Crustal structure and evolution of the southern Vøring Basin and Vøring Transform Margin, NE Atlantic

The dominantly passive volcanic Vøring and Møre Margins, NE Atlantic, are separated by the 200 km long Vøring Transform Margin (VTM). The southern Vøring Basin and the VTM have been studied by use of four regional Ocean Bottom Seismograph (OBS) profiles, combined by gravity modelling. The models demonstrate a complex pattern of magmatism along the transform margin. The distribution of magmatism seems to be related to the existence and trend of a lower crustal 8+ km/s body, interpreted as eclogitized rocks, present in the southern Vøring Basin. Early Tertiary breakup related magmatic ‘leakage’ across the Continent–Ocean-Transition (COT) appears to be facilitated where this layer is absent. These results support earlier workers who have concluded that the Jan Mayen Fracture Zone originated from a Caledonian zone of weakness. We propose that partly eclogitized rocks were uplifted into the lower crust close to this zone during the Caledonian orogeny and that this body acted as a barrier to magma emplacement during the Late Cretaceous–Early Eocene phase of rifting/breakup. The eclogitized terrain also appears to have caused northeastward channeling of the Late Cretaceous–Early Tertiary intrusions within the Vøring Basin. An up to 10 km thick pre-Cretaceous sedimentary basin in the southern Vøring Basin may be genetically related to the NS-trending Late Paleozoic and Mesozoic rift basins in North-East Greenland.     

Crustal structure of the Lofoten–Vesterålen continental margin, off Norway

By compiling wide-angle seismic velocity profiles along the 400-km-long Lofoten–Vesterålen continental margin off Norway, and integrating them with an extensive seismic reflection data set and crustal-scale two-dimensional gravity modelling, we outline the crustal margin structure. The structure is illustrated by across-margin regional transects and by contour maps of depth to Moho, thickness of the crystalline crust, and thickness of the 7+ km/s lower crustal body. The data reveal a normal thickness oceanic crust seaward of anomaly 23 and an increase in thickness towards the continent–ocean boundary associated with breakup magmatism. The southern boundary of the Lofoten–Vesterålen margin, the Bivrost Fracture Zone and its landward prolongation, appears as a major across-margin magmatic and structural crustal feature that governed the evolution of the margin. In particular, a steeply dipping and relatively narrow, 10–40-km-wide, Moho-gradient zone exists within a continent–ocean transition, which decreases in width northward along the Lofoten–Vesterålen margin. To the south, the zone continues along the Vøring margin, however it is offset 70–80 km to the northwest along the Bivrost Fracture Zone/Lineament. Here, the Moho-gradient zone corresponds to a distinct, 25-km-wide, zone of rapid landward increase in crustal thickness that defines the transition between the Lofoten platform and the Vøring Basin. The continental crust on the Lofoten–Vesterålen margin reaches a thickness of 26 km and appears to have experienced only moderate extension, contrasting with the greatly extended crust in the Vøring Basin farther south. There are also distinct differences between the Lofoten and Vesterålen margin segments as revealed by changes in structural style and crustal thickness as well as in the extent of elongate potential-field anomalies. These changes may be related to transfer zones. Gravity modelling shows that the prominent belt of shelf-edge gravity anomalies results from a shallow basement structural relief, while the elongate Lofoten Islands belt requires increased lower crustal densities along the entire area of crustal thinning beneath the islands. Furthermore, gravity modelling offers a robust diagnostic tool for the existence of the lower crustal body. From modelling results and previous studies on- and off-shore mid-Norway, we postulate that the development of a core complex in the middle to lower crust in the Lofoten Islands region, which has been exhumed along detachments during large-scale extension, brought high-grade, lower crustal rocks, possibly including accreted decompressional melts, to shallower levels.     

