Crustal Thickness Map Research Articles

Formation and deformation of the Pannonian Basin: constraints from observational data

Abstract The past decade has witnessed spectacular progress in the collection of observational data and their interpretation in the Pannonian Basin and the surrounding Alpine, Carpathian and Dinaric mountain belts. A major driving force behind this progress was the PANCARDI project of the EUROPROBE programme. The paper reviews tectonic processes, structural styles, stratigraphic records and geochemical data for volcanic rocks. Structural and seismic sections of different scales, seismic tomography and magnetotelluric, gravity and geothermal data are also used to determine the deformational styles, and to compile new crustal and lithospheric thickness maps of the Pannonian Basin and the surrounding fold-and-thrust belts. The Pannonian Basin is superimposed on former Alpine terranes. Its formation is a result of extensional collapse of the overthickened Alpine orogenic wedge during orogen-parallel extrusion towards a 'free boundary' offered by the roll-back of the subducting Carpathian slab. As a conclusion, continental collision and back-arc basin evolution is discussed as a single, complex dynamic process, with minimization of the potential and deformational energy as the driving principle.

Crustal flow beneath Iceland

Theoretical and experimental studies indicate that when oceanic crust is hotter than about 800°C, variations in crustal thickness drive lower crustal flow. We investigate the nature of crustal flow beneath Iceland, where zero‐age crust varies in thickness from under 20 km to over 40 km over a distance of 100 km and temperatures exceed 1000°C below depths of a few kilometers. We model the regional characteristics of crustal flow using the two‐dimensional channel flow equation with depth‐dependent viscosity. The model predicts the observed decay in crustal thickness contrasts between zero‐age and off‐axis crust on shore Iceland and development of a sharp Moho step as the plate moves away from the rift axis. These features become locked in place as plate cooling increases the viscosity. Observations are best matched by model predictions when the solidus viscosity is of order 1018 Pa s, in agreement with viscosity estimates from deformation rates associated with deglaciation and plate boundary processes. Crustal flow acts to erase the crustal thickness memory of ridge‐plume interaction, so that maps of Icelandic crustal thickness can be used neither to derive a detailed plume flux history nor to test in detail whether the Iceland Plume center is fixed relative to other hot spots. Crustal flow beneath Iceland is an unusually clear example of the kind of flow postulated to explain continental phenomena including exhumation of metamorphic core complexes and associated development of low‐angle detachments within extensional terrains and sharp‐edged plateau topography in major collisional belts.

An efficient, probabilistic neural network approach to solving inverse problems: Inverting surface wave velocities for Eurasian crustal thickness

Nonlinear inverse problems usually have no analytical solution and may be solved by Monte Carlo methods that create a set of samples, representative of the a posteriori distribution. We show how neural networks can be trained on these samples to give a continuous approximation to the inverse relation in a compact and computationally efficient form. We examine the strengths and weaknesses of this approach and use it to determine the full a posteriori distribution of crustal thickness from surface wave velocities. The solution to this inverse problem shows significant asymmetry and large uncertainties due to trade‐off with shear velocity structure around the Moho. We produce maps of maximum likelihood crustal thickness across Eurasia which are in agreement with current knowledge about the crust; thus we provide an independent confirmation of these models. In this application, characterized by repeated inversion of similar data, the neural network algorithm proves to be very efficient.

The role of deep structures in the distribution of some major ore deposits in Iran, ne of the Zagros thrust zone

Examination of isostatic, crustal thickness, residual gravity, gravimetric geoid, complete Bouguer and earthquake intensity maps of Iran, NE of the Zagros thrust zone suggests the presence of major deep boundaries in this region. To test the model of block structure in Iran, the meridional and latitudinal boundaries constructing a rectangular grid were plotted. A comparison with the mineral distribution maps of Iran indicates that the deep structural boundaries played a role in the genesis and distribution of endogenous ore deposits, NE of the Zagros thrust zone. The endogenous deposits and ore-rich regions are concentrated mainly along or close to deep boundaries (lineaments) or at their intersections. The porphyry copper deposits and other endogenous deposits of the Kerman region are also associated with an upper mantle updoming and a felsic mass manifest in the pattern of residual gravity, crustal thickness, complete Bouguer, and earthquake intensity maps of Iran. One of the most important features is the presence of a very significant meridional lineament band occurring approximately at 56°E longitude. The most important ore deposits are located along or close to this meridional band which is associated with more than eight east–west trending structural boundaries in the upper mantle. The two anomalous areas manifesting the mantle updoming and the felsic mass are also located along this band. Several target areas have been selected for mineral exploration.

