australia Seismic Survey (TASS) and other seismic refraction data

The recordings made during 1972 from large explosions at Kunanalling (W.A.), Mount Fitton (S.A.). and Bass Strait have added considerably to seismic refraction data measured over distances of 1000 km in continental Australia. Taken together with data from the 1956 Maralinga atomic bomb and 1970–71 Ord Dam explosions they show the existence of a refractor with apparent P‐wave velocity in the range 8.26–8.29 km/s, which is interpreted as the Moho under shield regions, at a depth of 34 km under Kalgoorlie and deepening eastward to 39 km under Maralinga. In northern South Australia and farther north and east this refractor is evident as a sub‐Moho refractor at a depth of about 60 km; the Moho refractor is also evident, with an apparent P velocity of 8.04 ± 0.04 km/s at a depth of 40 km. Two computer models (TASS‐1a and 2a) match the observed data. The subsequent arrivals recorded are consistent with the velocity of 8.53 km/s in a refractor at 165 km depth interpreted from the Ord Dam; there is little conclusive evidence for a low‐velocity zone above this depth.     

Interaction of metamorphism, deformation and exhumation in large convergent orogens

Coupled thermal‐mechanical models are used to investigate interactions between metamorphism, deformation and exhumation in large convergent orogens, and the implications of coupling and feedback between these processes for observed structural and metamorphic styles. The models involve subduction of suborogenic mantle lithosphere, large amounts of convergence (≥ 450 km) at 1 cm yr^?1, and a slope‐dependent erosion rate. The model crust is layered with respect to thermal and rheological properties — the upper crust (0–20 km) follows a wet quartzite flow law, with heat production of 2.0 μW m^?3, and the lower crust (20–35 km) follows a modified dry diabase flow law, with heat production of 0.75 μW m^?3. After 45 Myr, the model orogens develop crustal thicknesses of the order of 60 km, with lower crustal temperatures in excess of 700 °C. In some models, an additional increment of weakening is introduced so that the effective viscosity decreases to 10¹⁹ Pa.s at 700 °C in the upper crust and 900 °C in the lower crust. In these models, a narrow zone of outward channel flow develops at the base of the weak upper crustal layer where T≥600 °C. The channel flow zone is characterised by a reversal in velocity direction on the pro‐side of the system, and is driven by a depth‐dependent pressure gradient that is facilitated by the development of a temperature‐dependent low viscosity horizon in the mid‐crust. Different exhumation styles produce contrasting effects on models with channel flow zones. Post‐convergent crustal extension leads to thinning in the orogenic core and a corresponding zone of shortening and thrust‐related exhumation on the flanks. Velocities in the pro‐side channel flow zone are enhanced but the channel itself is not exhumed. In contrast, exhumation resulting from erosion that is focused on the pro‐side flank of the plateau leads to ‘ductile extrusion’ of the channel flow zone. The exhumed channel displays apparent normal‐sense offset at its upper boundary, reverse‐sense offset at its lower boundary, and an ‘inverted’ metamorphic sequence across the zone. The different styles of exhumation produce contrasting peak grade profiles across the model surfaces. However, P–T–t paths in both cases are loops where P_max precedes T_max, typical of regional metamorphism; individual paths are not diagnostic of either the thickening or the exhumation mechanism. Possible natural examples of the channel flow zones produced in these models include the Main Central Thrust zone of the Himalayas and the Muskoka domain of the western Grenville orogen.     

Crustal Structure across the Northwestern Margin of South China Sea: Evidence for Magma‐poor Rifting from a Wide‐angle Seismic Profile