Crustal thickening in an extensional regime: application to the mid-Norwegian Vøring margin

The structure of the mid-Norwegian volcanic Vøring margin at the onset of the Maastrichtian–Paleocene extension phase reflects the cumulative effect of earlier consecutive rifting events. Lateral structural differences present on the margin at that time are a consequence of migration of the location of maximum extension in time between Norway and Greenland. The most important imprints (Moho depth, thermal structure) of these events on the lithosphere are incorporated in a numerical simulation of the final extension phase. We focus on a possible mechanism of formation of the Vøring Marginal High and address the relationship between spatial and temporal evolution of crustal thinning and thickening, uplift of the surface and strength of the lithosphere.It is found that the Vøring Basin formed the strongest part of the margin which explains why the Maastrichtian–Paleocene rift axis was not located here but instead jumped westward with respect to the earlier rift axes locations. The modeling study predicts that local crustal thickening during extension can be expected when large lateral thermal variations are present in the lithosphere at the onset of extension. Negative buoyancy induced by lateral temperature differences increases downwelling adjacent to the rifting zone; convergence of material at the particular part of the margin is mainly taken up by the lower crust. The model shows that during the final phase of extension, the crust in the Vøring Marginal High area was thickened and the surface uplifted. It is likely that this dynamic process and the effects of magmatic intrusions both acted in concert to form the Marginal High.     

Modelled palaeo-temperature on Vøring, offshore mid-Norway — The effect of the Lower Crustal Body

The Vøring area, offshore mid-Norway has a complex geological history, has experienced several extensional phases and was significantly influenced by the break-up of the North Atlantic. We have modelled a cross-section over the Vøring Basin aiming to 1) reconstruct the basin evolution in a realistic way, and 2) to investigate the heat flow and temperature history in the basin.For the modelling we used the following tectonic events: the opening of the basin during the Permo-Triassic, an event during the Middle to Late Cretaceous and an event in the Late Cretaceous/Paleocene. The theoretical effects of the lithospheric stretching are depending on the palaeo-water depths of the area. We present a prediction of the palaeo-water depth, and a sensitivity analysis of the influence of the palaeo-water depth on the estimated beta-factors in the area.A lower crustal high-velocity body found in the area is often interpreted as magmatic underplating related to the break-up of the Norwegian Sea. We show the temperature history calculated by models that were run with and without assumption of underplating by a magmatic body emplaced during the Early Tertiary. The observed vitrinite reflectance in the Vøring area is best explained by numerical calculations of the vitrinite reflectance without a magmatic underplating. Our conclusion is that the Lower Crustal Body is not related to magmatic underplating or to significant sill intrusions. The body may consist of older mafic rocks, or a mixture of old continental crust and mafic intrusions.     

Geological events during the early formation of a passive margin

The concept of plate tectonics implies that the normal sea floor spreading stage is preceded by a sequence of events associated with the break-up of continental crust. Thus, evidence of the early development of “non-failed” rifts is to be found at passive continental margins. Of special interest is the question of the extent of the continental crust and the structural and compositional changes associated with the change in crustal type. In addressing these topics, we have focused attention on the Norwegian margin between the Jan Mayen and Senja fracture zones (66°–70°N) in an attempt to understand its history of rifting and early sea floor spreading. p ]The southern part of this rifted margin is characterized by a wide shelf and the marginal Vøring Plateau interrupts a gentle slope at a level of about 1500 m. However, the margin becomes progressively narrower towards the north and a typical narrow shelf and steep slope emerge off the Lofo—tenVesterålen Islands (Fig. 1). In a reconstructed pre-opening configuration (Talwani and Eldholm, 1977) the narrowest part of the juxtaposed EastGreenland margin is found in the south and a wide shelf and slope corresponds to the Lofoten-Vesterålen margin.The most prominent structural element is a buried basement high underneath the Vøring Plateau. The high is bounded landward by the Vøring Plateau Escarpment, a major structural boundary which defines typical changes in the geophysical parameters. These are: (1) a sudden increase of depth to acoustic basement; (2) changes in the velocity-depth function; (3) a gravity gradient; and (4) a magnetic edge anomaly separating sea-floor spreading type anomalies from a quiet zone on the landward side (Talwani and Eldholm, 1972). These observations were interpreted in terms of a sharp ocea—ncontinent crustal transition along the escarpment with sea-floor spreading commencing between anomaly 24 and 25 time (56–58 m.y. B.P.). Alternatively, the concept of ancient oceanic crust landward of this escarpment and the possible existence of continental crust under the outer basement high have been argued and we refer to Eldholm et al. (1979) for a detailed discussion.     

