Evidence for a synchronous circum-Iberian subsidence event and its relation to the African-Iberian plate convergence in the Late Cretaceous

Subsidence curves from Mesozoic sedimentary basins at the southern Iberian margin (Betic Cordilleras) display pronounced changes in subsidence rates around 85 Ma (chron 34, Late Cretaceous, Santonian to earliest Campanian). The subsidence events correlate with changes in the bulk and clay mineral composition in these basins, as well as with an Eoalpine high-pressure metamorphic event in the western Mediterranean region. The synchroneity with subsidence events observed in basins around the Iberian microplate suggests a causal relationship with the regional plate tectonic setting. We propose that the circum-Iberian subsidence event was largely controlled by the convergence and incipient collision of the Iberian microplate with Africa.     

Mesozoic palaeomagnetism of the transdanubian central mountains and its tectonic implications

The Mesozoic apparent polar wandering (APW) of the Transdanubian Central Mountains, determined from thermally isolated natural remanences at 13 localities, shows a remarkable similarity to the Mesozoic APW of Africa in that they both exhibit the same loop-like movement. Moreover, the difference between the two APW's can practically be eliminated by a 35° clockwise rotation of the palaeodeclinations. It is concluded, therefore, that the region of the Transdanubian Central Mountains was part of the African (-Adriatic) plate up to some time in the Cenozoic when it moved to its present position, resulting in a 35° anticlockwise rotation relative to Africa.     

Mantle diapirism and the formation of newly formed basins and surrounding centrifugally vergence orogens in the Mediterranean and Caribbean regions

Mantle diapirism contributes considerably to the formation of newly formed basins and surrounding centrifugally vergence fold-thrust belts in the Mediterranean and Caribbean regions. Mantle diapirism results from density inversion in the geosphere of astenosphere+lithosphere geosystem. Such inversion has become a driving force in the background of increasing heat flow caused by the heat-resistant convergence of Africa and Eurasia (in the case of the Mediterranean region) and North and South Americas (Caribbean region) in the Cenozoic. Mantle diapirism is caused by unstable gravity in the periods of tectonomagmatic activations. The analytical solution of the problem yields the critical parameters coupling the mantle flow dynamics and surface relief evolution. The difference between the structures and evolutions for Mediterranean and Caribbean regions is the following. In the Mediterranean region, the mantle diapirism produces newly formed basins of intercontinental seas at the final stage of Africa–Eurasia convergence (in the Cenozoic). In the Caribbean region, intensive mantle diapirism first disjoined the North and South Americas in the Mesozoic, and then played the same role as in the Mediterranean for the convergence of these continents in the Cenozoic.     

中亚地区中生代以来的地貌巨变与岩石圈动力学

中亚地区的地貌自中生代以来发生过两次巨变：一次是青藏高原的隆起,另一次是中生代中国东部高原及其西侧共存的中亚准平原的兴衰。青藏高原的隆起引起了全球气候和中亚环境的巨变。对此,自80年代以来开展的国际合作已经在地质学和地球物理学等研究领域取得了丰硕的成果。不过,在解释高原隆升-气候变化-剥蚀作用的相互关系方面仍存歧见。相比之下,中亚地区中生代的地貌巨变尚属新的研究课题。人们认识到,中亚地区在中侏罗世至新近纪曾存在一个准平原,而在中国东部则存在一个中生代高原。这一中生代地貌巨变引发出许多新的思考,如：为什么这一中生代准平原能保存长达150 Ma？中国东部高原是怎样形成的,又是怎样消失的？这两次地貌巨变及其相关的岩石圈动力学将是“TOPO?CENTRAL?ASIA”这一国际岩石圈计划项目的研究主题。     

Paleomagnetic implications on the evolution of the tethys belt from the atlantic ocean to the pamirs since the triassic

We first re-examined the apparent polar wander curves for stable Eurasia and Africa since the Triassic. These curves were then combined together with curves of North and South America according to the kinematics of the Atlantic ocean and a synthetic polar wander curve was given. Then, most of the paleomagnetic results from the Tethys mobile belt, from the Atlantic to the Pamirs, were analysed.Several groups of plates, microplates and blocks can be seen. First, relatively stable regions like Maghreb and Sicily, which have not moved much. Then we have a group formed by Iberia, Sardinia, Italy and, to a lesser extent, Corsica and the Western and Central Alps. For these blocks, movements are anticlockwise rotations chiefly driven by the anticlockwise rotation of Africa, but they are sometimes stronger.To the east, a major change takes place. The north of the Aegean Sea and the Ionian zone are clockwise rotated and these rotations are recent: Oligocene-Miocene for the first part, Pliocene to the present for the second part.A major problem arises in Turkey, Caucasus and Iran. Paleomagnetic results indicate a position far to the south of Eurasia, and, at the same time, geological evidence is in favour of a position close to Eurasia. We discuss these discrepancies.     

