Search

The Barrovian type metamorphism affecting the peri-Siberian tract of the Central Asian Orogenic Belt is mostly dated indirectly on zircon from (syn-tectonic) magmatic rocks as Late Proterozoic - Early Cambrian. However, in-situ monazite geochronology in micaschists and migmatite gneisses at the northern part of the Precambrian Baidrag block, central Mongolia, revealed that the Baikalian Late Proterozoic - Early Cambrian cycle overprints an earlier Tonian phase of metamorphism. The apparent Barrovian-type zoning ranging from garnet, staurolite, kyanite to kyanite/sillimanite migmatitic gneisses is thus false and points to hidden metamorphic discontinuities and mixed metamorphic histories from different times. Therefore, to decipher and interpret the record of different tectono-metamorphic events it is necessary to unreveal complete P-T-t paths from individual samples. Two localities with Tonian-age monazite show anticlockwise P-T paths: 1) Grt−Sil−Ky gneiss records burial to the sillimanite stability (~720°C, 6.0 kbar) followed by burial to the kyanite stability (~750°C, 9 kbar) and, 2) The Grt−St schist records burial to the staurolite stability field (~620°C, 6 kbar), further followed by almost isothermal burial (~590°C, 8.5 kbar). Based on monazite textural positon, internal zoning, and REE patterns, the time of prograde burial under a thermal gradient of 27-32°C/km is estimated at c. 890−853 Ma and further burial under a geothermal gradient of 18-22°C/km is dated at c. 835−815 Ma. On the other hand three localities with Late Proterozoic to Cambrian monazite ages show clockwise metamorphic paths at variable P-T gradients: 3) P-T conditions of the Grt schist reaches ~5 kbar and 500 °C and 4) the Grt−St−Ky schist reaches conditions of 9 kbar and 670 °C, indicating burial under a geothermal gradient of 20-26 °C/km. 5) Grt-Sil gneiss shows peak of 6-7 kbar and 700-750 °C, indicating melting conditions at 30-32 °C/km gradient. Monazite included in porphyroblasts and in the matrix indicate that these P-T conditions reached under variable geothermal gradient were semi-contemporaneous and occurred between 570 and 520 Ma. By correlation with published zircon ages of 600-530 Ma from granitoid magmatic rocks we suggest that the areas with higher geothermal gradient may be explained by closer vicinity of magmatic intrusions. These P−T and geochronology data from a continuous Barrovian metamorphic section suggest that anticlockwise P−T evolution from c. 930 to 750 Ma can be interpreted as a result of thickening of peri-Rodinian supra-subduction extensional and hot edifice. This metamorphic event was followed by a clockwise P−T evolution from c. 570 to 520 Ma possibly related to the collision of the Baidrag continental active margin with peri-Siberian continental mass further north.

The processes leading to the assembly of the Rodinia supercontinent through Grenvillian collisional orogeny are relatively well known. In contrast, accretionary orogenic processes occurring at the supercontinent periphery following Rodinia assembly are poorly understood. To fill this gap, we have identified metamorphic rocks in the Mongolia collage of the Central Asian Orogenic Belt, where numerous data testify for Meso- to Neo-proterozoic magmatic reworking. The tectono-metamorphic evolution of the peri-Siberian tract of the Central Asian Orogenic Belt is mainly characterized by the late Proterozoic-early Cambrian (Baikalian) cycle. However, we document here a Tonian age metamorphism at the northern part of the Precambrian Baidrag block, previously considered as a typical example of the Baikalian metamorphic belt. This study incorporates zircon and in-situ monazite geochronology linked to P-T modelling of Grt-Sil-Ky migmatite gneiss and Grt-St micaschist. Grt-Sil-Ky gneiss records initial burial to the sillimanite stability field at ~720 °C and 6.0 kbar followed by further burial to the kyanite stability field at ~750 °C and ~9 kbar and decompression to ~650 °C and ~8 kbar. The Grt-St schist records initial burial to the staurolite stability field at ~620 °C and 6 kbar, followed by further burial to ~590 °C and 8.5 kbar. The monazite data yield a continuum of 207Pb-corrected 238U/206Pb dates of ca. 926−768 Ma in the Grt-Sil-Ky gneiss, and ca. 937−754 Ma in the Grt-St schist. Based on monazite textural positon, internal zoning, and REE patterns, the time of prograde burial to 6.0 kbar under a thermal gradient of 27-32 °C/km is estimated at ca. 890−853 Ma. It is not clear whether such high-grade conditions prevailed until a phase of further burial under a geothermal gradient of 18-22 °C/km dated at ca. 835−815 Ma. The late monazite recrystallization at ca. 790 Ma is related to decompression. Additionally, monazite with dates of ca. 568−515 Ma occur as whole grains or as rims with sharp boundaries on Tonian monazite in Grt-St schist suggesting a minor Baikalian overprint. Metamorphic zircon rims with Th/U ratios of ~0.01-0.06 in Grt-Sil-Ky gneiss with 877±7 Ma age, together with lower intercepts of detrital zircon discordia lines in both Grt-Sil-Ky gneiss and Grt-St schist further support the Tonian age of high-grade metamorphism. The anticlockwise P-T evolution is interpreted as a result of thickening of a supra-subduction extensional and hot edifice - probably of back-arc or arc type. This kind of prograde metamorphism has so far only been described on the northern part of the Tarim block and was interpreted to be a result of initiation of peri- Rodinian subduction of the Mirovoi Ocean. The geodynamic consequences of a unique discovery of Tonian metamorphism are discussed in terms of tectonic switch related to initiation of peri-Rodinian oceanic subduction during supercontinent assembly, followed by strong mechanical coupling potentially related to onset of Rodinia dispersal.

