Identification of Crustal Thickness in Central Part of Sumatra Using Teleseismic Receiver Function Method

The Central part of Sumatra is a region that has a high potential for earthquakes. This research intended to determine the crustal thickness of the earth, P and S wave velocity profiles, and vp/vs value in the Central part of Sumatra using stacking H-k and inversion techniques based on the analysis of receiver function. This study utilized teleseismic earthquake data with a distance of 30° to 90° from the station and magnitude more than 6 (M>6). The stations used in this study were 3 BMKG broadband stations located in 3 zones, the fore arc ridge zone (SISI), the volcanic zone (PLSI) and the back arc zone (TPRI). The crustal thickness varies in the fore arc ridge zone (SISI) estimated 17.8 km, volcanic zone (PLSI) reaches 29.7 km and the back arc zone (TPRI) reaches 34 km. The crustal thickness is quite thick under the PLSI and thicker beneath TPRI station. These possibly due to the effect of topography and isostatic compensation in the station. However, whether there is a correlation between crustal thickness and topography needs further research using more stations. The highest vp/vs value was found in the volcanic zone of 1.9, that might be associated with the presence partial melting beneath the station. Meanwhile, the vp/vs value in the back arc zone is 1.72, indicating a relatively more homogeneous structure.

Isostasy amplifies relative sea-level change on continental-scale deltas

<p>The nature and contribution of flexural isostatic compensation to subsidence and uplift of passive margin deltas remains poorly understood. We performed a series of simulations to investigate flexural isostatic responses to high frequency fluctuations in water and sediment load associated with climatically-driven sea-level changes. We use a parallel basin and landscape dynamics model, BADLANDS, (an acronym for BAsin anD LANdscape DynamicS) that combines erosion, sedimentation, and diffusion with flexure, where the isostatic compensation of the load is computed by flexural compensation. We model a large drainage basin that discharges to a continental margin to generate a deltaic depocenter, then prescribe synthetic and climatic-driven sea-level curves of different frequencies to assess flexural response. Results show that flexural isostatic adjustments are bidirectional over 100-1000 kyr time-scales and mirror the magnitude, frequency, and direction of sea-level fluctuations, and that isostatic adjustments play an important role in driving along-strike and cross-shelf river-mouth migration and sediment accumulation. Our findings demonstrate that climate-forced sea-level changes set up a feedback mechanism that results in self-sustaining creation of accommodation into which sediment is deposited and plays a major role in delta morphology and stratigraphic architecture.</p>

Extreme gravity: Geologic effects

The maximum positive combined effect on the earth's gravity field of all geologic sources present in a given area, including any isostatic compensation, is limited by the ability of the lithosphere to support load. Bouguer gravity is one possible proxy for this effect. Bouguer maxima close to +350 mGal have been measured on Chichijima Island and Penrhyn Atoll in the Pacific Ocean. These may be the highest values anywhere on the planet. Suggestions of even higher values in the northernmost segment of the Colombian Andes rely on extrapolation of trends established in areas surrounding the Santa Marta Massif and must be treated with caution.

The Tibet lithosphere is not all hot

<p>We calculated the thermal lithosphere structure of Tibet and adjacent regions based on the new thermal isostasy method. Moho depth is constrained by the published receiver function results. The calculated surface heat flow in the surrounded Tarim, North China, and Yangtze cratons have a good match with the real measurements of surface heat flow. We recognize the northern Tibet anomaly where has a relatively thin lithosphere with a thermal thickness of <80 km and surface heat flow of >80 - 100 mW/m 2 may cause by the removal of lithospheric mantle and upwelling of asthenosphere. In Lhasa Block, the cold and thick lithosphere (>200 km) with a surface heat flow of 40 - 50 mW/m 2. In the east Tibet, the heterogeneous thermal lithosphere does not follow the widely spread large scale strike-slip faults and suggested that the faults do not cut down to the lithosphere. The surrounding cratons have different thermal lithosphere features. The Tarim and Yangtze cratons show typical cold and thick lithosphere with a lithosphere of >200km and surface heat flow of <50 mW/m2. The western North China Craton has an intermated lithosphere with a thickness of 120-200km and surface heat flow of 45-60 mW/m2. Our result suggested that high and flat Tibet has different isostatic compensation in different blocks. The heterogeneous lithosphere thermal structure of the Tibet suggested that the uplife force drive are difference in Tibet.</p><div> <div> </div> </div>

Surface Topography Anomalies Induced by Geodynamic Processes in the Southeast Carpathians