A moderate melting model for the Vøring margin (Norway) based on structural observations and a thermo-kinematical modelling: Implication for the meaning of the lower crustal bodies

High P-wave velocities (7.1–7.8 km/s) lower crustal bodies (LCBs) imaged along volcanic margins are commonly interpreted as plume and breakup-related thick mafic underplating. This interpretation is partly challenged in this paper based on new seismic observations and modelling of the outer Vøring Basin (Norway). An exceptional strong amplitude reflection, the T Reflection, is particularly well defined below the North Gjallar Ridge (NGR) between 7and 8 s TWT. The T Reflection is located near the volcanic lava flows emplaced during the NE Atlantic breakup ( 55–54 Ma ago) and coincides with the top of the LCB, forming a mid-crustal dome. Based on structural and temporal relationships, we show that the dome clearly influences the structural development of the NGR and predates the continental breakup at least by 10–15 Ma. Using a thermo-kinematical model, we tried also to investigate and quantify the relationships between the extension, LCB and the magmatic production. Modelling suggests that significant Paleocene–Early Eocene magmatism can be produced without any temperature anomaly in the mantle if differential stretching occurs during the breakup initiation. The conclusion of 2D thermo-kinematical parametric analysis is that the magmatic model predicts, either little extension (β < 2) with no melting or high extension (β > 5) with significant melting along the outer Vøring Basin. We suggest that the continental part of the LCB could not necessarily be breakup-related and so magmatic, as has often been stated previously. It is concluded here that the continental part of the LCB observed beneath the outer Vøring Basin may be partly (or fully) attributed to inherited, high-pressure granulite/eclogite lower crustal rocks. The real amount of mafic material emplaced along the outer Vøring Basin could be 20–40% less than thought.     

Vp/Vs ratio along the Vøring Margin, NE Atlantic, derived from OBS data: implications on lithology and stress field

A total of 13 regional Ocean Bottom Seismograph (OBS) profiles with an accumulated length of 2207 km acquired on the Vøring Margin, NE Atlantic have been travel time modelled with regards to S-waves. The V_p/V_s ratios are found to decrease with depth through the Tertiary layers, which is attributed to increased compaction and consolidation of the rocks. The V_p/V_s ratio in the intra-Campanian to mid-Campanian layer (1.75–1.8) in the central Vøring Basin is significantly lower than for the layers above and beneath, suggesting higher sand/shale ratio. This layer was confirmed by drilling to represent a layer of sandstone. This mid-Cretaceous ‘anomaly’ is also present in the northern Vøring Basin, as well as on the southern Lofoten Margin further north. The V_p/V_s ratio in the extrusive rocks on the Vøring Plateau is estimated to be 1.85, conformable with mafic (basaltic) rocks. Landward of the continent/ocean transition (COT), the V_p/V_s ratio in the layer beneath the volcanics is estimated to be 1.67–1.75. These low values suggest that this layer represents sedimentary rocks, and that the sand/shale ratio might be relatively high here. The V_p/V_s ratio in the crystalline basement is estimated to be 1.67–1.75 in the basin and on the landward part of the Vøring Plateau, indicating the presence of granitic/granodioritic continental crust. In the lower crust, the V_p/V_s ratio in the basin decreases uniformly from southwest to northeast, from 1.85–1.9 to 1.68–1.73, suggesting a gradual change from mafic (gabbroic) to felsic (granodioritic) lower crust. Significant (3–5%) azimuthal S-wave anisotropy is observed for several sedimentary layers, as well as in the lower crust. All these observations can be explained by invoking the presence of liquid-filled microcracks aligned vertically along the direction of the present day maximum compressive stress (NW–SE).     