The tectonometamorphic evolution of the peri-Siberian tract of the Central Asian Orogenic Belt is mainly characterized by Baikalian Late Proterozoic - Early Cambrian cycle related to amalgamation of Proterozoic oceanic and continent fragments to Siberain landmass. Here we present in-situ monazite geochronology linked to P−T modelling of micashischsts and migmatite gneisses at the northern part of the Precambrian Baydrag block (central Mongolia) previously considered as a part of Baikalian metamorphic belt. Garnet-sillimanite-kyanite gneiss records first burial to the sillimanite stability at ~725 °C and 6.5 kbar, followed by burial to the kyanite stability at ~650 °C and ~8 kbar. The garnet-staurolite schist records burial to the staurolite-stability at ~620 °C and 6 kbar, followed by a nearly isothermal burial to ~580 °C and 9 kbar. The monazite data yield a continuum of 207Pb-corrected 238U/206Pb dates of c. 926−768 Ma in the Grt−Sil−Ky gneiss, and c. 937−754 Ma in the Grt-St schist. Based on monazite textural positon and internal zoning, the time of prograde burial and peak under a thermal gradient of 28-32 °C/km is estimated at c. 870−890 Ma. It is not clear whether such high grade conditions prevailed until a phase of further burial under a geothermal gradient of 18-22 °C/km and dated at 800−820 Ma. Additionally, monazite with dates of c. 568−515 Ma occurs as whole grains or as rims with sharp boundaries on Grenvillean monazite in Grt-St schist testifying for minor Baikalian overprint. Metamorphic zircon rims with Th/U ratio ~0.01-0.06 in Grt−Sil−Ky gneiss with 877 ± 7 Ma age, together with lower intercepts of zircon discordia lines in both Grt-Sil-Ky gneiss and Grt-St schist further support the Tonian age of high grade metamorphism. The P−T and geochronology data show anticlockwise P−T evolution from c. 930 to 750 Ma which is interpreted as a result of thickening of suprasubduction extensional and hot edifice - probably of back arc or arc type. This kind of prograde metamorphism was so far described only on the northern part of the Tarim block and interpreted as a result of initiation of peri-Rodinian subduction of Mirovoi Ocean. Here, we further discuss geodynamic consequences of a unique discovery of Tonian metamorphism in term of tectonic switch related to initiation of peri-Rodinian oceanic subduction during supercontinent assembly followed by strong mechanical coupling potentially related to onset of Rodinia splitting.

Post-collisional mafic dykes crosscut the Paleozoic metamorphic basement and late-Variscan plutons in Les Guilleries massif (Catalan Coastal Ranges, NE Iberia). The predominance of mafic phenocrysts, porphyritic texture, abundant amphibole, high MgO and volatile content, together with crustal-like trace-element patterns indicate that the dykes correspond to calc-alkaline lamprophyres, mainly spessartites. Their enrichment in LILE, HFSE and REE and initial Sr-Nd isotopic compositions (87Sr/86Sri between 0.70851 and 0.71127, ε Ndi between -5.23 and -4.63) are consistent with an enriched subcontinental lithospheric mantle source. U-Pb ages of matrix titanite crystals yield concordia ages of 262±7Ma, congruent with crosscutting relationships. Post-magmatic processes are evidenced by intense chloritization and albitization of the lamprophyres, together with systematic variations of Na2O vs SiO2, K2O, CaO, Ba, Rb, Cs, Pb, Sr, Tl, and Zn, and possibly the removal of F. The geochemical and geochronological data support an orogenic geochemical affinity, in accordance with the transitional tectonic regime between Variscan compression/transpression and post-collisional transtension/extension, related to the fragmentation of Pangea and thinning of the lithosphere. The lamprophyre dykes studied could represent the youngest pulse of Variscan orogenic magmatism and, therefore, mark its end in NE Iberia before the onset of the generalized Triassic extension. Calc-alkaline lamprophyre. Post-collisional dykes. Sr-Nd isotopes. Les Guilleries Massif. Catalan Coastal Ranges. Variscan Orogeny.KEYWORDSABSTRACTCitation: Mellado, E., Corbella, M., Navarro-Ciurana, D., Kylander-Clark, A., 2021. The enriched Variscan lithosphere of NE Iberia: data from post-collisional Permian calc-alkaline lamprophyre dykes of Les Guilleries.

It is well documented that the interplay between crustal thickening and surface processes determines growth of continent-continent collision orogens from small and cold to large and hot. Additionally, studies have demonstrated that the structural style of a mountain belt is strongly influenced by inherited (extensional) structures, the pattern of erosion and deposition, as well as the distribution of shallow detachment horizons. However, the factors controlling distribution of shortening and variable structural style as a function of convergence and surface process efficiency remain less explored. We use a 2D upper-mantle scale plane-strain thermo-mechanical model (FANTOM) coupled to a planform, mass conserving surface-process model (Fastscape), to investigate the long-term evolution of mountain belts and the influence of lithospheric pull, extensional inheritance, surface processes efficiency, and decoupling between thin-and thick-skinned tectonics. We establish an evolutionary shortening distribution for orogenic growth from a mono-vergent wedge to an orogenic plateau, and find that internal crustal loading is the main factor controlling the large scale evolution, while lithospheric pull modulates the plate driving force for orogenesis. Limited foreland-basin filling and minor exhumation of the orogen core are characteristic for low surface-process efficiency, while thick foreland-basin fill, and profound exhumation of the orogen core are characteristic for high surface-process efficiency. Utilizing a force balance analysis, we show how inherited structures, surface processes, and decoupling between thin-and thick-skinned deformation influence structural style during orogenic growth. Finally, we present a comparison of our generic modeling results with natural systems, with a particular focus on the Pyrenees, Alps, and Himalaya-Tibet. publishedVersion

The general structure of the Chinese Altai has been traditionally regarded as being formed by five tectono-stratigraphic ‘terranes’ bounded by large-scale faults. However, numerous detrital zircon studies of the Paleozoic volcano-sedimentary sequences shown that the variably metamorphosed Cambro-Ordovician sequence, known as the Habahe Group, is present at least in four ‘terranes’. It structurally represents deepest rocks unconformably covered by Devonian and Carboniferous sedimentary and volcanic rocks. Calc-alkaline, mostly Devonian, granitoids that intruded all the terranes revealed their syn-subduction related setting. Geochemistry and isotope features of the syn-subduction granitoids have shown that they originated mainly from the melting of youthful sediments derived from an eroded Ordovician arc further north. In contrast, Permian alkaline granitoids, mostly located in the southern part of the Chinese Altai, reflect a post-subduction intraplate setting. The metamorphic evolution of the metasedimentary sequences shows an early MP-MT Barrovian event, followed by two Buchan events: LP-HT mid-Devonian (ca. 400–380 Ma) and UHT-HT Permian (ca. 300–270 Ma) cycles. The Barrovian metamorphism is linked to the formation of a regional sub-horizontal possibly Early Devonian fabric and the burial of the Cambro-Ordovician sequence. The Middle Devonian Buchan type event is related to intrusions of the syn-subduction granitoids during an extensional setting and followed by Late Devonian-Early Carboniferous NE-SW trending upright folding and crustal scale doming during a general NW-SE shortening, responsible for the exhumation of the hot lower crust. The last Permian deformation formed NW-SE trending upright folds and vertical zones of deformation related to the extrusion of migmatites, anatectic granitoids and granulite rocks, and to the intrusions of gabbros and granites along the southern border of the Chinese Altai. Finally, the Permo-Triassic cooling and thrust systems affected the whole mountain range from ca. 265 to 230 Ma. In conclusion, the Chinese Altai represents different crustal levels of the lower, middle and upper orogenic crust of a single Cambro-Ordovician accretionary wedge, heterogeneously affected by the Devonian polyphase metamorphism and deformation followed by the Permian tectono-thermal reworking event related to the collision with the Junggar arc. It is the interference of Devonian and Permian upright folding events that formed vertical boundaries surrounding the variously exhumed and eroded crustal segments. Consequently, these crustal segments should not be regarded as individual suspect terranes.