<p>Southeast Carpathians with deep basins (e.g., Transylvania and Focsani) and the mountain chain (SE Carpathians Mountain with ~1.5 km elevation) are characterized by unique morphological features.  The highly-variable subsurface structures (e.g., Vrancea slab) related to post-collisional tectonics are imaged by geophysical studies. Numerical modeling studies are performed to understand the deformation linked with active geodynamic processes developing in the east part of the region. Here, we present our multi-dimensional (2D-3D) thermo-mechanical modeling results with varying temperatures and crustal configurations. We analyze modeling results together with the observations in terms of possible mantle flow components of the surface topography in Southeast Carpathians. In addition to residual topography calculations, non-isostatic compensation of the elevation is interpreted based on admittance functions between free-air gravity and topography. Our results indicate that mantle flow induced dynamic forces beneath the region modify the elevation with positive amplitudes over the Transylvania Basin (0.8-1 km) and the high SE Carpathian Mountains (~ 1 km) and subsidence of the Focsani Basin (0.5-1 km).</p>

Gravity analysis of the offset between crustal structure and topography in the Liupan Shan, northeast Tibetan Plateau

Dense gravity/GPS measurements collected around the southwest margin of the Ordos Block in 2014 and 2017 were used to obtain three gravity anomaly profiles across the Liupan Shan Mountains. The Liupan Shan is located in a Bouguer gravity anomaly (BGA) transitional zone; however, low BGAs to the west do not correspond to the topography of the Liupan Shan in that region, with a maximum offset of approximately 34 km. This offset is also found in inverted crustal density structures whereby the interface between the upper and lower crust is notably concave to the west of the Liupan Shan. Flexural analysis indicates that the effective elastic thickness is 5 km and the uplift is caused by oblique subduction of surface materials. According to this uplift mechanism, the offset suggests that Liupan Shan migrated eastward following partial isostatic compensation. Therefore, we suggest that Liupan Shan has experienced an uplift–compensation–uplift tectonic process.

Isostasy, LAB and some properties of asthenosphere in chosen regions of Poland

We use programs from the package LABWA2015 to determine position of Lithosphere-Asthenosphere Boundary (LAB) and some properties of lower lithosphere in chosen sites in Poland. Seismic, topographic, thermal and petrological data are used together with assumption about isostasy. Moreover we investigate the role of assumption about the steady state temperature distribution. We have found that sometimes this assumption for continental lithosphere can be unjustified but usually does not lead to significant errors. We have found also that in chosen sites, the thermal LAB is in the depth range 85-95 km. The average value of thermal conductivity of mantle is ~4 W m-1 K-1. Just below MOHO, a level of approximate isostatic compensation is found. More precise compensation is found in the asthenosphere at ~110 km but its position is sensitive to the density distribution.

Methods For Restoring and Describing Ancient Clinoform Surfaces

Clinoform geometries demarcate a relative change from shallow to deeper waters and are therefore routinely used as paleogeography and paleobathymetry indicators. Distinct segments of the clinoform surface are defined based on their discrete breaks in slope, and these points define parameters which are used to describe and compare clinoform geometries from different basins and ages. In most cases, the breaks in slope are readily interpreted, but placement of the various breaks in slope remains mostly subjective and not strictly uniform. For clinoform geometries with gentle transitions and less accentuated breaks in slope, e.g., mud-prone systems, distinct breaks in slope can vary over many kilometers depending on selection criteria and detection method. The older and more complex history of the investigated strata, the harder it becomes to correctly place the different breaks in slope. This study proposes a common reference frame using an upper regional datum, in which the geometries of ancient clinoform surfaces can be restored. Restoration is performed with standard decompaction techniques, but we compare isostatic compensation using Airy isostasy with different scenarios of flexural support for the sediment load. Regression is used to fit a surface function to georeferenced points along the restored clinoform surface, and derivatives of this surface function are used to objectively and accurately describe and measure parameters such as relief, length, and gradient, in addition to numerically defining the bottomset, foreset, and topset segments of the clinoform surface. This enables comparison with modern counterparts unaltered by postdepositional subsidence and compaction, and the proposed procedure can be applied to 2D profiles from seismic, outcrop, or closely spaced well logs. It can also help in the analyses of 3D surfaces and the trajectory of discrete breaks in slope on successive clinoform surfaces. Accurate reconstructions and objective parameterization directly affect interpretation of shelf–slope sediment partitioning and depositional environment.