Crustal thickness map of the western United States by partitioned waveform inversion

The partitioned waveform inversion method [Nolet, 1990] was used to obtain a new crustal thickness map of the western United States. We fitted 200 seismograms containing higher‐ and fundamental mode Rayleigh waves for events and stations in the western United States, which provided average crustal thickness constraints between each source‐receiver pair. All these constraints were combined in a linear inversion to obtain a crustal thickness map. Local crustal thickness estimates from published seismic reflection/refraction work were used as additional constraints to overcome deficiencies of path coverage in some areas. The resulting crustal thickness map shows thicker (∼45 km) crust beneath the Cascades, thick (∼40 km) crust beneath the Sierra Nevada, thin (∼30 km) crust beneath the Basin and Range, and thick (greater than 45 km) crust beneath the Colorado Plateau. The boundary between the Colorado Plateau and the southern leg of the Basin and Range is a prominent feature of the model. Thick crust is seen at the bend of the San Andreas fault near Los Angeles. Our model is broadly consistent with the crustal thickness map of Mooney and Weaver [1989]. A quantitative comparison with the crustal thickness model 3SMAC [Nataf and Ricard, 1996] was performed by inverting for a model that deviates least from it. Results show that we are able to retain most of the features of that model while maintaining a good fit to the surface wave and point constraint data.

Finite extension across the South Vietnam basins from 3D gravimetric modelling: relation to South China Sea kinematics

We derive a crustal thickness map over the south Vietnam basins from a 3D inversion of gravity data that takes into account the sedimentary infill and its density variation with depth. This map is used to estimate the total amount of stretching across these basins, that varies from 190 km to the east (at 111°E) to 30 km to the west (at 107°E). Comparison with South China Sea kinematics (amount and timing of oceanic opening) implies large decoupling with respect to the Indochina block and supports the hypothesis of formation of the South China Sea in upper Oligocene to lower Miocene by southward subduction of the proto-South China Sea.

The lunar crust: Global structure and signature of major basins

New lunar gravity and topography data from the Clementine Mission provide a global Bouguer anomaly map corrected for the gravitational attraction of mare fill in mascon basins. Most of the gravity signal remaining after corrections for the attraction of topography and mare fill can be attributed to variations in depth to the lunar Moho and therefore crustal thickness. The large range of global crustal thickness (∼20–120 km) is indicative of major spatial variations in melting of the lunar exterior and/or significant impact‐related redistribution. The 61‐km average crustal thickness, constrained by a depth‐to‐Moho measured during the Apollo 12 and 14 missions, is preferentially distributed toward the farside, accounting for much of the offset in center‐of‐figure from the center‐of‐mass. While the average farside thickness is 12 km greater than the nearside, the distribution is nonuniform, with dramatic thinning beneath the farside, South Pole‐Aitken basin. With the global crustal thickness map as a constraint, regional inversions of gravity and topography resolve the crustal structure of major mascon basins to half wavelengths of 150 km. In order to yield crustal thickness maps with the maximum horizontal resolution permitted by the data, the downward continuation of the Bouguer gravity is stabilized by a three‐dimensional, minimum‐slope and curvature algorithm. Both mare and non‐mare basins are characterized by a central upwarped moho that is surrounded by rings of thickened crust lying mainly within the basin rims. The inferred relief at this density interface suggests a deep structural component to the surficial features of multiring lunar impact basins. For large (>300 km diameter) basins, moho relief appears uncorrelated with diameter, but is negatively correlated with basin age. In several cases, it appears that the multiring structures were out of isostatic equilibrium prior to mare emplacement, suggesting that the lithosphere was strong enough to maintain their state of stress to the present.