The Pelusium Line—a major transcontinental shear

The Pelusium Line, which was defined by Neev (1975) off the Mediterranean coast of Israel, is suggested to form a transcontinental arcuate shear which extends along the following three segments:

1. (A) from Anatolia along the eastern Mediterranean down to the eastern limit of the Nile Delta;

2. (B) across Africa down to the Niger Delta; and

3. (C) across the Mid-Atlantic Ridge along the equatorial fracture zones.

A “Central Plate”, composed of South and East Africa and the Arabian and Sinai subplates, has been left laterally shifted along the Pelusium Line relative to the Northwest African Plate.     

Overview of the Late Tertiary–Recent tectonic and palaeo-environmental development of the Mediterranean region

The Late Tertiary history of the Mediterranean region exemplifies processes of ocean basin closure and continental collision, as determined from integrated land and marine evidence. During the Mesozoic–Early Tertiary, tectonic settings were dominated by evolution of Neotethys. This ocean generally widened eastwards, with a number of oceanic strands in the Eastern Mediterranean area. Great diversity of tectonic settings and palaeo-environments developed during the Tertiary closure history of these oceanic basins. In the Eastern Mediterranean region, more northerly Neotethyan strands were closed by the Mid Tertiary, while oceanic crust remained in the south in the present Eastern Mediterranean Sea area. Northwards subduction of the remaining southerly Neotethyan strand was probably active by the Early Miocene. Different areas exhibit different stages of convergence and ocean basin closure. In the east, the amalgamated Eurasian plate had collided with the Arabian margin (Africa) by the Late Miocene, while oceanic crust still persisted further west. Steady-state subduction during the Late Tertiary gave rise to the Mediterranean ridge, as a substantial mud-dominated accretionary wedge. In the Aegean area, sufficient northward subduction took place to activate arc volcanism and pervasive back arc extension, short of marginal basin opening. In the easternmost Mediterranean, only limited subduction took place, associated with supra-subduction zone extension (e.g. in Cyprus). Today, steady state-subduction continues only locally, where vestiges of Neotethys remain (e.g. Herodotus abyssal plain). In the Western Mediterranean area, suturing of the African and Eurasian plates initially took place in the Betic region (Early–Mid Tertiary), where the Neotethys had existed only as a narrow connection with the Central North Atlantic. In the Central Mediterranean region, where the Western Neotethys was wider, northward subduction was active, apparently as early as the Late Cretaceous. In a widely accepted interpretation, an Andean-type magmatic arc developed along the southern margin of Europe and was then rifted off in the Late Oligocene-Early Miocene, to form the Corsica-Sardinia Block, opening the North Balearic marginal basin in its wake. The migrating subduction zone and microcontinent then collided diachronously with North Africa-related continental units (North Africa and Apulia) from Late Oligocene-Early Miocene, giving rise to collisional thrust belts in the Northern and Southern Apennines and along the North African continental margin (i.e. the Maghrebian chain) to the Betic-Rif area. From the Early Miocene onwards, a separate subduction system became active, related to removal of Neotethyan oceanic crust to the southeast (Ionian Sea), fueling suprasubduction zone extension and opening of the Tyrrhenian Sea. ‘Orogenic collapse’ is an alternative mechanism of such extension, and is widely believed to have caused divergent thrusting in the Betic and Rif regions of the westernmost Mediterranean, at the same time as crustal extension and subsidence of the Alboran Sea.     