To understand the polycyclic Palaeozoic tectono-metamorphic evolution of the Southern Chinese Altai, petrological and structural studies together with thermodynamic modelling and dating were carried out in the Cambro-Ordovician metapelitic sequence of the Kalasu area. The sequence is divided into upper, middle and lower crustal orogenic levels according to their metamorphic grade and structural patterns. Metamorphism increases from low to high-grade towards the deeper crustal levels with garnet-biotite schists in the upper crustal level, sillimanite-garnet and staurolite-garnet-sillimanite schists and gneisses in the middle crustal level, and cordierite-sillimanite-K-feldspar migmatites in the lower crustal level. Structural succession involves a subhorizontal S1 foliation folded by NE-SW open to tight and upright F2 folds (with no metamorphism associated), reworking by an orthogonal D3 deformation, characterized by NW-SE open to close F3 folds with moderately plunging axes, steeply dipping S3 axial planes and S3 cleavage. Early Devonian calc-alkaline granitoids intruded the sequence parallel to S1 foliation, whereas Permian undeformed gabbroic bodies were emplaced in the lower crust and granites in the upper crust coevally with D3. The P-T-t-d paths indicate that the crystalline rocks underwent a clockwise evolution marked by Early Devonian burial associated with heating, followed by Permian decompression, in agreement with studies from other parts of the Chinese Altai. The burial is recorded in the middle and lower levels by the presence of g-st-ky-ru relics within the S1 fabric. This stage is related to crustal thickening, whereas heating is related to intrusions of Devonian granite sheets during an extensional setting. A subsequent decompression (around 3-5 kbar) is recorded in all crustal levels, associated with intrusions of gabbro and granite along the southern border of the Chinese Altai and coeval with the last Permian deformation. This last stage is related to the collision between the Junggar arc system and the Chinese Altai orogenic belt.

Structural analysis and U–Pb geochronological study on zircons from the southern Chinese Altai (the Kalasu area, SE of the Altai city) show that the Cambro-Ordovician accretionary wedge (ca. 520–492 Ma) underwent four major geological events: 1) emplacement of Early Devonian magmas (ca. 410–400) associated with formation of a volcano-sedimentary cover, 2) major Middle Devonian (ca. 390–374 Ma) tectono-metamorphic event, 3) Late Devonian-Early Carboniferous folding without apparent metamorphism, and 4) a regional folding with localized Early Permian high- to ultrahigh-temperature reworking (ca. 300–280 Ma). The Early Devonian magmatism is characterized by emplacement of mafic rocks and granitoids in the centre of the NE-SW profile, coevally with granitoid magmatism and rhyolite volcanism in the southwest and northeast, respectively. The whole volcano-sedimentary and magmatic edifice was transposed by sub-horizontal metamorphic fabric associated with variable metamorphic degrees in different areas ranging from greenschist facies in the northeast (mu + bi±g) to amphibolite facies in the southwest (st + g ± sill) and granulite facies in the centre (g + sill+kfs). This metamorphic architecture, distribution of magmatism and character of metamorphic zircon populations allow to correlate these areas with upper, middle and lower orogenic crust that developed during important vertical shortening and horizontal flow in Middle Devonian. Subsequently, the whole edifice was affected by regional NE-SW trending upright (possibly Late Devonian-Early Carboniferous) folding. Finally, Early Permian shortening produced NW-SE trending regional upright folds in the southwest and northeast and a crustal-scale vertical, tabular deformation zone in the centre. The Permian deformation is accompanied by granulite facies (kfs + cd + sill+g) metamorphism and anatexis reworking the Devonian lower orogenic crust, with extensive resetting and growth of new zircons and with intrusions of Permian granites and gabbros. This study suggests that the Early Permian event was related to massive perturbation of thermal structure of the mantle lithosphere due to the collision of the Junggar arc with the Chinese Altai terrane.

A detailed geological and structural mapping was carried out in the Kalasu area located in the southern Chinese Altai, a small segment of the giant Altai accretionary orogenic wedge, in order to understand its polycyclic evolution. The Kalasu area is predominated by felsic metavolcanic rocks in the northeastern domain and mainly by a metapelitic sequence with subordinate metavolcanic layers, intruded by gneissic granitoids cropping out over the whole area. Structural investigations reveal the presence of three deformation events with variable intensity and degree of preservation. D1 is characterized by a metamorphic S1 foliation moderately to steeply dipping to the NW or locally to the SE, preserved in the northeastern and southwestern domain. The D2 deformation is responsible mainly for the variability in dip of the originally sub-horizontal S1 fabric due to NNW-SSE F2 folding, which is not associated with penetrative metamorphic reworking. Such a structural template was subsequently reworked by orthogonal D3 deformation. This last deformation is characterized by NW-SE steeply plunging isoclinal to close F3 folds, steeply dipping S3 axial planes and S3 foliation, which is delineated by cordierite-K-feldspar leucosome in the central domain where the S1 foliation is entirely transposed. To establish the absolute age of magmatic and metamorphic events, several samples were selected for U-Pb isotope analysis on zircon using the LA-ICPMS technique.

