Éditeur(s) : Oficina Tip. de la Secretaria de Fomento. Oficina Tip. de la Secretaria de Fomento. ( Mexico )
Résumé : (Statement of Responsibility) Instituto Geologico de Mexico.
(Dates or Sequential Designation) No. 1 (1895).
(Numbering Peculiarities) Suspended 1932-45.
(Issuing Body) No. 1-51 issued for Instituto Geologico de Mexico.
Title varies: Boletin del Instituto geologico de Mexico (7-36).
Description based on: No 7/8/9 (1897).
Mexico
Mexico

225628280

Éditeur(s) : HAL CCSD Seismological Society of America
Résumé : With the assistance of the ASTER team (M. Arnold, G. Aumaître, D. Bourlès, and K. Keddadouche)
International audience
The Bulnay earthquake of 23 July 1905 (Mw 8.3–8.5), in north‐central Mongolia, is one of the world’s largest recorded intracontinental earthquakes and one of four great earthquakes that occurred in the region during the twentieth century. The 375 km long surface rupture of the left‐lateral, strike‐slip, N095°E‐trending Bulnay fault associated with this earthquake is remarkable for its pronounced expression across the landscape and for the size of features produced by previous earthquakes. Our field observations suggest that in many areas the width and geometry of the rupture zone is the result of repeated earthquakes; however, in those areas where it is possible to determine that the geomorphic features are the result of the 1905 surface rupture alone, the size of the features produced by this single earthquake are singular in comparison to most other historical strike‐slip surface ruptures worldwide. Along the 80 km stretch, between 97.18° E and 98.33° E, the fault zone is characterized by several meters width and the mean left‐lateral 1905 offset is 8.9±0.6  m with two measured cumulative offsets that are twice the 1905 slip. These observations suggest that the displacement produced during the penultimate event was similar to the 1905 slip. Morphotectonic analyses carried out at three sites along the eastern part of the Bulnay fault allow us to estimate a mean horizontal slip rate of 3.1±1.7  mm/yr over the Late Pleistocene–Holocene period. In parallel, paleoseismological investigations show evidence for two earthquakes prior to the 1905 event, with recurrence intervals of ∼2700–4000  yrs.
ISSN: 0037-1106

hal-01179837
https://hal.archives-ouvertes.fr/hal-01179837
https://hal.archives-ouvertes.fr/hal-01179837v2/document
https://hal.archives-ouvertes.fr/hal-01179837/file/rizza2015.pdf
DOI : 10.1785/0120140119

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
We carried out morphotectonic studies along the left-lateral strike-slip Bogd Fault, the principal structure involved in the Gobi-Altay earthquake of 1957 December 4 (published magnitudes range from 7.8 to 8.3). The Bogd Fault is 260 km long and can be subdivided into five main geometric segments, based on variation in strike direction. West to East these segments are, respectively: the West Ih Bogd (WIB), The North Ih Bogd (NIB), the West Ih Bogd (WIB), the West Baga Bogd (WBB) and the East Baga Bogd (EBB) segments. Morphological analysis of offset streams, ridges and alluvial fans-particularly well preserved in the arid environment of the Gobi region-allows evaluation of late Quaternary slip rates along the different faults segments. In this paper, we measure slip rates over the past 200 ka at four sites distributed across the three western segments of the Bogd Fault. Our results show that the left-lateral slip rate is similar to 1 mm yr(-1) along the WIB and EIB segments and similar to 0.5 mm yr(-1) along the NIB segment. These variations are consistent with the restraining bend geometry of the Bogd Fault.;Our study also provides additional estimates of the horizontal offset associated with the 1957 earthquake along the western part of the Bogd rupture, complementing previously published studies. We show that the mean horizontal offset associated with the 1957 earthquake decreases progressively from 5.2 m in the west to 2.0 m in the east, reflecting the progressive change of kinematic style from pure left-lateral strike-slip faulting to left-lateral-reverse faulting. Along the three western segments, we measure cumulative displacements that are multiples of the 1957 coseismic offset, which may be consistent with a characteristic slip. Moreover, using these data, we re-estimate the moment magnitude of the Gobi-Altay earthquake at M-w 7.78-7.95.;Combining our slip rate estimates and the slip distribution per event we also determined a mean recurrence interval of similar to 2500-5200 yr for past earthquakes along the different segments of the western Bogd Fault. This suggests that the three western segments of the Bogd Fault and the Gurvan Bulag thrust fault (a reverse fault bounding the southern side of the Ih Bogd range that ruptured during the 1957 earthquake) have similar average recurrence times, and therefore may have ruptured together in previous earthquakes as they did in 1957. These results suggest that the western part of the Bogd Fault system, including the Gurvan Bulag thrust fault, usually behaves in a 'characteristic earthquake' mode.
ISSN: 0956-540X

hal-00639467
https://hal.archives-ouvertes.fr/hal-00639467
https://hal.archives-ouvertes.fr/hal-00639467v2/document
https://hal.archives-ouvertes.fr/hal-00639467/file/rizzaGJI2011.pdf
DOI : 10.1111/j.1365-246X.2011.05075.x