北非地区中生代盆地区域沉积中心发育机制新认识及油气差异富集效应

北非地区为世界上油气富集地区之一,区内油气分布表现出极大的不均匀性,以往研究对这一油气差异性富集控制因素的探讨较为薄弱。本研究重点从中生代期间发育的多个区域沉积中心的演化和形成机制的角度,探讨这一科学问题。对已有的基础地质和油气勘探资料的综合再分析表明,北非地区冈瓦纳大陆北缘发育维德迈尔—佩拉杰、苏尔特、东地中海三个彼此孤立存在的中生代沉积中心,这些沉积中心在空间上处于阿拉拉隆起、苏尔特隆起、黎凡特隆起三个海西运动中形成的NE向古隆起之上,具有“古隆起塌陷反转”的形成机理;沉积中心均靠近新特提斯洋边缘,总体呈现受海西运动形成的古隆起和新特提斯洋开启背景下的伸展作用联合控制。三个中生代沉积中心为中生代优质烃源岩发育区和油气富集区;受海西期塑造的古构造、海西构造剥蚀对砂岩储层的控制以及中生代烃源岩发育等有利因素所控,这些塌陷形成的中生代沉积中心及围区成为最为重要的油气富集区带。中生代盆地的这一形成过程为该区油气差异富集的重要控制因素。     

Tertiary compressional deformation of the Iberian plate

The Tertiary deformation of the Iberian plate is here interpreted as the result of changes in the coupling between the Iberian–African plates. During the early stages of the Africa/Iberia subduction (Palaeocene), deformation was confined at the Betic plate boundary. From the Eocene, during the collision in the southern plate margin, compressional deformation delocalized and distributed throughout the Iberian plate. First, in the Pyrenees, where the main stage of thrusting occurred during the Late Eocene – Early Oligocene. Then (mainly Oligocene – Late Miocene), in the inner part of the Iberian plate, forming basement uplifts in the Iberian Chain and the Central System, in correspondence of pre-existing (Mesozoic and Variscan) structures. Finally, during the decay of compression inside the Iberian plate, extension took place the Mediterranean margin and the Alboran Sea.     

Adria,the African promontory,in mesozoic Mediterranean palaeogeography

The orogenic belts encircling the present-day Adriatic Sea are the deformed Mesozoic continental margin of an area known as Adria, the outline of which began to take shape during Middle Triassic continental rifting. Early Jurassic oceanic rifting was usually close to, but not coincident with, sites of earlier continental rifting. The Triassic rifted zones were usually incorporated into the continental margin of Adria, profoundly influencing its subsequent development. The Mesozoic platform/basin morphology of this margin can be correlated along the length of the belt.Palaeomagnetic data from autochthonous outcrops of the foreland of Adria do not indicate relative rotation and moreover suggest that this foreland has moved in coordination with Africa since the Early Mesozoic. Seismic soundings indicate that thick Mesozoic sedimentary sequences which can be correlated with sections on the African platform are continuous beneath the eastern Mediterranean seas. The concept of Adria as having behaved as a promontory of the African plate is tested by correlation of the main tectonic events in the belt with the spreading history of the Atlantic. The simplest model which adequately accounts for available data comprises a continuous Mesozoic continental margin from the Magrebids of Tunisia, through the Apennines, Alps, Dinarides and Hellenides to the alpine belt of Turkey. This margin was the southern margin of the Mesozoic Tethys and its foreland was more or less continuous with the African platform. Some structural and geochemical features of the double ophiolitic belt on the eastern side of Adria may be explained in terms of more external oceanic branches giving a more diversified continental margin of Adria. The present undulations of the Periadriatic belt are mainly a product of Late Cretaceous to recent deformation, which severely modified the shape of this margin by continental collision and by subsequent development of back-arc features.     

Reassessment of historical sections from the Paleogene marine margin of the Congo Basin reveals an almost complete absence of Danian deposits