[spa] Los experimentos de multi-sísmicas realizados en el marco de la iniciativa del proyecto ALCUDIA incluyen una incidencia normal y sísmica gran angular de los conjuntos de datos de reflexión sísmica. Estos limitan las estructuras internas, la estructura detallada de la corteza y litosfera y, proporcionan el conocimiento necesario de las propiedades físicas a lo largo de la Zona Central Ibérica. Las imágenes sísmicas de incidencia normal se presentan de Sur a Norte: las estructuras de la Zona Ossa-Morena (Unidad Central), el ondulante domino Ibérico Central, los cuerpos intrusivos graníticos (Granitos de Mora, complejo Anatético de Toledo, etc.) hasta la falla de Toledo, justo al Sur de la cuenca del Tajo. La zona de estudio está caracterizada por una topografía ondulada debido a una serie de sinclinales y anticlinales. La imagen muestra una corteza inferior débilmente reflectante, ~13-18 km de espesor. La topografía sinodal refleja los pliegues, los sistemas de cabalgamientos imbricados, y fallas extensionales lístricas observados por los datos de sísmica de incidencia normal. A niveles más profundos las imágenes revelan estructuras arqueadas y laminares que sugieren estructuras como boundinage. Estas características son interrumpidas hacia el Sur, muy probablemente debido a la existencia de la zona de sutura. La expresión más profunda del afloramiento de la zona de sutura, está ligeramente desplazada hacia el Norte con respecto a su localización en superficie y, esta es vista como un cabalgamiento profundo de una fina capa de corteza inferior. Hacia el límite norte del perfil, debajo la falla de Toledo, una banda de reflectividad buzando hacia norte sugiere un adelgazamiento de la corteza inferior hacia el Norte. La discontinuidad de Mohorovicic se delimita como una fuerte disminución de la reflectividad a la base de la corteza inferior, localizada a unos ~31-33 km de profundidad (10.5 s tempo doble). Los datos de gran ángulo de sísmica de reflexión adquiridos a principios de mayo de 2012, tenían como objetivo proporcionar las distribuciones de las propiedades físicas (velocidades P y S, densidades, y el coeficiente de Poisson) de la corteza bajo la zona de estudio. Este perfil fue diseñado para ser casi coincidente con el perfil de incidencia normal. Los modelos de velocidad de P y S obtenidos revelan importantes variaciones laterales de velocidad en la corteza superior, que son coherentes con la observación de los afloramientos en superficie. La corteza inferior se identifica por un fuerte aumento de la velocidad a ~13 km y ~20 km de profundidad en Zona Central Ibérica y en la Cuenca del Tajo, respectivamente. La base de la corteza está fuertemente limitada por grandes amplitud de las llegadas de PmP y SmS y se encuentra a ~31 km en el extremo sur y a ~35,5 km por debajo del extremo norte del perfil de sísmica de gran ángulo. La sísmica de gran ángulo también se utilizó para producir low-fold stacks para las ondas P y las S. Esto se hizo para tomar una mirada más cercana a la discontinuidad de Moho, y para ser capaces de estimar la estructura interna y generar sismogramas sintéticos donde las fases PmP y SmS serían cualitativamente comparables a las fases observadas en los registros sísmicos. Esto dio lugar restricciones clave en la estructura de la discontinuidad de Moho bajo la Zona Central Ibérica y la Cuenca del Tajo en el orógeno Varisco del Macizo Central Ibérico. Las imágenes revelan una profundización gradual de ~31 km (10,2 s) en la Zona Central Ibérica a ~35,5 km (11,8 s) debajo de la cuenca del Tajo para la discontinuidad de Moho. Los datos sísmicos de gran ángulo y los resultados derivados de ellos son compatibles con las imágenes de incidencia normal., [eng] The multi-seismic experiments carried out under the ALCUDIA research initiative include a normal incidence and a wide-angle seismic reflection datasets. These constrain the internal architecture, the detailed crustal and lithospheric structure and, provide the necessary knowledge on physical properties across the Central Iberia Zone. The normal incidence seismic images from south to north: the suture zone of Ossa-Morena Zone (the Central Unit), the undulating Central Iberian domain, the intruded granitic bodies (the Mora Granite, the Toledo Anatectic complex, etc) up to the Toledo Fault, just south of the Tajo Basin. The study area is characterized by undulating topography characterized by a series of synclines and anticlines. The image shows a weakly reflective upper crust, ~13-18 km thick. The sinusoidal topography reflects the folding, the imbricate thrust systems, and listric extensional faults imaged by the normal incidence data. At deeper levels the image reveals arcuate and laminated events suggesting boudinage like structures. These features are disrupted in the south most probably due to the existence of the suture zone. The deep expression of the outcropping suture zone, is slightly displaced to the north with respect to its surface position and, it is imaged as an under-thrusting of a thin slice of lower crust. Towards the northern end of the transect, beneath the Toledo fault, a north dipping band of reflectivity suggest a significant thinning of the lower crust to the north. The Mohorovicic discontinuity is delineated as a sharp decrease in reflectivity at the base of the lower crust, located at ~31-33 km depth (10.5 s two-way travel-time). The wide-angle seismic reflection data acquired in early May 2012, was aimed to provide the distributions of the physical properties (P- and and S-wave velocities, densities, and Poisson’s ratio) of the crust beneath the study area. It was designed to be nearly coincident with the normal incidence profile. The obtained P- and S- wave velocity models reveal significant lateral velocity variations in the upper crust, which are consistent with the observation from the surface outcrops. The lower crust is identified by a sharp increase in velocity at ~13 km and ~20 km beneath the Central Iberian Zone and the Tajo Basin, respectively. The base of the crust is strongly constrained by the high amplitude PmP and SmS arrivals and it is located at ~31 km in the southern end and at ~35.5 km below the northern end of the wide-angle transect. The wide-angle was also used to produce low-fold P- and S-wave wide-angle stacks. This was done to take a closer look to the Moho discontinuity, and to be able to estimate the internal structure and generate synthetic seismograms in which the PmP and SmS would be qualitatively comparable to the phases observed in the shot records. This place key constraints on the structure of the Moho discontinuity beneath the Central Iberian Zone and the Tajo Basin in the Variscan orogen of the central Iberian Massif. The images reveal gradual deepening from ~31 km (10.2 s) in the Central Iberian Zone to ~35.5 km (11.8 s) beneath the Tajo Basin for the Moho discontinuity. The wide-angle seismic data and the results derived from it are compatible with the normal incidence images.