Ar–Ar ages of intrusions in East Greenland: Rift-to-drift transition over the Iceland hotspot

Sixteen ⁴⁰Ar–³⁹Ar ages are presented for alkaline intrusions to appraise prolonged post-breakup magmatism of the central East Greenland rifted margin, the chronology of rift-to-drift transition, and the asymmetry of magmatic activity in the Northeast Atlantic Igneous Province. The alkaline intrusions mainly crop out in tectonic and magmatic lineaments orthogonal to the rifted margin and occur up to 100 km inland. The area south of the Kangerlussuaq Fjord includes at least four tectonic lineaments and the intrusions are confined to three time windows at 56–54 Ma, 50–47 Ma and 37–35 Ma. In the Kangerlussuaq Fjord, which coincides with a major tectonic lineament possibly the failed arm of a triple junction, the alkaline plutons span from 56 to 40 Ma. To the north and within the continental flood basalt succession, alkaline intrusions of the north–south trending Wiedemann Fjord–Kronborg Gletscher lineament range from 52 to 36 Ma.

We show that post-breakup magmatism of the East Greenland rifted margin can be linked to reconfiguration of spreading ridges in the Northeast Atlantic. Northwards propagation of the proto-Kolbeinsey ridge rifted the Jan Mayen micro-continent away from central East Greenland and resulted in protracted rift-to-drift transition. The intrusions of the Wiedemann Fjord–Kronborg Gletscher lineament are interpreted as a failed continental rift system and the intrusions of the Kangerlussuaq Fjord as off-axis magmatism. The post-breakup intrusions south of Kangerlussuaq Fjord occur landward of the Greenland–Iceland Rise and are explained by mantle melting caused first by the crossing of the central East Greenland rifted margin over the axis of the Iceland mantle plume (50–47 Ma) and later by uplift associated with regional plate-tectonic reorganization (37–35 Ma). The Iceland mantle plume was instrumental in causing protracted rift-to-drift transition and post-breakup tholeiitic and alkaline magmatism on the East Greenland rifted margin, and asymmetry in the magmatic history of the conjugate margins of the central Northeast Atlantic.     

Evolution of oceanic crust on the Kolbeinsey Ridge, north of Iceland, over the past 22 Myr

The evolution of oceanic crust on the Kolbeinsey Ridge, north of Iceland, is discussed on the basis of a crustal transect obtained by seismic experiment from the Kolbeinsey Ridge to the Jan Mayen Basin. The crustal model indicates a relatively uniform structure; no significant lateral velocity variations are observed, especially in the lower crust. The uniform velocity structure suggests that the postulated extinct axis does not exist over the oceanic crust formed at the Kolbeinsey Ridge, but supports a model of continuous spreading along the ridge after oceanic spreading started west of the Jan Mayen Basin. The oceanic crust formed at Kolbeinsey Ridge is 1–2.5 km thicker than normal oceanic crust due to hotter-than-normal mantle from the Iceland Mantle Plume. The observed generally uniform thickness throughout the transect might also indicate that the temperatures of the astheno-spheric mantle ascending along the Kolbeinsey Ridge have not changed significantly since the age of magnetic anomaly 6B.     

The structure of the lower crust at the Argentine continental margin, South Atlantic at 44°S