The early Paleogene is critical for understanding global biodiversity patterns in modern ecosystems. During this interval, Southern Hemisphere continents were largely characterized by isolation and faunal endemism following the breakup of Gondwana. Africa has been proposed as an important source area for the origin of several marine vertebrate groups but its Paleogene record is poorly sampled, especially from sub-Saharan Africa. To document the early Paleogene marine ecosystems of Central Africa, we revised the stratigraphic context of sedimentary deposits from three fossil-rich vertebrate localities: the Landana section in the Cabinda exclave(Angola), and the Manzadi and Bololo localities in western Democratic Republic of Congo.We provide more refined age constraints for these three localities based on invertebrate and vertebrate faunas, foraminiferal and dinoflagellate cyst assemblages, and carbon isotope records. We find an almost complete absence of Danian-aged rocks in the Landana section, contrary to prevailing interpretations over the last half a century(only the layer 1, at the base of the section, seems to be Danian). Refining the age of these Paleocene layers is crucial for analyzing fish evolution in a global framework, with implications for the early appearance of Scombridae(tunas and mackerels) and Tetraodontiformes(puffer fishes). The combination of vertebrate fossil records from Manzadi and Landana sections suggests important environmental changes around the K/Pg transition characterized by an important modification of the ichthyofauna. A small faunal shift may have occurred during the Selandian. More dramatic is the distinct decrease in overall richness that lasts from the Selandian to the Ypresian. The Lutetian of West Central Africa is characterized by the first appearance of numerous cartilaginous and bony fishes. Our analysis of the ichthyofauna moreover indicates two periods of faunal exchanges: one during the Paleocene, where Central Africa appears to have been a source for the European marine fauna, and another during the Eocene when Europe was the source of the Central Africa fauna. These data indicate that Central Africa has had connections with the Tethyian realm.     

TECTONICS OF THE JUNCTION ZONE BETWEEN THE CENTRAL PAMIRS TROUGH AND NORTH PAMIRS UPLIFT

The Central Pamirs trough, trending E - W, lies between the North Pamir uplift and the Pamir- Hindukush uplift. The North Pamir uplift Was a positive Area during the Mesozoic, while continuous deposition took place in the trough. Within the trough the structures are Alpine. The North Pamir uplift is divisible into three zones. Adjacent to the trough and separated from it by a major northdipping thrust is a zone of highly contorted Ordovician strata. To the north, the central zone, also bounded by thrusts, has gently folded Paleozoic strata in the west, passing eastward into complex structures where the zone is narrowest. The Paleozoics of the northernmost zone lie in broad open folds, thrust southward over the central zone and northward over Permian volcanics and sediments. The northern part of the Central Pamirs trough is an area of broad open folds in Mesozoic and Paleozoic sediments, bounded on the north by a belt of generally north-dipping thrust slices of Paleozoic rocks, paralleling the fold trends. Thrust synclines have been thrust both northward and southward over the intervening anticlines, in some cases with superposition of younger beds over older. The structures described can only be the result' of tangential compression. They do not support the current (Russian) theory that the tectonics Of the area are due to gravitational movement of rock masses from uplifts to troughs. -- P. B. Jones.     

钨矿床的时空分布及研究现状

本文在查阅国内外钨矿资料的基础上,总结了钨矿床的时空分布、矿床类型、成因以及白钨矿在同位素测年方面的应用,阐述了钨矿床的成因研究现状及其发展趋势。     

First evidence of a distal early Holocene ash layer in Eastern Mediterranean deep-sea sediments derived from the Anatolian volcanic province

A hitherto unknown distal volcanic ash layer has been detected in a sediment core recovered from the southeastern Levantine Sea (Eastern Mediterranean Sea). Radiometric, stratigraphic and sedimentological data show that the tephra, here termed as S1 tephra, was deposited between 8970 and 8690 cal yr BP. The high-silica rhyolitic composition excludes an origin from any known eruptions of the Italian, Aegean or Arabian volcanic provinces but suggests a prevailing Central Anatolian provenance. We compare the S1 tephra with proximal to medial-distal tephra deposits from well-known Mediterranean ash layers and ash fall deposits from the Central Anatolian volcanic field using electron probe microanalyses on volcanic glass shards and morphological analyses on ash particles. We postulate a correlation with the Early Holocene ‘Dikkart?n’ dome eruption of Erciyes Da? volcano (Cappadocia, Turkey). So far, no tephra of the Central Anatolian volcanic province has been detected in marine sediment archives in the Eastern Mediterranean region. The occurrence of the S1 tephra in the south-eastern part of the Levantine Sea indicates a wide dispersal of pyroclastic material from Erciyes Da? more than 600 km to the south and is therefore an important tephrostratigraphical marker in sediments of the easternmost Mediterranean Sea and the adjacent hinterland.     

