Tectonic geomorphology and hydrocarbon induced topography of the Mid-Channel Anticline, Santa Barbara Basin, California

The geomorphology of the western sector of the Mid-Channel Anticline (MCA), Santa Barbara, southern California suggests the actively growing fold is laterally propagating to the west. The presence of fold scarps and cross faults that segment the structure suggests that buried faults that are producing the folding are present at shallow depths. The summit area of the anticline at the Last Glacial Maximum (22 to 19 ka) was probably a small late Pleistocene island. Evidence for presence of the island includes what appears to be terrestrial erosion and is supported by assumption of sea level change and rates of uplift and subsidence.Pockmarks and domes ranging in diameter from  10 to 100 m, and several meters deep are present along the crest and flanks of the MCA. These features appear to be the result of hydrocarbon emission. Their formation has significantly modified the surface features, producing simple to complex erosional and/or constructional topography. A large pockmark near the anticline crest dated by two calibrated AMS radiocarbon dates of 25.3 and 36.9 ka continues to emit hydrocarbon gases. We term the topography produced by hydrocarbon emission as Hydrocarbon Induced Topography (HIT).     

Three-dimensional seismic structure beneath the Australasian region from refracted wave observations

The earthquakes in the seismicity belt extending through Indonesia, New Guinea, Vanuatu and Fiji to the Tonga–Kermadec subduction zone recorded at the 65 portable broad-band stations deployed during the Skippy experiment from 1993–1996 provide good coverage of the lithosphere and mantle under the Australian continent, Coral Sea and Tasman Sea.
The variation in structure in the upper part of the mantle is characterized by deter-mining a suite of 1-D structures from stacked record sections utilizing clear P and S arrivals, prepared for all propagation paths lying within a 10° azimuth band. The azimuth of these bands is rotated by 20° steps with four parallel corridors for each azimuth. This gives 26 separate azimuthal corridors for which 15 independent 1-D seismic velocity structures have been derived, which show significant variation in P and S structure.
The set of 1-D structures is combined to produce a 3-D representation by projecting the velocity values along the ray path using a turning point approximation and stacking into 3-D cells (5° by 50 km in depth). Even though this procedure will tend to underestimate wave-speed perturbations, S -velocity deviations from the ak135 reference model exceed 6 per cent in the lithosphere.
In the uppermost mantle the results display complex features and very high S -wave speeds beneath the Precambrian shields with a significant low-velocity zone beneath. High velocities are also found towards the base of the transition zone, with high S -wave speeds beneath the continent and high P -wave speeds beneath the ocean. The wave-speed patterns agree well with independent surface wave studies and delay time tomography studies in the zones of common coverage.     

Earthlearningidea—two million and counting: bringing Earth science ideas to teachers across the world

As one door closes, another one opens—sometimes. This is certainly true in the history of the development of the Earth science education website, http://www.earthlearningidea.com . In the expectation that funding would be available to help teachers in a range of developing countries during the UN's International Year of Planet Earth in 2008, a series of overseas workshops in Earth science was proposed. Undeterred by the probable absence of funding, Chris King, of Keele University, co‐opted Elizabeth Devon and Peter Kennett to initiate a web‐based series of Earth science activities, which could be freely accessed by anyone in the world. This was, and is, run on a purely voluntary basis. Up until 2014, the website was generously hosted, free of charge, by the Applied and Environmental Geophysics Research Group at Keele University, but the site now runs on a FreeBSD server situated in the London Telehouse and managed by Martin Devon. The minimal monthly costs of this are covered by a grant from the Geologists' Association's Curry Fund.     