It is well established that the Argentine passive margin is of the rifted volcanic margin type. This classification is based primarily on the presence of a buried volcanic wedge beneath the continental slope, manifested by seismic data as a seaward dipping reflector sequence (SDRS). Here, we investigate the deep structure of the Argentine volcanic margin at 44°S over 200 km from the shelf to the deep oceanic Argentine Basin. We use wide-angle reflection/refraction seismic data to perform a joint travel time inversion for refracted and reflected travel times. The resulting P-wave velocity-depth model confirms the typical volcanic margin structure. An underplated body is resolved as distinctive high seismic velocity (v_p up to 7.5 km/s) feature in the lower crust in the prolongation of a seaward dipping reflector sequence. A remarkable result is that a second, isolated body of high seismic velocity (v_p up to 7.3 km/s) exists landward of the first high-velocity feature. The centres of both bodies are 60 km apart. The high-velocity lower-crustal bodies likely were emplaced during transient magmatic–volcanic events accompanying the late rifting and initial drifting stages. The lateral variability of the lower crust may be an expression of a multiple rifting process in the sense that the South Atlantic rift evolved by instantaneous breakup of longer continental margin segments. These segments are confined by transfer zones that acted as rift propagation barriers. A lower-crustal reflector was detected at 3 to 5 km above the modern Moho and probably represents the lower boundary of stretched continental crust. With this finding we suggest that the continent–ocean boundary is situated 70 km more seaward than in previous interpretations.     

Mohorovičić discontinuity beneath Fennoscandia and adjacent parts of the Norwegian Sea and the North Sea

A preliminary contour map showing the Mohorovičić discontinuity (Moho) beneath Fennoscandia, adjacent parts of the Norwegian Sea and the North Sea has been compiled on the basis of published information from deep seismic soundings.The Moho contour map shows a 10 km thick crust beneath the investigated basin-region of the Norwegian Sea. It seems that the Vøring Plateau has at least in part a continental structure even if the Moho-depth is only 15 km. A shallow Moho (28–30 km) all along the Norwegian coast is a well established feature. A good correlation between the surface elevation of the mountain range running through Norway and parts of Sweden and the depth of the Moho is also well established. The Gulf of Bothnia is a region of a great Mono-depression.     

Crustal structure of the continental margin of Korea in the East Sea (Japan Sea) from deep seismic sounding data: evidence for rifting affected by the hotter than normal mantle

Despite the various opening models of the southwestern part of the East Sea (Japan Sea) between the Korean Peninsula and the Japan Arc, the continental margin of the Korean Peninsula remains unknown in crustal structure. As a result, continental rifting and subsequent seafloor spreading processes to explain the opening of the East Sea have not been adequately addressed. We investigated crustal and sedimentary velocity structures across the Korean margin into the adjacent Ulleung Basin from multichannel seismic (MCS) reflection and ocean bottom seismometer (OBS) data. The Ulleung Basin shows crustal velocity structure typical of oceanic although its crustal thickness of about 10 km is greater than normal. The continental margin documents rapid transition from continental to oceanic crust, exhibiting a remarkable decrease in crustal thickness accompanied by shallowing of Moho over a distance of about 50 km. The crustal model of the margin is characterized by a high-velocity (up to 7.4 km/s) lower crustal (HVLC) layer that is thicker than 10 km under the slope base and pinches out seawards. The HVLC layer is interpreted as magmatic underplating emplaced during continental rifting in response to high upper mantle temperature. The acoustic basement of the slope base shows an igneous stratigraphy developed by massive volcanic eruption. These features suggest that the evolution of the Korean margin can be explained by the processes occurring at volcanic rifted margins. Global earthquake tomography supports our interpretation by defining the abnormally hot upper mantle across the Korean margin and in the Ulleung Basin.     

Local to regional scale structural controls on mineralisation and the importance of a major lineament in the eastern Mount Isa Inlier, Australia: Review and analysis with autocorrelation and weights of evidence

Although major crustal lineaments may play an important role in mineralisation, the relationship between lineaments and mineral deposits can be quite cryptic, and structural controls may vary as a function of scale along lineaments. Major lineaments alone may be of limited use for detailed target generation. The Cloncurry Lineament in the Eastern part of the Mount Isa Inlier is a crustal scale structure defined by potential field-derived ‘worms’. Weights-of-evidence quantifies the association between mineral occurrences and this lineament. Autocorrelation is used to recognise structural controls on mineralisation at different scales, by progressively limiting the lengths of the vectors between mineral occurrence points in the autocorrelation plot. The weights-of-evidence analysis shows that Au, Au–Cu, Cu–Au and Cu deposits have a positive spatial correlation to the Cloncurry Lineament, which suggests it that acted as a primary crustal scale control on the localisation of Cu and Au through focussing mineralisation systems on a broad scale. However, autocorrelation defines a variety of local structural controls, which can be interpreted as shear zones, variably oriented fault sets, en echelon fault arrays, and potentially the orientation of bedding and/or iron formations which localise fluid flow and mineral deposition at finer scales. The results suggest that major lineaments defined by geophysical contrasts can be used in conjunction with techniques of spatial analysis for targeting structurally controlled mineralisation in areas under thin cover adjacent to mineralised terrains such as the Mt Isa Inlier.     

