Victor Mocanu

Detailed look at final stage of plate break‐off is target of study in Romania

Geophysical experiments next year in Romania may provide insight into a common but short-lived seismic process that can be observed and understood at only one spot on Earth at present. About 150 stations will be set up in the Vrancea area in the southeast Carpathian Mountains to, in effect, record the terminal phase of the detachment of a subducting slab of oceanic lithosphere. This is a major regional tomographic study using a large number of broadband seismometers, which will operate for 6 months. Images will be used for hazard assessment as well as for a delineation of detachment history. Active subduction of oceanic lithosphere at convergent plate boundaries involves earthquakes, magmatism, metamorphism, and deformation—some of the most vivid manifestations of any plate tectonic process. The initiation and termination of subduction, however, remains relatively poorly understood. When convergence of lithospheric plates ceases and the suction force of the subducting plate becomes negligible, the subducting slab moves into an almost vertical position. If subduction occurs in an arcuate geometry, the slab is likely to be segmented.

Lateral variations in lithosphere strength in the Romanian Carpathians: constraints on basin evolution

Lithospheric strength profiles constructed through extrapolation of rock mechanics data and constrained by crustal geophysics data, demonstrate the existence of important spatial variations of thermomechanical properties in different domains of the Romanian Carpathians, the Pannonian basin and the Transylvanian depression. These models show important lateral variations in lithospheric rigidity along the Southern and Eastern Carpathian foreland, compatible with inferences from flexural modelling studies. The modelling predicts an absence of mantle strength and the presence of weak lithosphere in the Pannonian and Transylvanian basins. A NWSE-trending rheological transect is presented, connecting the Moesian platform, the Vrancea region of the Carpathians, the Transylvanian basin and the Apuseni Mountains with the Romanian part of the Pannonian basin. This transect demonstrates the existence of a close correlation of the inferred spatial variations in mechanical properties of the Romanian lithosphere with observed patterns in large-scale gravity anomalies and with tectonic units. A notable feature of the rheological models is the presence of an intra-Carpathian weak zone. The combination of a strong foreland lithosphere with a weak lithosphere in the inner part of the Carpathian/Transylvanian/Pannonian system sheds light on a number of key features observed in the evolution of sedimentary basins in these areas. Remarkable differences in rheology predictions between flexural models and depth-dependent rheology models indicate that the crustal lithosphere is detached from the mantle part of the lithosphere in the Carpathian foreland.

Subduction of the Chile Ridge: Upper mantle structure and flow

We deployed 39 broadband seismometers in southern Chile from Dec. 2004 to Feb. 2007 to determine lithosphere and upper mantle structure in the vicinity of the subducting Chile Ridge. Body-wave travel-time tomography clearly shows the existence of a long-hypothesized slab window, a gap between the subducted Nazca and Antarctic lithospheres. P-wave velocities in the slab gap are distinctly slow relative to surrounding asthenospheric mantle. Thus, the gap between slabs visible in the imaging appears to be filled by unusually warm asthenosphere, consistent with subduction of the Chile Ridge. Shear wave splitting in the Chile Ridge subduction region is very strong (mean delay time ~3 s) and highly variable. North of the slab windows, splitting fast directions are mostly trench parallel, but, in the region of the slab gap, splitting fast trends appear to fan from NW-SE trends in the north, through ENE-WSW trends toward the middle of the slab window, to NE-SW trends south of the slab window. We interpret these results as indicating flow of asthenospheric upper mantle into the slab window.

Source-side shear wave splitting and upper mantle flow in the Chile Ridge subduction region

The actively spreading Chile Ridge has been subducting beneath Patagonian Chile since the Middle Miocene. After subduction, continued separation of the faster Nazca plate from the slow Antarctic plate has opened up a gap—a slab window—between the subducted oceanic lithospheres beneath South America. We examined the form of the asthenospheric mantle flow in the vicinity of this slab window using S waves from six isolated, unusual 2007 earthquakes that occurred in the generally low-seismicity region just north of the ridge subduction region. The S waves from these earthquakes were recorded at distant seismic stations, but were split into fast and slow orthogonally polarized waves at upper mantle depths during their passage through the slab window and environs. We isolated the directions of fast split shear waves near the slab window by correcting for upper mantle seismic anisotropy at the distant stations. The results show that the generally trench-parallel upper mantle flow beneath the Nazca plate rotates to an ENE trend in the neighborhood of the slab gap, consistent with upper mantle flow from west to east through the slab window.

Study of recent crustal movements in Romania: A review

Abstract An historical overview of geomorphological, geological and geodetical surveys carried out in Romania is presented. These studies have provided information concerning the recent crustal movements on the Romanian territory. Special attention was paid to the geodetical surveys carried out in “specific study areas” during the last 25 years in order to verify some geodynamic hypotheses. In this way, the uplift tendency of the Carpathian orogen has to be emphasized. There is a clear separation between the northern part of Eastern Carpathians where the highest vertical velocities are pointed out (+ 5 mm/y) and the rest of the mountain chain, characterized by a moderate upheaval (+ 2 mm/y). The bending zone of the Carpathian foredeep is affected by a relatively strong subsidence (− 2 mm/y). The Carpathian foreland units appear as a relatively stabilized area. The Transylvanian basin occurs as an almost stable zone. The Romanian sector of the Pannonian basin is characterized by continuous subsidence movements, the intensity increasing from east to west. The western part of the Moesian Platform is relatively stable. Unlike it, the eastern sector with a large surface extension occurs as an important subsidence area, the sinking movements being more intense to the east, near the Black Sea coast.