Dynamic Earth: crustal and mantle heterogeneity

The dynamic processes within the Earth leave their record in geophysical and geochemical variation about the general stratification with depth. A snapshot of current structure is provided by geophysical evidence, whereas geochemical information provides a perspective over the age of the Earth. The combination of information on the distribution of heterogeneity from geophysical and geochemical sources provides enhanced insight into likely geodynamic processes. A variety of techniques can be used to examine crustal structure, but the major source of information on seismic heterogeneity within the Earth comes from tomographic studies, exploiting surface waves for the lithosphere and body waves for the bulk Earth. A powerful tool for examining the character of mantle heterogeneity is the comparison of images of bulk-sound and shear-wave speed extracted in a single inversion, since this isolates the dependencies on the elastic moduli. Such studies are particularly effective when a common path coverage is achieved for P and S as, for example, when common source and receiver pairs are extracted for arrival times of the phases. The relative behaviour of the bulk-sound and shear-wave speeds allows the definition of heterogeneity regimes. For subduction zones, a large part of the imaged structure comes from S-wave speed variations. The narrow segments of fast wave speeds in the lower mantle, in the depth range 900 – 1500 km, are dominated by S variations, with very little bulk-sound contribution, so images of P-wave speed are controlled by shear-wave variability. Deep in the mantle, there are many features with high seismic-wave speed without an obvious association with subduction in the last 100 million years, which suggests long-lived preservation of components of the geodynamic cycle. The base of the Earth's mantle is a complex zone with widespread indications of heterogeneity on many scales, discontinuities of variable character, and shear-wave anisotropy. Discordance between P- and S-wave speed anomalies suggests the presence of chemical heterogeneity rather than just the effect of temperature.     

New constraints on the current stress field and seismic velocity structure of the eastern Yilgarn Craton from mechanisms of local earthquakes

The Yilgarn Craton has hosted some of the largest earthquakes within the Australian continent in the last 100 years. Earthquakes have mainly been studied in the western part of the craton, and are thought to result from the reactivation of Precambrian structures in an E–W compressive regional stress field imposed by plate-scale processes. Here we present moment tensor solutions for three recent moderate-sized earthquakes around the town of Kalgoorlie that are inconsistent with E–W compression, but instead suggest E–W extension in the eastern Yilgarn Craton. Waveforms of earthquakes at Boulder (M_W = 4.0, 20 April 2010), Kalgoorlie (M_W = 4.3, 26 February 2014) and Coolgardie (M_W = 3.9, 31 October 2014) were inverted for moment tensors. All three earthquakes were shallow (centroid depth ≤4 km) normal-faulting events that occurred along roughly N–S-striking planes, either with a steep westward or a relatively shallow eastward dip. The robustness of the retrieved mechanisms has been thoroughly tested, employing different earth models, assuming different locations for the earthquakes and using different period bands for the inversion. The fit of synthetic long-period waveforms to the observations was in all cases substantially improved by assuming a two-layered crust with high S wavespeeds (about 3.9–4 km/s) overlying substantially slower material. Since there is independent evidence from active source profiles for a P velocity increase between the upper and lower crust, a large difference in v_p/v_s ratio between upper and lower crust is the only way to explain both lines of evidence. This vertical contrast could represent a dominance of felsic material in the upper crust, and substantially more mafic material in the lower crust. Taken together, our results also appear to imply that the regional stress field is E–W extensive in the Kalgoorlie area, and possibly for the entire Kalgoorlie Terrane. This is contrary to current assumptions from continent-scale stress modelling. That the orientations of rupture planes roughly align with the regional structural grain could indicate that Archean structures are reactivated in response to the current stress field.     

Structure of the Mt Isa region from seismic ambient noise tomography

We use seismic tomography, exploiting group velocities derived from ambient noise, to delineate the crustal structure beneath Mt Isa and the surrounding blocks and basins. The depth extent of the blocks can be traced into the mid-crust and the spatial extent of the associated velocity anomalies mapped over an area of approximately 500 km by 500 km. The Proterozoic Mt Isa block is imaged as a region of elevated seismic velocities comparable to the Yilgarn craton in Western Australia, while the surrounding basins have relatively low velocities. Seismic velocity anomalies display correlations with the regional Bouguer gravity data and with high crustal temperatures in the region. There are a number of isolated low-velocity anomalies under the Millungera basin that suggest either previously unknown thermal anomalies or zones with high permeability, which can also produce lowered velocities.     

