Themes: The main goal of this SPP is to understand how re-organizations of Earth’s mantle during the collision of tectonic plates are related to processes that ultimately affect the Earth’s surface in mountain belts. This will be enabled by generating the first truly 4D image of a mountain belt. Imaging mountains in four dimensions is a radical departure from traditional 2D and current 3D approaches which rely on classical seismic profiling and geological section-balancing to reconstruct crustal evolution, in most cases without regard for the mantle. 4D imaging combines leading-edge seismic imaging technology (e.g., full waveform inversion applied to dense, wide-aperture station arrays) with state-of-the-art geodynamic modeling and geological constraints on motion history (kinematic and geodetic analyses) to trace changes of this complex structure back in time. The resulting 4D image is a model that allows us to explore in exquisite detail the physical processes that have given rise to mountains and that are still actively shaping them today. This includes the study of transient processes acting on the scale of an entire mountain range, which is vital for better assessments of natural hazard and resources in orogens worldwide. 4D imaging will revolutionize solid-earth science in much the same way that CAT scans improved diagnostic medicine.

4D-MB is oriented around four research themes in order to foster interaction of the different subdisciplines required to realize the scientific objectives agreed on at the DFG round-table discussion in Hofgeismar (5-6. June, 2013) and described below.

Theme 1: Large-scale reorganizations of the lithospherewill shed deciding light on debates over whether the Eastern Alps are the site of a switch in subduction polarity and whether the subducted lithospheric slab beneath the Western Alps is in the process of breaking off. This will involve applying and developing new techniques to image the Alps from the surface down to the mantle transitional zone, and beyond. Specifically, it will entail defining the shape and orientation of subducted slabs, as well as of the lithosphere beneath the Alpine forelands. A detailed view of these deep structures will improve our knowledge of the rheology of the lithosphere, particularly as it relates to coupling of the surface and mantle.

Theme 2: Surface response to changes in deep structure on different time scalesaddresses an ongoing controversy over the competing roles of climate and tectonics in mountain building. Specifically, we will test the hypothesis that recent denudation and uplift rates partly reflect long-term, deep-seated processes imaged by AlpArray, including slab-tearing and –breakoff. A related aspect of this theme is comparing the impacts of faulting and seismicity where there is active convergence (eastern Southern Alps) and orogen-parallel extension (Eastern Alps) with areas undergoing little or no convergence (Western Alps). A challenge will be to distinguish the geomorphic impact of glaciation and tectonically induced events.

Theme 3: Rock trajectories and deformation during mountain buildingwill resolve the question of whether mantle and crustal structures manifest early stages of mountain-building (subduction, collision) or primarily preserve the imprint of later events (indentation, lateral escape) up to the present. This theme is concerned with the internal structure of faults, the processes underlying this structure and its relationship to seismic anisotropy. This also pertains to the rates of structural and chemical change, especially the way in which fragments of continental and oceanic lithosphere are subducted and preserved during their return to the surface.

Theme 4: Motion and seismicity from the present backwards in timeis aimed at explaining the enigma of why the upper crust and foreland of the Alps are seismic, whereas the lower crust and mantle lithosphere, including slabs beneath the Alps, are largely aseismic. This will involve relating patterns of deformation and seismicity to the overall motion picture of the Alps since the onset of subduction and collision. Establishing connections between structures exposed at the surface and imaged in the lithosphere may help us to see whether current seismicity and fault motion are linked to the deep structure of the Alps or to a newer tectonic regime. These studies will also improve assessments of seismic hazard in the Alps.

4D-MB tests the idea that reorganizations of Earth’s mantle have both immediate and long-lasting effects on earthquake distribution, crustal motion, and landscape evolution. Research will focus on the Alps, a proven testing ground for new ideas with revolutionary implications for mountain building on a global scale.

The AlpArray seismic network – a milestone in probing the deep structure of mountain belts

The scientific objectives outlined above can only be realized if the AlpArray seismic network is built; Germany is a key player in this international endeavor. This network is a state-of-the-art, multi-component instrument that will deliver the high-resolution 3D images which form the starting point for 4D modelling of the Alps and is a requisite for all four of the research themes outlined above.