[cat] Aquesta tesi doctoral descriu la cinemàtica dels Pirineus Basco-Cantàbrics, més concretament de la Plataforma Burgalesa, mitjançant la integració de i) geologia de superfície; ii) interpretació de línies sísmiques i pous; i iii) estudis paleomagnètics de rotacions d'eix vertical. La integració d'aquest conjunt de dades permet comprendre la relació entre la Plataforma Burgalesa i les conques d'avantpaís de l'Ebre i del Duero durant la orogènia Pirenaica. Les principals contribucions de la tesi són: i) Presentar un model estructural, el qual honora tots les restriccions imposades per l'estratigrafia, la preconfiguració de l'àrea prèvia a la deformació contractiva i les dades tant superficials com profundes. Aquest model permet comprendre com és la transició entre la deformació pel·licular present a l'est de la zona d'estudi i la deformació no pel·licular present a l'oest. ii) Reinterpretació de l'estructura de Zamanzas localitzada al nord de la Plataforma Burgalesa mitjançant el treball combinat entre geologia de superfície i les dades aportades per els pous d'exploració. iii) Aportar noves dades de vint-i-sis estacions de paleomagnetisme repartides per a la zona d'estudi i que permeten caracteritzar el patró de rotacions d'eix vertical. El resultat final és el de proposar un nou model per a l'evolució de la Plataforma Burgalesa que honra les restriccions imposades tant per la geologia de superfície com les dades d'exploració profunda disponibles fins a la data. Aquest model està caracteritzat per presentar deformació no pel·licular al sector oest mentre que quan el nivell de desenganxament augmenta de potència els encavalcaments es desenganxen i deformen la cobertora presentant un estil estructural pel·licular. Aquesta tesi ha demostrat que la pre-configuració tant de la conca extensiva pròpiament dita com la del seu voltant, tenen una forta implicació en el desenvolupament i estructuració de la Plataforma Burgalesa durant la Orogènia Pirenaica. És durant aquesta etapa en la qual es produeixen rotacions tant horàries com antihoràries en els diferents sectors mostrejats., [eng] This PhD Thesis describes the kinematics of the Burgalesa Platform in the Basque-Cantabrian Pyrenees by the integration of i) surface geology; ii) seismic and well data; and iii) vertical axis rotation paleomagnetic studies. The integration of all the data allow to understand the relationship between the Burgalesa Platform and the Ebro and Duero foreland basins during the Pyrenean Orogeny. The principal contribution of the Thesis is to present a new structural model, that fits all the constraints imposed by the stratigraphy, the pre-configuration of the area prior to the contractive deformation and both the deep and the surface data. This model allows to understand the transition between thin-skinned and the thick-skinned tectonics. The combined work between surface geology and well data has allowed to reinterpret the Zamanzas structure located at the northern boundary of the Burgalesa Platform. Previously, this structure had been interpreted as a set of extensional faults. This Thesis pointed out the mismatches and reinterpret the Zamanzas structure as a popup generated during the Pyrenean Orogeny. The combined interpretation of wells and seismic sections allow to partially comprehend the structure of the area during the extensional event as well as the result of the Pyrenean Orogeny occurred later. This study allowed to determine the distribution of the Lower Cretaceous depocenters during the North Atlantic and the Bay of Biscay opening. Moreover, during this stage salt mobilisation triggered by extension generated areas where the Triassic salt layer was thickened as demonstrated by the well data and the outcropping diapirs. This process accentuated the development of the depocenters. The extensional event also generated the onlap sedimentary geometries in the syn-rift sediments. The extensional model proposed is a combined model with the development of cover forced folding with salt migration at the southern boundary. The Triassic salt decoupled the extension between the basement and the cover and contractional structures were developed such as the back-thrust in the Huidobro area that produced the duplication of the Mesozoic. The early stages of the Pyrenean Orogeny resulted in the development of the San Pedro Structure, with north-directed and basement-involved thrusts, later fossilised by the Duero foreland sediments. To the north, the Burgalesa Platform, characterised by south-directed thrusts rooted into the Triassic salts, was southwards displaced until the San Pedro act as a backstop forcing the Burgalesa Platform to extrude laterally towards the southeast overriding the Ebro Foreland Basin. The paleomagnetic study focussed on determine the possible vertical axis rotations along the Pyrenees, denotes that the amount of sites that some areas have is not present in the Burgalesa Platform and vicinities that lack of sites. For that reason, another objective of this Thesis is to provide new paleomagnetic sites for the study area. The study carried out is made up of twenty-six new sites widely distributed and that allow to characterise the vertical axis rotation pattern. The results obtained reveals that i) the magnetisation of the Cenozoic sediments is prior to deformation and that ii) clockwise and counter clockwise rotation occurred in the Burgalesa Platform and adjacent areas. The evolution models previously proposed for the Burgalesa Platform were based either from surface geology of from geophysical data thus fitting some of the constraints. Because any model integrated all the observations and available data, a new model explaining the Burgalesa Platform evolution has been proposed. This model is characterised by presenting thick-skinned deformation at the western sector whereas, thin-skinned at the eastern one when the Triassic salt layer thickens point in which the thrusts are detach the cover from the basement. The results of this Thesis demonstrate that the pre-configuration of the extensional basin and its vicinities have a strong implication in the development and evolution of the Burgalesa Platform during the Pyrenean Orogeny. Finally, during this evolution, clockwise and counter clockwise rotations occurred in the different sectors as shown by the paleomagnetic studies carried out during this Thesis.