The emplacement of giant ophiolite nappes I. Mesozoic—cenozoic examples

The occurrence of ophiolite nappes has been considered evidence for the siting of ancient subduction zones. A study of the detailed stratigraphy and plate motions associated with Upper Mesozoic to Pliocene ophiolite nappes of the Pacific, Indian and Mediterranean shows that transcurrent faulting during changes in relative plate motions is the major cause of initial ophiolite nappe emplacement. Giant ophiolite nappes are not related to subduction zones or island arcs.     

Origin of the Eastern Mediterranean basin: a reevaluation

The origin of the Eastern Mediterranean basin (EMB) by rifting along its passive margins is reevaluated. Evidence from these margins shows that this basin formed before the Middle Jurassic; where the older history is known, formation by Triassic or even Permian rifting is indicated. Off Sicily, a deep Permian basin is recorded. In Mesozoic times, Adria was located next to the EMB and moved laterally along their common boundary, but there is no clear record of rifting or significant convergence. Farther east, the Tauride block, a fragment of Africa–Arabia, separated from this continent in the Triassic. After that the Tauride block and Adria were separate units that drifted independently. The EMB originated before Pangaea disintegrated. Two scenarios are thus possible. If the configuration of Pangaea remained the same throughout its life span until the opening of the central Atlantic Ocean (configuration A), then much of the EMB is best explained as a result of separation of Adria from Africa in the Permian, but this basin was modified by later rifting. The Levant margin formed when the Tauride block was detached, but space limitations require this block to have also extended farther east. Alternatively, the original configuration (A2) of Pangaea may have changed by 500 km of left-lateral slip along the Africa–North America boundary. This implies that Adria was not located next to Africa, and most of the EMB formed by separation of the Tauride block from Africa. Adria was placed next to the EMB during the transition from the Pangaea A2 to the Pangaea A configuration in the Triassic. Both scenarios raise some problems, but these are more severe for the first one. Better constraints on the history of Pangaea are thus required to decipher the formation of the Eastern Mediterranean basin.     

http://www.sciencedirect.com/science/article/pii/S1674987112000084

Seismic anisotropy and its main features along the convergent boundary between Africa and Iberia are detected through the analysis of teleseismic shear-wave splitting.Waveform data generated by 95 teleseismic events recorded at 17 broadband stations deployed in the western Mediterranean region are used in the present study.Although the station coverage is not uniform in the Iberian Peninsula and northwest Africa,significant variations in the fast polarization directions and delay times are observed at stations located at different tectonic domains.Fast polarization directions are oriented predominantly NW-SE at most stations which are close to the plate boundary and in central Iberia;being consistent with the absolute plate motion in the region.In the northern part of the Iberian Peninsula,fast velocity directions are oriented nearly E—W;coincident with previous results.Few stations located slightly north of the plate boundary and to the southeast of Iberia show E—W to NE-SW fast velocity directions,which may be related to the Alpine Orogeny and the extension direction in Iberia.Delay times vary significantly between 0.2 and 1.9 s for individual measurements,reflecting a highly anisotropic structure beneath the recording stations.The relative motion between Africa and Iberia represents the main reason for the observed NW-SE orientations of the fast velocity directions.However,different causes of anisotropy have also to be considered to explain the wide range of the splitting pattern observed in the western Mediterranean region.Many geophysical observations such as the low Pn velocity,lower lithospheric Q values,higher heat flow and the presence of high conductive features support the mantle How in the western Mediterranean,which may contribute and even modify the splitting pattern beneath the studied region.     

Volcanism in the rift‐valley system that evolved into the western margin of Australia

The Mesozoic forerunner of the western margin of Australia has been regarded tectonically as an ancient analogue of the multiple rift‐valley system of East Africa, which comprises two arms: volcanic on the E, and virtually non‐volcanic on the W. The abundance of widespread volcanics recently dredged and cored along the outermost margin, which corresponds with the volcanic arm of the East African system, contrasts with the apparent scarcity of volcanics inshore, in the inner arm of the rift system. We tested the possibility that volcanogenic material has been overlooked inshore by a petrographic study of the Late Jurassic to earliest Cretaceous Yarragadee Formation of the Perth Basin; only rare possible pyroclasts of quartz and glass (probably emplaced by air‐fall from the volcanic outer arm) were found, confirming the contrast in volcanism between the arms. This petrological evidence, together with the appropriate range of composition of the volcanism, from silicic to mafic, including alkaline and peralkaline members, reinforces the analogy with East Africa.     

