2. Wochenbericht FS Maria S. Merian MSM71

LOBSTER, Las Palmas – Heraklion, 07.02.2018 – 27.02.201

3. Wochenbericht FS Maria S. Merian MSM71

LOBSTER, Las Palmas – Heraklion, 07.02.2018 – 27.02.201

1. Wochenbericht SO244/2

FS SONNE Reise SO244, Antofagasta – Antofagasta 27.11.15 – 13.12.15, 1. Wochenbericht vom 29. Nov. 201

1. Wochenbericht M86/3

FS METEOR Reise 86, Fahrtabschnitt 3 Brindisi –Dubrovnik 1. Wochenbericht, 20.01.12 – 22.01.1

2. Wochenbericht M111

FS METEOR Reise M111 Catania – Catania 10.10.14 – 01.11.14 2. Wochenbericht vom 24. Okt. 201

Statistical separation strategy to analyze velocity structure obtained by seismic tomography

General solutions of inverse problems often can be obtained by introducing probability distributions to sample the model space. We have developed a simple approach to define an a priori space in a tomographic study and retrieve the velocity/depth posterior distribution by a Monte Carlo method. Utilizing a fitting routine designed for very low statistics to set up and analyze the obtained tomography results, we can statistically separate the velocity/depth model space derived from inverting seismic refraction data. A profile acquired in the Lesser Antilles subduction zone reveals the effectiveness of this approach. Resolution analysis of the structural heterogeneity includes a divergence analysis that can dissect long, wide-angle profiles for deep crust and upper mantle studies. The complete information of any parameterized physical system is contained in the a posteriori distribution. Methods for analyzing and displaying key properties of the a posteriori distributions of highly nonlinear inverse problems are therefore essential in the scope of any interpretation. It is possible to map velocity variations in their extent and structure by measuring the total as well as relative divergence of the velocity structure in the a posteriori space. We have applied the divergence analysis to a part of the transect where a backstop structure has been identified, and the method resolves shallow features and returns information concerning the confidence level of results. Assuming a relationship between forearc and backstop, we can obtain a structural image in accordance with previous interpretations. ©2010 Society of Exploration Geophysicist

Backstop geometry and accretionary mechanics of the Sunda margin

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

Magnetotelluric image of the fluid cycle in the Costa Rican subduction zone

Fluids entering the subduction zone are a key factor in the subduction process. They determine the onset of melting, weakening and changes in the dynamics and thermal structure of subduction zones and trigger earthquakes when being released from the subducting plate in a series of metamorphic processes. However, the amount of water carried into the subduction zone and its distribution are not well constrained by existing data and are subject of vigorous current research in SFB574 (Volatiles and Fluids in Subduction Zones: Climate Feedback and Trigger Mechanisms for Natural Disasters). Electromagnetic methods like magnetotellurics have been used widely to recognize fluid release and melt production through enhanced electrical conductivities. Here we present an image of the hydration and dehydration cycle down to 120 km depth in one setting derived by an onshore-offshore transect of magnetotelluric soundings in Costa Rica. An electrically conductive zone in the incoming plate outer rise is associated with sea water penetrating down extensional faults and cracks into the upper mantle possibly causing serpentinization. Along the downward subducting plate distinct conductive anomalies identify fluids from dehydration of sediments, crust and mantle. A conductivity anomaly at a depth of approx. 12 km and at a distance of 65 km from the trench is associated with a first major dehydration reaction of minerally-bound water. This is of importance in the context of mid-slope fluid seeps which are thought to significantly contribute to the recycling of minerally-bound water. The position of the conductivity anomaly correlates with geochemical and seismic evidence stating that mid-slope fluids are originated at >=12 km depth before rising up through deep faults to the seeps. The conductivity anomaly is therefore associated with a fluid accumulation feeding the mid-slope seeps. Another fluid accumulation is revealed by a conductivity anomaly at 20-30 km depth and a distance of approximately 30 km seaward from the volcanic arc. This lower crustal fluid accumulation could likely be caused by trapping of fluids released due to de-serpentinization processes or due to other mineral dehydration processes. While we are at the moment not able to attribute one specific process causing the anomaly based on electromagnetic data alone, this feature is however of fundamental importance. A comparison with other electromagnetic studies from subduction zones around the world reveal that such a conductivity anomaly is a global feature suggesting the presence of a global fluid sink. Based on very simplified assumptions we are able derive rough estimates for the amount of water being stored in the overriding plate. Relating seismic evidence as well as petrological results collected in the multi-disciplinary study on the Costa Rican subduction zone we introduce budget estimations for the water cycle in the subduction zone