[spa] Las cuencas sinorogénicas de Cuba se desarrollaron como consecuencia de la convergencia entre las placas del Caribe y Norteamericana. Dicha convergencia evolucionó desde un contexto de subducción-acreción durante el Cretácico tardío hasta un contexto de acreción-colisión durante el Paleógeno, y finalizó con la sutura del Orógeno Cubano a la Placa Norteamericana. En la evolución de las cuencas se distinguen dos estadios sinorogénicos y uno post-orogénico. Los estadios sinorogénicos están relacionados con la subducción-acreción del terreno Caribeana, que subdujo bajo el Arco Volcánico Caribeño quedando parcialmente acrecionado a éste durante el Campaniense Superior y el Maastrichtiense, y con la colisión entre el arco y el margen continental norteamericano durante el Paleógeno. El estadio post-orogénico se inició diacrónicamente durante el Eoceno, siendo más joven hacia el Este. El análisis tectonoestratigráfico de las cuencas constituye, conjuntamente con el estudio del magmatismo y del metamorfismo, una de las principales herramientas para definir la evolución del Orógeno Cubano. En Cuba hemos diferenciado cuencas contraccionales, extensionales y de desgarre que se originaron sincrónicamente. Las cuencas contraccionales se desarrollaron en la parte norte del orógeno y pueden interpretarse como un sistema de cuencas de antepaís que incluye cuencas transportadas y de antefosa. Las cuencas transportadas evolucionaron desde un contexto de antearco (Arco Volcánico Caribeño) en el Cretácico tardío hasta un contexto de colisión durante el Paleógeno. Las cuencas de antefosa se desarrollaron hacia el norte del cinturón orogénico como resultado de la flexión litosférica asociada al peso del orógeno. La sedimentación sinorogénica en las cuencas contraccionales comenzó durante el Cretácico tardío (Campaniense) y finalizó después del Eoceno Inferior en Cuba occidental, del Eoceno Medio en Cuba central, y del Eoceno Superior en Cuba oriental. Ésta se produjo principalmente en medios marinos e incluye secuencia olistostrómicas y flyschoides que registran la evolución de la convergencia. Por otra parte, las cuencas extensionales estuvieron relacionadas con la apertura de la cuenca de Yucatán y se formaron en la parte sur del orógeno durante el Paleógeno. Estas cuencas evolucionaron inicialmente como cuencas de intra-arco y luego fueron incorporadas al orógeno como cuencas intramontanas. La sedimentación sinorogénica en las cuencas extensionales se produjo en ambientes marinos profundos e incluye series turbidíticas derivadas de la erosión de rocas volcánicas y metamórficas. Finalmente, las cuencas de desgarre pueden interpretarse como cuencas poligenéticas asociadas a corredores tectónicos que son oblicuos al orógeno. El relleno sedimentario sinorogénico de las cuencas de desgarre indica altas tasas de subsidencia e incluye series olistostrómicas y turbidíticas que presentan grandes cambios laterales de facies y de espesores. La formación coetánea de todos estos tipos de cuencas sugiere que diversos regímenes tectónicos (de compresión, de extensión y de desgarre) se produjeron durante la formación del Orógeno Cubano. Este hecho explica varias particularidades estructurales y evolutivas del orógeno, entre ellas la diferenciación de dominios estructurales, la partición de la deformación, y la segmentación del orógeno en bloques que evolucionaron de manera independiente. Tomando como referencia las cuencas de Cuba central (bloque de Las Villas), se ha logrado distinguir diferentes estilos estructurales en el Orógeno Cubano. Se reconocen tres dominios estructurales diferentes: (1) el Cinturón de Complejos Metamórficos, (2) la Zona Axial, y (3) el Cinturón de Deformación del Norte. La evolución estructural del Cinturón de Complejos Metamórficos incluye una fase compresional desarrollada durante el Cretácico tardío que es seguida por una fase extensional durante el Paleógeno. La compresión dio lugar a la formación de un prisma de acreción que fue parcialmente subducido, y la extensión produjo su exhumación en contextos de ante-arco e intra-arco. La Zona Axial fue intensamente deformada y acortada desde el Cretácico tardío hasta el Eoceno. La compresión se produjo en una fase inicial y a continuación tuvo lugar una deformación transpresiva durante el Eoceno Medio. El Cinturón de Deformación del Norte consiste en un sistema imbricado que se formó desde el Paleoceno hasta el Eoceno Medio. Los pliegues y las fallas se produjeron siguiendo una secuencia normal, con el transporte tectónico dirigido hacia el NNE. Algunas estructuras SO–NE son coetáneas con el sistema imbricado que se extiende en dirección NO–SE, formándose corredores tectónicos y/o fallas de transferencia que facilitaron un régimen de partición de la deformación mientras se producía la colisión. La sincronía de la compresión en el norte con la extensión en el sur es coherente con la apertura de la cuenca de Yucatán. La evolución desde regímenes de compresión-extensión hasta regímenes de transpresión está en consonancia con el aumento de la oblicuidad de la colisión entre las placas del Caribe y Norteamericana. Particularmente, la zona de fallas La Trocha ha actuado como una zona de transferencia siniestra que separa los bloques de Las Villas y Camagüey en Cuba central. Las estructuras que conforman dicha zona de fallas (fallas La Trocha, Zaza-Tuinicú, Cristales y Taguasco) son consistentes con la rotación en sentido horario de la convergencia y del acortamiento en Cuba central. Desde el Paleoceno hasta el Eoceno Inferior, un acortamiento en dirección SSO–NNE produjo transtensión en la falla La Trocha y transpresión en la falla Zaza-Tuinicú. Posteriormente, durante el Eoceno Medio, el acortamiento rotó hacia una dirección SO–NE, dando como resultado una componente normal en la falla La Trocha y transpresión en las fallas Zaza-Tuinicú y Cristales. A partir del Eoceno Superior Cuba central ha estado soldada a la Placa Norteamericana. La deformación post-soldadura ha producido transtensión en las fallas La Trocha y Taguasco, y es consistente con un acortamiento en dirección OSO–ENE que refleja la actividad del límite transformante de Caimán. La cinemática de las placas y la evolución estructural de la zona de fallas La Trocha indican que la Cuenca Central es una cuenca de desgarre poligenética y que la formación de este sistema (es decir, zona de fallas – cuenca de desgarre) fue una consecuencia de la colisión oblicua que ocurrió durante el Paleógeno entre el Arco Volcánico del Caribe y el margen de las Bahamas (Placa Norteamericana). La formación de los principales corredores tectónicos cubanos fue coetánea con la orogenia, lo que conllevó a la segmentación del orógeno en una serie de bloques estructurales que evolucionaron de manera independiente. La edad y evolución de la deformación sugieren que dichos corredores actuaron como importantes límites tectónicos y que evolucionaron en correspondencia con la rotación de la convergencia entre las placas. La segmentación del orógeno está también soportada por la sedimentación sinorogénica, que abarca intervalos de tiempo diferentes en cada bloque, lo que sugiere que la soldadura del orógeno a la Placa Norteamericana se produjo en una secuencia O–E (más joven hacia el Este). La segmentación del Orógeno Cubano y la evolución diferenciada de sus partes sugieren que el Caribe noroccidental evolucionó como un sistema de microplacas durante su transferencia desde la Placa del Caribe hasta la Placa Norteamericana. Asimismo, se sugiere que el límite de placas activo migró en dirección Este durante la convergencia. A partir del análisis tectonoestratigráfico de las cuencas sinorogénicas, y de sus implicaciones estructurales y tectónicas, se ha podido establecer una serie de criterios y/o recomendaciones para la exploración de hidrocarburos en Cuba. En este sentido, se sugiere que las características de los sistemas petroleros cubanos están fuertemente controladas por la estructura del orógeno. Se han diferenciado tres sistemas de plays principales; los cuales están asociados al cinturón plegado cubano, a las estructuras de desgarre principales, y al sistema de antepaís respectivamente. Se sugiere que los yacimientos por descubrir en los sistemas de plays asociados al cinturón plegado y a las estructuras de desgarre pueden contener crudos de cualquier calidad en función de las características primarias y la madurez de la roca madre, del tipo y magnitud de la migración, de la superposición o no de distintos sistemas petroleros, y/o de la ocurrencia de procesos secundarios. Asimismo, se estima que dichos yacimientos serán mayoritariamente pequeños en cuanto al volumen de sus reservas y estarán vinculados a trampas de tipo estructural; dúplex, zonas triangulares y retrocabalgamientos en el sistema de plays del cinturón plegado, y a anticlinales fallados, estructuras en flor y sellos contra falla en el sistema de plays asociado a estructuras de desgarre. En cambio, se sugiere que los yacimientos no descubiertos en el sistema de plays del antepaís podrán tener crudos de alta calidad y grandes volúmenes de reservas. No obstante, se debe considerar que, aunque el sistema de plays asociado al antepaís es actualmente el que mayor interés atrae por su valoración de riesgo/recompensa, las características geoquímicas y estructurales del Orógeno Cubano sugieren que las otras áreas no deben ser descartadas., [eng] The synorogenic basins of Cuba are a consequence of the convergence between the Caribbean and North American plates. This convergence evolved from a subduction-accretion setting during the Late Cretaceous to an accretion-collision setting during the Paleogene, and ended with the welding of the Cuban Orogen to the North American Plate. Two synorogenic and one postorogenic stages are distinguished in the evolution of these basins. The synorogenic stages are related to the subduction-accretion of Caribeana, which was subducted beneath the Caribbean Volcanic Arc during the Late Campanian-Maastrichtian, and to the collision between the volcanic arc and the North American continental margin during the Paleogene. The postorogenic stage started during the Eocene and developed diachronically (younger eastward). The tectono-stratigraphic analysis of sedimentary basins is, in conjunction with the study of magmatism and metamorphism, a major tool in order to define the evolution of the Cuban Orogen. In Cuba, contractional, extensional and strike-slip basins evolved synchronously. Contractional basins evolved in the northern part of the orogen and may be interpreted as a foreland basin system that includes piggyback and foredeep basins. Piggyback basins evolved from a forearc setting (Caribbean Volcanic Arc) in the latest Cretaceous to a Paleogene collision setting. Foredeep basins developed northward of the orogenic belt as a consequence of the lithospheric flexion linked to the orogenic load. Synorogenic sedimentation in contractional basins started during the latest Cretaceous (Campanian) and ended after the Early Eocene in western Cuba, the Middle Eocene in central Cuba, and the Late Eocene in eastern Cuba. It occurred mainly in deep-marine environments and includes olistostromic and flyschoid series that record the evolution of convergence. Extensional basins were related to the opening of the Yucatan Basin and were formed in the southern part of the orogen during the Paleogene. These basins formerly evolved as intra-arc basins and then were incorporated to the orogen as hinterland basins. Synorogenic sedimentation in the extensional basins occurred in deep-marine environments and includes flyschoid series, which resulted from erosion of volcanic and metamorphic rocks. Strike-slip basins may be interpreted as polygenetic basins linked to major oblique tectonic corridors. Syntectonic record of the strike-slip basins suggests high subsidence rates. It includes olistostromic and flyschoid series that show large lateral changes in facies and thicknesses. The coeval formation of contractional, extensional and strike-slip basins suggests that different tectonic regimes occurred during the formation of the Cuban Orogen, explaining several structural features of the orogen as the differentiation of structural domains, the strain-partitioning, and the different evolution of some blocks. Based on a tectono-stratigraphic analysis of the sedimentary basins of central Cuba (Las Villas block), three structural domains are differentiated in the Cuban Orogen: (1) the Metamorphic Complexes Belt, (2) the Axial Zone, and (3) the Northern Deformation Belt. The structural evolution of the Metamorphic Complexes Belt includes a latest Cretaceous compressional phase followed by a Paleogene extensional phase. Contraction created an accretionary prism that was partially subducted, and extension produced exhumation in intra-arc and forearc settings. The Axial Zone was strongly deformed and shortened from the latest Cretaceous to the Eocene. Compression occurred in an initial phase and subsequent transpressive deformation took place in the Middle Eocene. The Northern Deformation Belt consists of a thin-skinned thrust fault system formed during the Paleocene to the Middle Eocene; folding and faulting occurred in a piggyback sequence with tectonic transport towards the NNE. Some major SW–NE structures are coeval with the Cuban NW–SE striking folds and thrusts, and form tectonic corridors and/or transfer faults that facilitated strain-partitioning regime attending the collision. The synchronicity of compression in the north with extension in the south is consistent with the opening of the Yucatan Basin. The evolution from compression–extension to transpression is in keeping with the increase in obliquity in the collision between the Caribbean and North American plates. Particularly, the La Trocha fault zone acted as a major left-lateral transfer zone, constituting a limit between the Las Villas and Camagüey blocks in central Cuba. Some faults that are included in this fault zone (La Trocha, Zaza-Tuinicú, Cristales and Taguasco faults) were consistent with the clockwise rotation of convergence and shortening in central Cuba. From the Paleocene to the Early Eocene, a SSW-NNE shortening produced transtension in the La Trocha fault and transpression in the Zaza-Tuinicú fault. Subsequently, during the Middle Eocene, shortening shifted to a SW-NE direction, resulting in the normal component of the La Trocha fault and transpression in the Zaza-Tuinicú and Cristales faults. Since the Late Eocene, central Cuba has been welded to the North American Plate. The post-welding deformation gave rise to transtension of the La Trocha and Taguasco faults. This deformation is consistent with a WSW-ENE shortening and reflects activity in the transform boundary of the Cayman Trough. Plate-kinematics and the structural evolution of the La Trocha fault zone indicate that the related Central Basin is a strike-slip polygenetic basin and that the formation of this system (i.e., fault zone – strike-slip basin) was a consequence of the Paleogene oblique collision between the Caribbean Volcanic Arc and the Bahamas Borderland (North American plate). The formation of the Cuban tectonic corridors was coeval with the orogeny, which led to the segmentation of the orogen into a number of structural blocks that evolved in different way. Age and evolution of deformation suggest that the Cuban corridors acted as important tectonic limits that evolved in accordance with rotation of the convergence between the Caribbean and North American plates. Segmentation of the orogen is also supported by the synorogenic sedimentation, which embraces different time spans in each block, suggesting that welding of the orogen to the North American Plate progressed from W to E. The segmentation of the orogen and different evolution of the Cuban blocks lend support to the evolution of the NW-Caribbean as a dynamic microplate system and to the eastward migration of the active transform plate boundary during the convergence. Based on the tectono-stratigraphic analysis of the Cuban basins, we were able to establish a set of criteria and/or recommendations for hydrocarbon exploration in Cuba. In this direction, it is suggested that the major features of the Cuban petroleum systems are strongly controlled by the structure of the orogen. Three major play systems are differentiated, which are linked to the Cuban fold-and-thrust belt, to the major strike-slip structures, and to the foreland basin respectively. The undiscovered oil fields in the plays linked to the fold-and-thrust belt and to the strike-slip structures could contain different oil types depending on the primary features and maturity of the source rocks, the migration type, the occurrence and superposition of different petroleum systems, and/or the occurrence of secondary processes. It is also estimated that these oil fields will be mostly small in volume of resources and will be linked to structural traps; duplexes, triangle zones and backthrusts in the plays related to the fold-and-thrust belt, and faulted anticlines, flower structures and seals-faults in the plays linked to the strike-slip structures. By contrast, the undiscovered oil fields in the plays linked to the foreland could contain high-quality oils and large volumes of resources. However, we should be borne in mind that, despite the risk-reward assessment of the play system linked to the foreland is attractive, geochemical and structural features of the Cuban Orogen suggest that other regions and play systems should not be dismissed.