Seismicity of the Norwegian Sea: The Jan Mayen Fracture Zone

The seismicity of the Jan Mayen Island region is re-examined, and a new and improved seismicity map is presented. These data, together with four previously published and one new and very precise fault-plane solution, have been interpreted in the light of other geophysical information bearing on the area. The data support the hypothesis that the transform portion of the Jan Mayen Fracture Zone consists of a system of en-échelon faults. A separate seismicity area immediately northeast of the Jan Mayen Island itself is quite pronounced on the new seismicity map. Moreover, from the available earthquake data the suggested Iceland—Jan Mayen ridge offset is located at 70.75°N.     

Crustal structure of the East African Rift zone

A review of seismological data on the crustal structure of the East African Rift zone is presented. The only refraction line is that along the Gregory Rift, which indicates a 7.5 km/sec refractor which is presumed to be the Moho. The bulk of data is provided by surface-wave dispersion studies. Some preliminary measurements of crustal and sub-Moho velocities using the University of Durham array at Kaptagat in Kenya are included.

There is now a growing body of evidence that the crust is generally of shield type over the whole rift zone. The exception is along the axis of the Gregory Rift, where a low-velocity Moho and some crustal modification is apparent. This is presumably the result of magma intrusions and suggests some crustal separation along this section of the rift. Sub-Moho velocities are probably normal outside the rifts themselves, though anomalously low upper-mantle velocities are to be associated with rifting. There is firm evidence for thinning of the lithosphere along the eastern branch of the rift. A cross-section of the Gregory Rift which is consistent with the current data is presented.     

Effect of local variations of vertical and horizontal stresses on the Cenozoic structuring of the mid-Norwegian shelf

The mid-Norwegian margin has a complex history and has experienced several phases of changing horizontal and vertical stresses on regional and local scale during the Cenozoic time. In addition to regional stresses related to the opening of the North Atlantic (i.e. ridge push), local variations in stress history may be important for development, distribution and reactivation of structures in the Vøring area in Cenozoic time. Presence and stability of flexural hinge zones between areas of relative uplift and subsidence have played an important role for focusing shallow horizontal stresses within the basins. Emplacement of lower crustal bodies during break-up will, whatever the nature of these bodies, have substantial isostatic effects, and modelling show that this could cause many hundred meters of temporal uplift above the lower crustal bodies, locally exceeding 1300 m of surface uplift. Effects of intra plate stress (IPS) are modelled along three 2D transects across the Vøring Basin. Modelling shows that IPS may have given substantial vertical motions in certain areas of the mid-Norwegian shelf, both with extensional IPS at the time of break-up, and later with compressive IPS during Tertiary time. The modelling assumes a strongly reduced effective elastic thickness (EET) due to lithospheric heating at break-up and later increasing EET as the lithosphere cooled towards present time. Our modelling takes into account the tectonic and isostatic effects of loading faulting and lithospheric thinning throughout the geological history, including several phases of extension prior to the Cenozoic compression. This approach emphasizes the importance of the deformation history of the lithosphere compared to other studies that only take into account the effects of Cenozoic processes of compression and loading on the sedimentary units. We do not state that isostatic uplift or intra plate stress are the most important causes for Cenozoic uplift and compressional deformation in this area, but point to the fact that these factors locally may have played an important role in focusing deformation caused by an interplay of different mechanisms.     

