M. Engels

Birth of an intraoceanic spreading center

The Cocos-Nazca spreading center is one of the few examples of the formation of a spreading center by splitting of oceanic lithosphere. It was created when the Farallon plate broke up in the early Miocene following the collision of the Pacific-Farallon spreading center with the North American continent. Much of the ancient Farallon plate corresponding to the area of opening is lost to subduction beneath Central America and South America, but new data from the conjugate area on the Pacific plate allow the first detailed reconstruction of the break-up process. The opening began after chron 7 (25 Ma) at a location of focused crustal extension caused by overlapping spreading centers that had evolved in response to a slight reorientation of a Pacific-Farallon ridge segment. Beginning at chron 6B (22.7 Ma), eastward progressing seafloor spreading started along an axis that most likely migrated toward the region of weak lithosphere created by the Galapagos hotspot. By chron 6 (19.5 Ma), plate splitting from the spreading center to the trench was complete, allowing the fully detached Cocos and Nazca plates to move independently. This kinematic change resulted in a significant ridge jump of the newly established Pacific-Nazca spreading center, a change in plate motion direction of the Nazca plate by 20° clockwise, and a large increase in Pacific-Cocos plate velocity in the middle Miocene.

Trans-dimensional Bayesian inversion of controlled-source electromagnetic data in the German North Sea

This paper presents the first controlled-source electromagnetic survey carried out in the German North Sea with a recently developed seafloor-towed electrical dipole–dipole system, i.e., HYDRA II. Controlled-source electromagnetic data are measured, processed, and inverted in the time domain to estimate an electrical resistivity model of the sub-seafloor. The controlled-source electromagnetic survey targeted a shallow, phase-reversed, seismic reflector, which potentially indicates free gas. To compare the resistivity model to reflection seismic data and draw a combined interpretation, we apply a trans-dimensional Bayesian inversion that estimates model parameters and uncertainties, and samples probabilistically over the number of layers of the resistivity model. The controlled-source electromagnetic data errors show time-varying correlations, and we therefore apply a non-Toeplitz data covariance matrix in the inversion that is estimated from residual analysis. The geological interpretation drawn from controlled-source electromagnetic inversion results and borehole and reflection seismic data yield resistivities of ?1 ?m at the seafloor, which are typical for fine-grained marine deposits, whereas resistivities below ?20 mbsf increase to 2–4 ?m and can be related to a transition from fine-grained (Holocene age) to unsorted, coarse-grained, and compacted glacial sediments (Pleistocene age). Interface depths from controlled-source electromagnetic inversion generally match the seismic reflector related to the contrast between the different depositional environments. Resistivities decrease again at greater depths to ?1 ?m with a minimum resistivity at ?300 mbsf where a seismic reflector (that marks a major flooding surface of late Miocene age) correlates with an increased gamma-ray count, indicating an increased amount of fine-grained sediments. We suggest that the grain size may have a major impact on the electrical resistivity of the sediment with lower resistivities for fine-grained sediments. Concerning the phase-reversed seismic reflector that was targeted by the survey, controlled-source electromagnetic inversion results yield no indication for free gas below it as resistivities are generally elevated above the reflector. We suggest that the elevated resistivities are caused by an overall decrease in porosity in the glacial sediments and that the seismic reflector could be caused by an impedance contrast at a thin low-velocity layer. Controlled-source electromagnetic interface depths near the reflector are quite uncertain and variable. We conclude that the seismic interface cannot be resolved with the controlled-source electromagnetic data, but the thickness of the corresponding resistive layer follows the trend of the reflector that is inclined towards the west.

From Subduction to Collision: The Sunda‐Banda Arc Transition

In the aftermath of the Mw 9.3 Indian Ocean earthquake and tsunami of 26 December 2004, which killed more than 250,000 people, numerous investigations have been commissioned near the epicenter offshore northern Sumatra to evaluate future earthquake and tsunami hazards. These projects have mapped seafloor morphology and imaged deep structures and faults in order to better understand the origin of megathrust earthquakes and tsunamis in the western portion of the Sunda Arc subduction system offshore northern Sumatra [e.g., Henstock et al., 2006]. In contrast, the eastern part of the arc has received relatively little attention, even though it may be just as hazardous. Our geophysical data from the eastern Sunda Arc and the transition to the Banda Arc (Figure 1) provide evidence for recent tectonic activity and thus for a similar earthquake and tsunami risk.

1D joint multi‐offset inversion of time‐domain marine controlled source electromagnetic data

The accurate estimation of sub-seafloor resistivity features from marine controlled source electromagnetic data using inverse modelling is hindered due to the limitations of the inversion routines. The most commonly used one-dimensional inversion techniques for resolving subsurface resistivity structures are gradient-based methods, namely Occam and Marquardt. The first approach relies on the smoothness of the model and is recommended when there are no sharp resistivity boundaries. The Marquardt routine is relevant for many electromagnetic applications with sharp resistivity contrasts but subject to the appropriate choice of a starting model. In this paper, we explore the ability of different 1D inversion schemes to derive sub-seafloor resistivity structures from time domain marine controlled source electromagnetic data measured along an 8-km-long profile in the German North Sea. Seismic reflection data reveal a dipping shallow amplitude anomaly that was the target of the controleld source electromagnetic survey. We tested four inversion schemes to find suitable starting models for the final Marquardt inversion. In this respect, as a first scenario, Occam inversion results are considered a starting model for the subsequent Marquardt inversion (Occam–Marquardt). As a second scenario, we employ a global method called Differential Evolution Adaptive Metropolis and sequentially incorporate it with Marquardt inversion. The third approach corresponds to Marquardt inversion introducing lateral constraints. Finally, we include the lateral constraints in Differential Evolution Adaptive Metropolis optimization, and the results are sequentially utilized by Marquardt inversion. Occam–Marquardt may provide accurate estimation of the subsurface features, but it is dependent on the appropriate conversion of different multi-layered Occam model to an acceptable starting model for Marquardt inversion, which is not straightforward. Employing parameter spaces, the Differential Evolution Adaptive Metropolis approach can be pertinent to determine Marquardt a priori information; nevertheless, the uncertainties in Differential Evolution Adaptive Metropolis optimization will introduce some inaccuracies in Marquardt inversion results. Laterally constrained Marquardt may be promising to resolve sub-seafloor features, but it is not stable if there are significant lateral changes of the sub-seafloor structure due to the dependence of the method to the starting model. Including the lateral constraints in Differential Evolution Adaptive Metropolis approach allows for faster convergence of the routine with consistent results, furnishing more accurate estimation of a priori models for the subsequent Marquardt inversion.