Crustal structure beneath the Blue Mountains terranes and cratonic North America, eastern Oregon, and Idaho, from teleseismic receiver functions

We present new images of lithospheric structure obtained from P-to-S conversions defined by receiver functions at the 85 broadband seismic stations of the EarthScope IDaho-ORegon experiment. We resolve the crustal thickness beneath the Blue Mountains province and the former western margin of cratonic North America, the geometry of the western Idaho shear zone (WISZ), and the boundary between the Grouse Creek and Farmington provinces. We calculated P-to-S receiver functions using the iterative time domain deconvolution method, and we used the H-k grid search method and common conversion point stacking to image the lithospheric structure. Moho depths beneath the Blue Mountains terranes range from 24 to 34 km, whereas the crust is 32–40 km thick beneath the Idaho batholith and the regions of extended crust of east-central Idaho. The Blue Mountains group Olds Ferry terrane is characterized by the thinnest crust in the study area, ~24 km thick. There is a clear break in the continuity of the Moho across the WISZ, with depths increasing from 28 km west of the shear zone to 36 km just east of its surface expression. The presence of a strong midcrustal converting interface at ~18 km depth beneath the Idaho batholith extending ~20 km east of the WISZ indicates tectonic wedging in this region. A north striking ~7 km offset in Moho depth, thinning to the east, is present beneath the Lost River Range and Pahsimeroi Valley; we identify this sharp offset as the boundary that juxtaposes the Archean Grouse Creek block with the Paleoproterozoic Farmington zone.

GPS probes the kinematics of the Vrancea Seismogenic Zone

In late 2001, the Surface Behavior and Dynamical Units of the Southeast Carpathian Tectonics (SUBDUCT) program was initiated by the Netherlands Research Center for Integrated Solid Earth Sciences (ISES), together with the faculty of geology and geophysics at the University of Bucharest, and the (Romanian) National Institute for Earth Physics. The aim of this program is to monitor, analyze, and interpret the surface motions occurring in response to active crust-lithosphere dynamics of the southeast Carpathians in Romania (Figure 1), using the Global Positioning System (GPS). For this region, observations of surface kinematics constitute a new and independent data source. In combination with other information, mainly obtained by geologic and geophysical studies, surface motions may help to unravel the intriguing tectonics of the region. Particularly SUBDUCT focuses on the dynamic processes of the Vrancea high-seismicity region.

Tectonics of the Alpine-Carpathian-Pannonian region: introduction

Research on the Alpine-Carpathian-Pannonian system intensified during the past decade because of several reasons. These include the following. (1) The specific nature of late-orogenic intracontinental extrusion processes with orogen-parallel displacement due to indentation by the rigid Southalpine/Apulian indenter, and formation of various sedimentary basins in the interior of the arcuate orogen. The ongoing research on Cenozoic/Neogene basins has shown the need for quantitative understanding of vertical motion of the underlying basement, as well as better understanding of the thermal and mechanical properties of the lithosphere underlying the various basins (Genser et al., 1996). At the same time, it has become apparaent for more structural studies to unravel the complex tectonic history of basins. (2) The pre-collisional history, which includes a complex system of oceanic and continental plate boundaries, is not fully understood. (3) The Phanerozoic history of the AlpineCarpathian-Balkanian orogenic belt that is entirely different its geological evolution from the adjacent Russian platform. Because of these factors, many international projects and workshops are dealing with various aspects of the considered region. Among these, the

Azimuthal anisotropy in the Chile Ridge subduction region retrieved from ambient noise

In the southern Andes, the oblique convergence of the Nazca plate and the subduction of an active oceanic ridge represent two major tec- tonic features driving deformation of the forearc in the overriding continental plate, and the relative effects of these two mechanisms in the stress fi eld have been a subject of debate. North of the Chile triple junction, oblique subduction of the Nazca plate is associated with the Liquine-Ofqui fault zone, an ~1000-km-long strike-slip fault that is partitioning the stress and deformation in the forearc. South of the Chile triple junction, the Antarctic plate converges normal to the trench, and several ridge segments have been colliding with the overriding plate since 14 Ma. Proposed effects of the collision include episodes of uplift, extension, and formation of a forearc sliver. Using ambient seismic noise recorded by the Chile Ridge Subduction Project seismic network, we retrieved azimuthal anisotropy from inversion of Rayleigh wave group velocity in the 6-12 s period range, mostly sensitive to crustal depths. North of the Chile triple junction in the forearc region, our results show a fast velocity for azimuthal anisotropy oriented subparallel to the Liquine-Ofqui fault zone. South of the Chile triple junction, anisotropy is higher, and fast velocity measurements present clockwise rotation south of the subducted ridge and counterclockwise rota- tion north of the ridge. These results suggest the presence of two main domains of deformation: one with structures formed during oblique convergence of the Nazca plate north of the Chile triple junction and the other with structures formed during normal convergence of the Antarctic plate, coupled with collision of the Chile Ridge south of the Chile triple junction. Low velocities and high anisotropy over the sub- ducted Chile Ridge and slab window could be an indication of anomalously high thermal conditions, yielding a more plastic deformation compared with the north, where conditions are more cold and rigid.

Analysis of the surface waves present on the passive seismic data set recorded in the Mizil area (Romania)

A passive seismic survey in April 2009 had two main goals: the use of seismic interferometry to retrieve the seismic signal (reflected waves) from the background noise and to demonstrate that passive surveys can cover spatial gaps between active profiles. Seismic interferometry works well if the passive records are not affected by surface waves with high amplitudes; therefore, such records are neglected in the application of seismic interferometry.