Questo volume è concepito in maniera nuova per condurre il lettore - attraverso una serie di esempi reali e artifici inconsueti - alla comprensione delle tappe evolutive subite da un territorio nel corso della propria storia geologica. Descrive e analizza i numerosi segni lasciati dalle forze che agiscono, tanto sulla superficie quanto nelle profondità della crosta terrestre, sgretolando, erodendo, smistando e accumulando, cementando, sospingendo, sollevando, piegando, affastellando, frammentando, e poi di nuovo sgretolando, erodendo... in un susseguirsi incessante e infinito di deposizioni, deformazioni e modellamenti. Il volume si rivolge non solo agli studenti che, alloro primo approccio con le scienze che descrivono e analizzano l'evoluzione geologico-morfologica di un territorio, intendono comprendere l'essenza e la ragione di molti tra i fenomeni che condizionano le modificazioni, grandi e piccole, del nostro pianeta, ma anche agli appassionati naturalisti che spesso si domandano la ragione di molti caratteri presenti nel territorio e nell'ambiente. Caratteri che sovente sono tanto evidenti nella loro manifestazione quanto incomprensibili nella loro genesi. Inoltre, ho voluto dare molto spazio alle didascalie delle oltre duecento figure (tra foto e disegni) al fine di esemplificare e concretizzare i concetti espressi nel testo. Per un escursionista abituato a percorrere sentieri e mulattiere sono tante le possibilità di interesse comunicate dagli aspetti sempre vari e affascinanti che la Natura riserva lungo i suoi infiniti percorsi. Curiosità botaniche, faunistiche, forestali, insieme a quelle archeologiche, storiche, di architettura rurale; senza considerare le infinite proposte di semplice appagamento estetico: cascate, tramonti, temporali, arcobaleni improvvisi, mari di nubi... Tra tutto questo l'aspetto geologico e morfologico dell'ambiente che si attraversa viene spesso del tutto trascurato. Ciò accade perché la divulgazione di tali aspetti naturali, molto attiva in altri paesi o addirittura considerata fondamentale nel mondo anglosassone, da noi è ancora ritenuta, a torto, di secondario interesse. La carenza di una tale educazione geologica s.l. fa sì che per la gran parte degli escursionisti le morfologie alpine e le rocce attraversate durante le escursioni siano pressoché tutte uguali, prive di significato e incapaci di trasmettere sensazioni. Eppure, se solo l'attenzione fosse maggiormente stimolata all'osservazione più particolare e diretta dei tanti strati e banchi rocciosi incontrati e calpestati durante le innumerevoli escursioni, si