Along-strike shear-sense reversal in the Vals-Scaradra Shear Zone at the front of the Adula Nappe (Central Alps,Switzerland)

The Adula Nappe in the Central Alps comprises pre-Mesozoic basement and minor Mesozoic sediments, overprinted by Paleogene eclogite-facies metamorphism. Peak pressures increase southward from ca. 1.2 GPa to values over 3 GPa, which is interpreted to reflect exhumation from a south-dipping subduction zone. The over- and underlying nappes experienced much lower Alpine pressures. To the north, the Adula Nappe ends in a lobe surrounded by Mesozoic metasediments. The external shape of the lobe is simple but the internal structure highly complicated. The frontal boundary of the nappe represents a discontinuity in metamorphic peak temperatures, between higher T in the Adula Nappe and lower T outside. A shear zone with steeply dipping foliation and shallowly-plunging, WSW-ENE oriented, i.e. orogen-parallel stretching lineation overprinted the northernmost part of the Adula Nappe and the adjacent Mesozoic metasediments (Vals-Scaradra Shear Zone). It formed during the local Leis deformation phase. The shear sense in the Vals-Scaradra Shear Zone changes along strike; from sinistral in the W to dextral in the E. Quartz textures also vary along strike. In the W, they indicate sinistral shearing with a component of coaxial (flattening) strain. A texture from the middle part of the shear zone is symmetric and indicates coaxial flattening. Textures from the eastern part show strong, single c-axis maxima indicating dextral shearing. These relations reflect complex flow within the Adula Nappe during a late stage of its exhumation. The structures and reconstructed flow field indicate that the Adula basement protruded upward and northward into the surrounding metasediments, spread laterally, and expelled the metasediments in front towards west and east.     

On the tectonic origin of Iberian topography

The present-day topography of the Iberian peninsula can be considered as the result of the Mesozoic–Cenozoic tectonic evolution of the Iberian plate (including rifting and basin formation during the Mesozoic and compression and mountain building processes at the borders and inner part of the plate, during the Tertiary, followed by Neogene rifting on the Mediterranean side) and surface processes acting during the Quaternary. The northern-central part of Iberia (corresponding to the geological units of the Duero Basin, the Iberian Chain, and the Central System) shows a mean elevation close to one thousand meters above sea level in average, some hundreds of meters higher than the southern half of the Iberian plate. This elevated area corresponds to (i) the top of sedimentation in Tertiary terrestrial endorheic sedimentary basins (Paleogene and Neogene) and (ii) planation surfaces developed on Paleozoic and Mesozoic rocks of the mountain chains surrounding the Tertiary sedimentary basins. Both types of surfaces can be found in continuity along the margins of some of the Tertiary basins. The Bouguer anomaly map of the Iberian peninsula indicates negative anomalies related to thickening of the continental crust. Correlations of elevation to crustal thickness and elevation to Bouguer anomalies indicate that the different landscape units within the Iberian plate can be ascribed to different patterns: (1) The negative Bouguer anomaly in the Iberian plate shows a rough correlation with elevation, the most important gravity anomalies being linked to the Iberian Chain. (2) Most part of the so-called Iberian Meseta is linked to intermediate-elevation areas with crustal thickening; this pattern can be applied to the two main intraplate mountain chains (Iberian Chain and Central System) (3) The main mountain chains (Pyrenees and Betics) show a direct correlation between crustal thickness and elevation, with higher elevation/crustal thickness ratio for the Central System vs. the Betics and the Pyrenees. Other features of the Iberian topography, namely the longitudinal profile of the main rivers in the Iberian peninsula and the distribution of present-day endorheic areas, are consistent with the Tertiary tectonic evolution and the change from an endorheic to an exorheic regime during the Late Neogene and the Quaternary. Some of the problems involving the timing and development of the Iberian Meseta can be analysed considering the youngest reference level, constituted by the shallow marine Upper Cretaceous limestones, that indicates strong differences induced by (i) the overall Tertiary and recent compression in the Iberian plate, responsible for differences in elevation of the reference level of more than 6 km between the mountain chains and the endorheic basins and (ii) the effect of Neogene extension in the Mediterranean margin, responsible for lowering several thousands of meters toward the East and uplift of rift shoulders. A part of the recent uplift within the Iberian plate can be attributed of isostatic uplift in zones of crustal thickening.