菲律宾海四国海盆地壳结构重力反演及其形成演化过程分析

四国海盆是位于菲律宾海板块内由岛弧张裂形成的弧后盆地,其深部地壳结构对认识伊豆小笠原岛弧的裂解和弧后盆地的扩张过程有重要的意义.在反射多道地震剖面和深部海底地震（OBS）探测剖面的约束下,结合磁异常条带数据,利用两条横穿四国海盆的重力测线数据对海盆的地壳物性结构反演,对比重力反演剖面与深部探测剖面地壳厚度和密度特征,得到更加精细的四国海盆地壳结构.研究结果显示,四国海盆洋壳厚度自西向东逐渐增厚,在残留扩张脊处莫霍面深度迅速增加.根据地壳密度和厚度将四国海盆分为:洋壳减薄区、洋壳增厚区、后扩张洋壳增生区,分别对应初始慢速张裂、单翼快速扩张、对称慢速扩张3期扩张活动.南北测线不同构造分区得到的扩张速率与由磁异常条带得到扩张速率相同;洋壳减薄区下地壳均有高密度体,与OBS剖面中下地壳高速体相对应,可能是由于洋壳慢速扩张过程中强烈拆离作用,地幔蛇纹石化导致.      

Permo-Triassic tectonic evolution and metallogenesis of the Rockhampton—Maryborough area, Queensland — A photogeological investigation

Structural analysis of remotely sensed data provides a method of assessing the tectonic significance of regional metallogenic lineaments in the New England Orogen of southeastern Queensland. Photogeological analysis of Landsat imagery and small-scale aerial photography reveals a pattern of WNW—NNW-oriented structures, which were apparently generated in response to Mesozoic crustal extension and reactivated during Early Tertiary block faulting. These structures tend to overprint arcuate late Palaeozoic to early Mesozoic trends and batholith belts, and exert a control over Middle to Late Triassic rifting and epizonal plutonism. The distribution of epigenetic base and precious metal deposits in the Rockhampton—Maryborough area is locally but not regionally related to identifiable structural lineaments.     

Tectono-stratigraphic modelling of the North Sicily continental margin (southern Tyrrhenian Sea)

A two-dimensional numerical modelling that simulate the kinematic and thermal response of the lithosphere to thinning was used for the quantitative reconstruction of the late Neogene to Recent times tectonic and stratigraphic evolution of the North Sicily continental margin (southern Tyrrhenian Sea). The numerical study of the evolution of the North Sicily margin builds on the crustal image and kinematic interpretation of the margin obtained by Pepe et al. [Tectonics 19 (2000) 241] on the basis of seismic data and gravity modelling. Tectonic modeling indicate that different segments of the margin were undergoing different vertical movements, which are mainly expression of the rifting and thinning of the lithosphere occurred during tectonic evolution of the southern Tyrrhenian Sea. A prediction of the pre-rift basement topography and the Moho along the margin converges to a value of 6.5 km for the depth of necking and a temperature-dependent EET (500° isotherm). The model fails to reproduce the morphology of the Solunto High confirming its non-extensional origin. A polyphase evolution is required to reproduce the observed syn- and post-rift stratigraphy. During the first rifting stage (between 9 and 5 Ma), crustal thinning factors reach maximum values of 1.27 in the Cefalù basin. A similar value is predicted for the subcrustal thinning around 60 km NNE of the profile margin. Crustal thinning factors increase during the second rifting stage (from 4 to 2 Ma) and reach values of 2 and up to 3.5 in the Cefalù basin and in the continent–oceanic transition zone, respectively. Similarly, subcrustal lithospheric thinning factors reach values up to 2.5 in the distal sector of the margin. An uplift of more than 100 m is predicted for the North Sicily shelf and surrounding onshore areas during the post-rift stage. The evolution of thermal structure with time is very sensitive to the partial thinning factors describing the evolution of the thinning itself during time. The lithosphere preserved part of its strength during extension. The effective elastic thickness (EET) along the margin through time is 24 km at the onset of rifting and reaches values less to 8 km during the second rifting stage in the northeastern end of the margin.