The Shape and Internal Structure of the Moon from the Clementine Mission

Global topographic and gravitational field models derived from data collected by the Clementine spacecraft reveal a new picture of the shape and internal structure of the moon. The moon exhibits a 16-kilometer range of elevation, with the greatest topographic excursions occurring on the far side. Lunar highlands are in a state of near-isostatic compensation, whereas impact basins display a wide range of compensation states that do not correlate simply with basin size or age. A global crustal thickness map reveals crustal thinning under all resolvable lunar basins. The results indicate that the structure and thermal history of the moon are more complex than was previously believed.

The Moho depth distribution in Fennoscandia and the regional tectonic evolution from Archean to Permian times

From a variety of seismic refraction and reflection profiling results, a new detailed Moho depth map for the Fennoscandian part of the Baltic Shield has been compiled. A complementary crystalline crustal thickness map was compiled for Denmark, the Skagerrak Sea, offshore western Norway and adjacent areas where sedimentary strata reach maximum thicknesses of 10 km and even more. These maps are discussed in relation to tectonic evolution and crustal ages of Fennoscandia, seismic velocities, Bouguer gravity and seismicity. Areas deviating from the proposed age-thickness relationships are the Kola Peninsula (Archean) and the Oslo Rift (Permian). The average crustal thickness in the former area is around 44 km, while the latter area is associated with a Moho elevation of 3–5 km. Maximum crustal thicknesses in excess of 50 km have been found in southwestern and Central Finland and correlate with a crust of Proterozoic age. The Caledonides of western Norway do not exhibit, as elsewhere, a crustal root. Sub-Moho P n and S n seismic velocities are relatively small in that area and Bouguer gravity values are strongly negative. Intracrustal P-velocity variations do not seem to reflect geological age or specific tectonic features. Although the seismicity is moderate in Fennoscandia, most of the earthquake activity and, in particular, the largest earthquakes are confined to areas where the crustal thinning is most pronounced.

Geophysical investigation of deep crustal structure of the Siberian Platform region

A sparse network of regional profiles developed in Siberia using deep seismic sounding methods (DSS) does not allow a valid three dimensional model of Siberian deep crustal structure to be constructed. To create such a model, interpretation methods of available geophysical and geological data are suggested. These are based on modelling of geological objects using potential fields. Information on DSS key profiles is considered to be key data.The results of such interpretation are shown in the crustal thickness map and in that of crystalline basement, as well as on a deep geological-geophysical profile across the West Siberian Plate and Siberian Platform. The crustal thickness in Siberia varies from 30 to 55 km. It is on average 4?6 km thicker on the Siberian Platform than the West Siberian Plate. The least crustal thickness of 30 to 33 km appears to be beneath regions with maximum thickness of Mesozoic deposits. Modern and Early Mesozoic rift zones are characterized by decreased crustal thickness of 36?39 km. Ancient shields of the Siberian Platform and adjacent folded mountain units are up to 45?55 km thick.The geometry of the top of crystalline basement in the young West Siberian Plate is sharply different from that of the ancient Siberian Platform. Within the West Siberian Plate structural features with 5 to 11 km of relief dominate, while on the Siberian Platform the structural relief varies from 2?3 to 14 km.The peculiarities of deep crustal structure have greatly influenced the formation and distribution of sedimentary units. The constructed section shows the depths and thicknesses of the Riphean, Vendian?Silurian, Devonian?Middle Triassic and Mesozoic?Cenozoic megacomplexes, most promising for oil and gas in Siberia. Data obtained on the deep structure of Siberian Platform regions are thus of great practical importance. They can be used to predict areas of petroleum potential and determine optimal trends of further oil and gas exploration by geophysical methods.

Crustal structure in the Western Somali Basin

As part of integrated marine geophysical studies in the Western Somali Basin, we performed 118 sonobuoy experiments to define better the crustal structure of the margins and basin created by the separation of Madagascar and Africa. After using T2/X2, conventional slope-intercept methods, and slant-stacked t-p techniques to analyse the data, we combined our solutions with all previous velocity information for the area. Velocity functions were derived for the sediment coiumn, and we detected a high-velocity (4.58 ± 0.29 km s–1) sediment layer overlying acoustic basement. We confirmed that the crust is indeed seismically oceanic, and that it may be considered either in terms of a layered model – layers 2B (5.42 ± 0.19 km s–1), 2C (6.23 ± 0.22 km s–1), 3 (7.03 ± 0.25 km s–1), and mantle (7.85 ± 0.32 km s–1) were identified – or a more complex gradient model in which layer 2 is marked by a steeper velocity gradient than underlying layer 3. Integrated igneous crustal thicknesses (1.62 ± 0.22 s, 5.22 ± 0.64 km) are significantly less than what is considered normal. We present a revised seismic transect across the East African margin, as well as total sediment thickness, depth to basement and crustal thickness maps.