We present results from a 484 km wide-angle seismic profile acquired in the northwest part of the South China Sea (SCS) during OBS2006 cruise. The line that runs along a previously acquired multi-channel seismic line (SO49-18) crosses the continental slope of the northern margin, the Northwest Subbasin (NWSB) of the South China Sea, the Zhongsha Massif and partly the oceanic basin of the South China Sea. Seismic sections recorded on 13 ocean-bottom seismometers were used to identify refracted phases from the crustal layer and also reflected phases from the crust-mantle boundary (Moho). Inversion of the traveltimes using a simple start model reveals crustal images in the study area. The velocity model shows that crustal thickness below the continental slope is between 14 and 23 km. The continental part of the line is characterized by gentle landward mantle uplift and an abrupt oceanward one. The velocities in the lower crust do not exceed 6.9 km/s. With the new data we can exclude a high-velocity lower crustal body (velocities above 7.0 km/s) at the location of the line. We conclude that this part of the South China Sea margin developed by a magma-poor rifting. Both, the NWSB and the Southwest Sub-basin (SWSB) reveal velocities typical for oceanic crust with crustal thickness between 5 and 7 km. The Zhongsha Massif in between is extremely stretched with only 6–10 km continental crust left. Crustal velocity is below 6.5 km/s; possibly indicating the absence of the lower crust. Multi-channel seismic profile shows that the Yitongansha Uplift in the slope area and the Zhongsha Massif are only mildly deformed. We considered them as rigid continent blocks which acted as rift shoulders of the main rift subsequently resulting in the formation of the Northwest Sub-basin. The extension was mainly accommodated by a ductile lower crustal flows, which might have been extremely attenuated and flow into the oceanic basin during the spreading stage. We compared the crustal structures along the northern margin and found an east-west thicken trend of the crust below the continent slope. This might be contributed by the east-west sea-floor spreading along the continental margin.     

Ichnological evidence of jökulhlaup deposit recolonization from the Touchet Beds,Mabton, WA,USA

The late Wisconsinan Touchet Beds section at Mabton, Washington reveals at least seven stacked jökulhlaup deposits, five showing evidence of post-flood recolonization by vertebrates. Tracemakers are attributed to voles or pocket mice (1–3 cm diameter burrows) and pocket gophers or ground squirrels (3–6 cm diameter burrows). The Mount St. Helens S tephra deposited between flood beds contains the invertebrate-generated burrows Naktodemasis and Macanopsis. Estimates of times between floods are based on natal dispersal distances of the likely vertebrate tracemakers (30–50 m median distances; 127–525 m maximum distances) from upland areas containing surviving populations to the Mabton area, a distance of about 7.9 km. Tetrapods would have required at least two to three decades to recolonize these flood beds, based on maximum dispersal distances. Invertebrate recolonization was limited by secondary succession and estimated at only a few years to a decade. These ichnological data support multiple floods from failure of the ice dam at glacial Lake Missoula, separated by hiatal surfaces on the order of decades in duration. Ichnological recolonization times are consistent with published estimates of refill times for glacial Lake Missoula, and complement the other field evidence that points to repeated, autogenically induced flood discharge.     

Steep bending of continental lithosphere without its stretching or plate collision: an indication for lithospheric failure and phase transitions

The Black Sea and Peri-Caspian basin include steep slopes which are 4–10 km high and only 20–30 km wide. They formed without extension-related normal faulting beneath the slopes and convergence in the adjacent regions. In the Peri-Caspian basin a steep bending of the lithosphere was associated with rapid crustal subsidence which evolved over a few million years. The large intensity and high rate of the deformations indicate a temporary loss of high rigidity in the lithospheric layer which occurred independently of plate-boundary-related processes. The origin of this phenomenon presents a special problem. A tentative explanation is that a lithospheric failure in cool intraplate regions can result from infiltration of volatiles from the asthenosphere and rapid metamorphism in mafic rocks in the lower crust.     

Tectonic rotations about vertical axes during the last 4 Ma in part of the New Zealand plate-boundary zone

The declinations of the primary magnetization of sedimentary rocks in the northern part of the New Zealand plate-boundary zone, after thermal or alternating field cleaning, have been used to determine tectonic rotations about vertical axes of rigid crustal blocks. The pattern of rotations during the last 4 Ma, combined with structural style and continuity, defines seven structural domains each ca 100 × 100–200 km across, which contain crustal blocks up to 100 km across and 20 km thick. Large crustal blocks (tens of km across) in two of these domains have rotated clockwise more than 20° relative to one of the margins of the plate-boundary zone in the last 4 Ma. Their behaviour appears to be controlled by the nature of the plate boundaries, such as the presence of an underlying subducted slab and the strength of the crust at the back of the overlying crustal wedge. Small crustal blocks (< 10 km across) may have rotated clockwise through angles greater than 20° during the last 4 Ma, floating on an underlying zone of more distributed deformation. The tectonic rotations of the large crustal blocks, and the nature of the deformation at their boundaries, combined with an interpolation of the relative plate positions, can be used to reconstruct the plate-boundary zone at ca 4 Ma.