The structure of the upper mantle beneath the Delamerian and Lachlan orogens from simultaneous inversion of multiple teleseismic datasets

Distant earthquake data recorded by seven sub-arrays of the ongoing WOMBAT rolling seismic array deployment in southeast Australia are combined for the first time to constrain 3-D variations in upper mantle P-wavespeed via teleseismic tomography. The seven arrays comprise a total of 276 short period recorders spaced at intervals of approximately 50 km, thus allowing unprecedented resolution of the upper mantle over a large region. In the mantle lithosphere immediately below the crust (~ 50 km depth), dominant variations in velocity tend to strike east–west, and share little resemblance to Palaeozoic boundaries in the shallow crust inferred from surface geology and potential field data. A broad region of elevated wavespeed beneath northern Victoria may represent the signature of underplated igneous rocks associated with detachment faulting during the break-up of Australia and Antarctica. A distinct low velocity anomaly in southern Victoria appears to correlate well with the Quaternary Newer Volcanic Provinces. Towards the base of the mantle lithosphere, the dominant structural trend becomes north–south, and five distinct velocity zones become apparent. Of particular note is a transition from higher wavespeed in the west to lower wavespeed in the east beneath the Stawell Zone, implying that the Proterozoic lithosphere of the Delamerian Orogen protrudes eastward beneath the Western subprovince of the Lachlan Orogen. This transition zone extends northwards from southern Victoria into central New South Wales (the northward limit of the arrays), and is one of the dominant features of the model. Further east, there is a transition from lower to higher wavespeeds in the vicinity of the boundary between the Western and Central subprovinces of the Lachlan Orogen, which has several plausible explanations, including the existence of a Proterozoic continental fragment beneath the Wagga–Omeo Zone. The presence of elevated wavespeeds beneath the Melbourne Zone in Victoria, although not well constrained due to limited data coverage, provides some support to the Selwyn Block model, which proposes a northward extension beneath Bass Strait of the Proterozoic core of Tasmania.     

Teleseismic tomography of the upper mantle beneath the southern Lachlan Orogen, Australia

With the goal of better understanding the deep structure and tectonic setting of the Lachlan Orogen, 50 short period seismic stations were deployed across the southern end of the Great Dividing Range in Victoria (southeast Australia) between 2005 and 2006 to record distant earthquakes. A total of 7452 relative P-wave arrival time residuals from 169 teleseismic events have been extracted from the continuous records using an adaptive stacking technique, which exploits the coherency of global phases across the array. These residuals are mapped as three-dimensional perturbations in P-wavespeed in the upper mantle beneath the array using a recently developed iterative non-linear tomographic procedure, which combines a grid based eikonal solver and a subspace inversion technique. The capability of the new scheme to include interface geometry is utilised in order to investigate the effects of a priori Moho topography on the resolution of upper mantle structure. The resultant images show a pattern of P-wavespeed anomalies that lacks a predominant orientation, and therefore does not favour a purely W–E subduction–accretion model for the formation of the Lachlan Orogen. One of the main features of the three-dimensional model is a zone of elevated wavespeed beneath the northern end of the array, which extends to a depth of approximately 150 km, and contrasts with significantly lower wavespeeds to the south. This anomaly, which does not appear to be an artifact of relative arrival time residual contributions from the adjoining mountainous terrane, may reflect the presence of a substantial piece of Proterozoic lithosphere incorporated within the Phanerozoic subduction–accretion setting of the Lachlan Orogen. Another key feature of the solution model is a zone of relatively low velocity beneath the newer volcanic province northwest of Melbourne, which extends from the crust to a depth of approximately 200 km. This is likely to represent the signature of elevated temperatures associated with a diffuse mantle source for the Quaternary volcanism in Victoria.