Physical properties and state of the crust and upper mantle of the Eastern Mediterranean Sea deduced from geophysical data

By evaluating the geophysical information available for the Eastern Mediterranean Sea (EMS), a crustal thickness map and some conclusions as to the nature of the crustal type were drawn. It was found that the Ionian Abyssal Plain is floored by an oceanic type of crust which is 14 km thick and covered by sediments of the order of 8 km thick. The Herodotus Abyssal Plain in the Levantine Sea is floored by a crust 22 km thick and covered by sediments ranging between 10 and 15 km thick. The Eastern Mediterranean Rise, extending more or less continuously from the Apulian Plateau to Rhodes and the south of the Antalya Basin, is of a thickness ranging between 28 and 34 km. The thickness of its sedimentary cover is of the order of 10 km. The borders of the Eastern Mediterranean Sea are built up of thick continental crust. For example the Hellenides of Western Greece range between 40 and 44 km thick and Western Turkey has a similar crustal thickness. The crust of Sicily also exceeds 40 km whereas the southern and eastern borders of the EMS are approximately 30–32 km thick. Tectonically the northern border of the EMS is built up of active continental margins defined by the Calabrian, Hellenic and Cyprus—Antalya Arcs, whereas the southern border represents a passive continental margin controlled mainly by subsidence and stretching. To the east, the Dead Sea Megashear limits the area. The deep basins of the Eastern Mediterranean Sea are tectonically controlled by subsidence and partial stretching. The Mediterranean Ridge or Rise is still an area of dispute, regarded by some authors as a subsiding continental fragment and separated from the Hellenic Arc by the Hellenic Trench, whereas others consider it to be a zone of crustal shortening, thickened mainly by thrusting. Both the western and southwestern borders of the Mediterranean Rise and the Hellenides are controlled by compressional processes similar to those deforming the Calabrian Arc. The Stravo and Pliny trenches to the east-southeast border of the Hellenic Arc are partly affected by strike-slip movements. Various plate-tectonic models of the area have been proposed by several authors, though the complexity of the deformation hardly permits reliable reconstructions. The lack of reliable paleomagnetic information and the absence of linear magnetic anomalies do not favour the application of plate tectonic concepts. Vertical processes due to the uprising of lithothermal systems below the Hellenic and Calabrian arcs may also be regarded as the driving force controlling the present-day deformation.

New global map of seismic crust

An up‐to‐date global compilation of the thickness of the earth's crust and a compilation of upper mantle seismic velocity measurements has been published recently by Soller, Ray, and Brown (Tectonics, 1, 125–149, 1982). The data base indicate 2508 crustal thicknesses and 1806 Pn (uppermost mantle) velocity values. The data are from seismic refraction and surface wave measurements. The compilation represents a significant expansion of available crustal thickness maps, with more uniform coverage. Thus, in agreement with current geophysical interpretations, numerous tectonic features are recognizable as local variations in crustal thickness. For example, mountain chains in isostatic equilibrium, such as the Alps, the Andes, and the Pamir and Tien Shan ranges, display a thickening of the crust and a deeper Mohorovicic Boundary. Beneath the Tibetan plateau, the crust is 80 km thick—the maximum observed. The area of Japan and the Kurile Islands are represented on the map as a thickened crustal ridge separating the thin oceanic crust of the Pacific plate from the thicker crust underlying the marginal basins of the Seas of Japan and Okhotsk. Contours define the Pacific‐Eurasion plate boundary, located southeast of the Kurile island arc. Very thin crust, often less than 4 km, occasionally outlines oceanic spreading centers, such as the Midocean Ridge in the North Atlantic.