In questa nota divulgativa si descrive l'origine e lo sviluppo delle catene montuose in relazione ai processi di mobilità crostale del pianeta.

SUMMARY Whereas the present-day true polar wander and the secular non-tidal acceleration of the Earth have usually been attributed to postglacial rebound, it has recently been suggested that non-glacially induced vertical tectonic movements taking place under non-isostatic conditions can also be effective in changing the Earth's rotation. We present a case study in which we analyse the effects of some simple uplift histories of the Himalayas and the Tibetan Plateau on the rotational axis and on the second-degree zonal harmonic of the geoid, for time-scales of up to a few million years. We first assume a permanent amount of overcompensation, which is consistent with observed geoid anomalies over the Himalayas, and then we model by means of the normal-mode techniques, the viscous relaxation in the mantle, with the elastic lithosphere supporting elastically 2 per cent of isostatic disequilibrium. In our normal-mode analysis, the Earth is divided into five layers: an effectively elastic lithosphere, a viscoelastic shallow upper mantle, transition zone and lower mantle characterized by the Maxwell rheology and an inviscid core. The readjustment of the equatorial bulge due to viscous flow in the mantle is taken into account in our studies by solving the linearized Liouville equations for the conservation of angular momentum, via the Love numbers formalism. Polar wander is sensitive to the rate of relaxation of the modes M1 and M2 due to the discontinuities between the three mantle layers, positioned at 420 and 670 kilometres depth. The rate of readjustment is sensitive to the viscosity of the transition zone whenever the lower mantle/shallow upper mantle viscosity ratio is small. The highest present-day velocity of polar wander due to Himalayan and Tibetan Plateau uplift is estimated to be 1° Myr−1 for an isoviscous mantle that has the same magnitude of the observed value, reduced to 0.1° Myr−1 for a factor 50 viscosity increase in the lower mantle. These numbers are about the same as those found from postglacial rebound that occurs on the short time-scale of a thousand years instead of the million years of our analysis, but represent upper bounds for mountain building, obtained only in the case in which a permanent deviation from isostasy of at least 2 per cent is assumed. In general, the proposed mechanism is less efficient in driving long-term rotation instabilities than deep-seated processes characterized by the same time-scale of a million years such as subduction; polar-wander velocity is extremely sensitive to the depth of the uncompensated anomalous root of the topography for the models in which full mantle relaxation is allowed.