The array comprises a network or backbone of 579 broad-band seismometers that will cover the Alps for 3 years, from the end of 2015 to the end of 2018 (white lines in Fig. 3, see Appendix III). The tomographic target volume of the backbone at the scale of the orogen extends from the surface down to the base of the Mantle Transition Zone (MTZ) at ~660 km depth. The backbone is broken down into 278 existing, permanent broad-band seismometers of the national networks, 268 mobile land-based seismometers from a multinational pool and 33 off-shore seismometers to be deployed temporarily in the Ligurian Sea (Fig. 3b). The German contribution to the backbone (Fig. 3b) includes 24 of the 33 ocean-bottom seismometers (OBS) in the Ligurian Sea (LOBSTER) and 68 land-based stations scraped together in a joint effort by 8 German universities (UNIBRA). The latter will be deployed in the central part of the Alps. From Autumn of 2017 onward, after acquisition of the 100 land-based stations of DSEBRA, the 68 university stations will be replaced and an additional 32 sites will be taken over from project partners (Fig. 3 of Appendix V).

The AlpArray backbone network will be augmented by several swaths of dense station deployments that will be deployed for 2 years across key areas of the orogen (Fig. 3a, see Appendix III). These are embedded in the AlpArray backbone and DSEBRA. Resolution generally decreases with depth due to defocussing effects, but increases with deployment time and decreased spacing between the stations; thus, the ~12 km spacing along the swaths will provide unprecedented 10-15 km resolution of structures down to ~200 km depth. Determining the conductivity structure of the crust and lithospheric mantle is desirable and will be prefaced by a feasibility study of magnotellurics (MT).

The German effort will concentrate on swaths C and D in order to complement previous campaigns (e.g., TRANSALP) across the gap in positive anomalies beneath the Central and Eastern Alps (Figs. 3, 4). This will help us to understand how active faults in the eastern part of the Southern Alps are linked to crustal and mantle structures that are accommodating ongoing Adria-Europe convergence. Combining dense, focused swaths with the AlpArray backbone network and DSEBRA will allow us to improve on the resolution of deep lithospheric images generated by US Array, the mobile seismological station network of the Earthscope project with 70 km backbone station spacing (Kerr 2013). This will be made possible by combining the new AlpArray data with leading-edge methods like full seismic wave-form inversion and seismic interferometry.

German stations of the backbone (DSEBRA and LOBSTER) – essential components of AlpArray

DSEBRA "Deutsches (German) Seismological Broadband Array" is a single array instrument of 100 broad- band, land-based stations and is dedicated to long-term, dense, large-aperture seismological field experiments, as described in detail in Appendix V. In this proposal (section VIII) we request funds to deploy and operate these mobile seismometers in order to fill large gaps in the permanent station network (Fig. 3a) in the middle of the AlpArray backbone (Fig. 3b). This part of the backbone will cover the central and eastern parts of the Alps, plus the Upper Rhine Graben and the Molasse Basin in southern Germany and Austria. The first mission of DSEBRA will therefore enable AlpArray to image slab geometries and properties, as well as deep structure in the vicinity of the Alps-Dinarides join (Themes 1, 3). This is where the Adriatic and European plates are actively converging, where surface effects are most pronounced (Theme 2) and where seismic hazard and risk are highest (Theme 4).

LOBSTER “Ligurian Ocean-Bottom Seismology & Tectonics Research” provides ship time to deploy OBS stations and to insure 24 broad-band OBSs from the German DEPAS pool. The proposal was granted by the DFG Senate Commission for Oceanography in January of 2014 and the commission has since confirmed the use of ship time and OBS rental in 2017 (Letter, Appendix VI). Data analysis and OBS insurance are not included in these proposals; therefore both points are part of the funding package requested in this SPP (therefore section VIII). The off-shore stations in the Ligurian Sea will provide crucial information on the 3D structure of the Alps-Apennines join by capturing surface and body waves emanating from earthquakes in the Pacific area and penetrating the Alpine slabs. Both DSEBRA and LOBSTER are keys for understanding how the surface and lithosphere are responding to changes in the polarity of subduction (Themes 1, 3).

Combining seismic array technology with innovative geophysical imaging methods in 4D-MB is an emerging field that will revolutionize our view of mountain building. The new German stations of DSEBRA will bring Germany to the forefront of international geodynamic research.