Mars topography harmonics and geophysical implications

An improved model of Martian global topography has been obtained by fitting a sixteenth‐degree harmonic series to occultation, radar, spectral, and photogrammetric measurements. The existing observations have been supplemented in areas without data by empirical elevation estimates based on photographic data. The mean radius is 3389.92 ± 0.04 km. The corresponding mean density is 3.9331 ± 0.0018 g cm−3. The center of figure is displaced from the center of mass by 2.50 ± 0.07 km toward 62° ± 3°S, 272° ± 3°W. The geometric flattening (fg = (6.12 ± 0.04) × 10−3) is too great and the dynamic flattening (fd = (5.22 ± 0.03) × 10−3) is too small for Mars to be homogeneous and hydrostatic. It is confirmed that the low‐degree gravity harmonics are produced primarily by surface height variations and only secondarily by lateral density variations. Maps of the data distribution, global topography, and Bouguer gravity anomaly are presented. These are interpreted in terms of a crustal thickness map which is consistent with gravity, topography, and recent preliminary Viking seismic results. From plausible density contrasts and an assumed zero crustal thickness at Hellas, the inferred minimum mean crustal thickness is 28 ± 4 km.

Crustal structure under Lasa from seismic refraction measurements

The Large-Aperture Seismic Array (Lasa) and an expanded array of temporary stations recorded 15 large explosions fired by the U.S. Geological Survey in September 1968. A three-dimensional arrangement permitted an attempt to strip the crust into layers by means of time-term calculations. The Pg first arrivals, having a velocity of 6.1 km/sec, were used to map time terms representing the sedimentary rock layering. Record sections were used to study later phases and their relation to the first arrivals. Multiply refracted arrivals following Pg indicate a gradual increase of velocity with depth in the sedimentary rocks and in the granitic basement. The arrivals P* and PiP have apparent velocities of 6.6–7.0 km/sec and provide information on the velocities of crustal rocks to a depth of 16 km. The pattern of time terms changes with increasing depth of the refracting horizons. We contend that this relationship implies a compensation process in the upper crust. Velocities in the lower crust were determined by means of P*, PmP, and P¯ later arrivals observed on the record sections, and an average function of velocity versus depth was derived for the entire crust. The function was used to map crustal thickness from PmP arrivals, reflected from the Mohorovicic (M) discontinuity. Numerous shot-receiver combinations were near critical distance ranges for these reflections. Variations in character and occurrence indicate change in crustal structure. Weak Pn arrivals, refracted along the top of the mantle, were also used to map crustal thickness, and the contours are similar to the higher-quality results from PmP events. A synclinal trough trends slightly north of due east under the center of Lasa. The depth to M varies from about 55 km in the deepest part to about 40 km to the south. A crustal thickness map was compiled from these results and a review of previous crustal studies.

Geologic History of Nicaraguan Rise: ABSTRACT

A study of refraction-seismic velocity data beneath the Caribbean Sea and nearby areas of the Atlantic Ocean reveals that the thickest crust is in the Antillean island belts and the Nicaraguan Rise. In the Nicaraguan Rise, the maximum thickness is about 22 km and is south of the present topographic crest. Isopach maps of total crustal thickness indicate that the Caribbean crust is intermediate between average oceanic and continental crust. The oldest dated rocks in the Caribbean are Jurassic, a fact which is consistent with the idea that the region had its origin in early Mesozoic time as a result of rifting and subsequent drift between the American and Afro-European continental blocks. Throughout Jurassic and Early Cretaceous time, the Nicaraguan Rise was a mobile belt associated with vast submarine lava flows and mafic intrusives. Volcanism gradually decreased during the Late Cretaceous. Terrigenous clastic and carbonate strata alternated with tuff, agglomerate, and mixed-origin volcanic-sedimentary units. The first phase of the Laramide orogeny occurred at the end of the Cretaceous, and it may have been at this time that a rift along the southern flank of the rise separated it from the Beata Ridge which now is south of Hispanola. Normal marine sedimentation--clastic and chemical--prevailed during the Tertiary. Islands emerged and sank as movement occurred between crustal blocks. Tectonism beginning in middle Miocene time markedly altered the topography and depositional pattern of the rise. The Cayman Trough rift formed as a result of left-lateral displacement of at least 250 mi. The Nicaraguan Rise was tilted southward, with the result that the topographic crest was shifted 50-100 mi north. Recent geophysical work related to petroleum prospecting has aided in interpreting the structure and stratigraphy of the rise. Cross sections and a map of depth to magnetic basement are presented. End_of_Article - Last_Page 2031------------