Éditeur(s) : HAL CCSD American Geophysical Union (AGU)
Résumé : The role of the Karakorum fault zone (KFZ) is debated. South of 33°N, ongoing dextral-oblique slip along the SW edge of the Gar basin exhumes metamorphic and magmatic rocks of the Ayilari range. Minerals have recorded a continuum of deformation from temperatures >600–400°C down to <250_C. The 40Ar/39Ar ages, the oldest being 21.2 ± 1.0 Ma, yield minimum estimates for the initiation of the KFZ. These are in agreement with the U-Th/Pb ages constraining the onset of deformation at ca 25–22 Ma. Thermochronologic results show slow cooling for the period ca 21– 14 Ma, followed by rapid cooling between 14 and 4 Ma. These data demonstrate that right-lateral motion was in progress in the early Miocene and that shear continued at least until 4 Ma, pointing to >20 Ma of deformation along the fault. Greenschist facies deformation superimposed upon the medium- to high-grade deformation marks a kinematic change from pure dextral to dextral-normal motion associated with the onset of rapid cooling. At the regional scale, the coexistence of transtension in the Gar basin with transpression documented along the Pangong range farther north suggests another example of the ‘‘zipper tectonics'' model developed along the Red River fault. The kinematic shift induced the rise of the Ayilari range starting at 16–12 Ma and the incision of major river courses. The Indus River might have become captive of the relief at this time. The river's 120 km of apparent offset implies dextral motion at a long-term rate of ca 8.5 ± 1.5 mm/yr.
ISSN: 0278-7407

hal-00197561
https://hal.archives-ouvertes.fr/hal-00197561
https://hal.archives-ouvertes.fr/hal-00197561/document
https://hal.archives-ouvertes.fr/hal-00197561/file/Valli_Karakorum_thermochron_Tectonics_07_pre_print.pdf
DOI : 10.1029/2005TC001913

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
This study analyzes the active tectonics within the northwestern and southeastern extensions of the Pambak-Sevan-Syunik fault (PSSF), a major right-lateral strike-slip fault cutting through Armenia. Quantifying the deformations in terms of geometry, kinematics, slip rates and earthquake activity, using cosmogenic 3He, OSL/IRSL and radiocarbon dating techniques, reveal different behaviors between the two regions. Within the northwestern extension, in the region of Amasia, the PSSF bends to the west and splits into two main WNW–ESE trending reverse faults defining a compressional pop-up structure. We estimate an uplift rate and a shortening rate of 0.5 ± 0.1 mm/y and 1.4 ± 0.6 mm/y, respectively. This suggests that most of the ∼2 mm/y right lateral movement of the PSSF seems to be absorbed within the Amasia pop-structure. Within the southeastern extension, the PSSF shows signs of dying out within the Tsghuk Volcano region at the southernmost tip of the Syunik graben. There, the tectonic activity is characterized by a very slow NS trending normal faulting associated with a slight right-lateral movement. Slip rates analyses (i.e. vertical slip rate, EW stretching rate at 90° to the fault, and right-lateral slip rate of ∼0.2 mm/y, ∼0.1 mm/y and ∼0.05 mm/y, respectively) lead to the conclusion that the right lateral movement observed further north along the PSSF is mainly transferred within other active faults further west within the Karabagh (Hagari fault or other structures further northwestwards). Comparing our slip rates with those estimated from GPS data suggests that most of the deformation is localized and seismic, at least within the Tsghuk region. The geometrical and kinematic pattern observed within the two terminations of the PSSF suggests that the fault and its surrounding crustal blocks are presently rotating anticlockwise, as also observed within the GPS velocity field. This is consistent with the recent kinematic models proposed for the Caucasus-Kura-South Caspian region and brings a new insight into the present geodynamics of Armenia.
ISSN: 1040-6182

hal-01307342
https://hal.archives-ouvertes.fr/hal-01307342
DOI : 10.1016/j.quaint.2015.05.021

Éditeur(s) : HAL CCSD American Association for the Advancement of Science
Résumé : 6 pages
International audience
Large impacts provide a mechanism for resurfacing planets through mixing near-surface rocks with deeper material. Central peaks are formed from the dynamic uplift of rocks during crater formation. As crater size increases, central peaks transition to peak rings. Without samples, debate surrounds the mechanics of peak-ring formation and their depth of origin. Chicxulub is the only known impact structure on Earth with an unequivocal peak ring, but it is buried and only accessible through drilling. Expedition 364 sampled the Chicxulub peak ring, which we found was formed from uplifted, fractured, shocked, felsic basement rocks. The peak-ring rocks are cross-cut by dikes and shear zones and have an unusually low density and seismic velocity. Large impacts therefore generate vertical fluxes and increase porosity in planetary crust.
ISSN: 0036-8075

hal-01399386
https://hal.archives-ouvertes.fr/hal-01399386
PUBMED : 27856906
DOI : 10.1126/science.aah6561

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Structural analysis along with 40Ar–39Ar and U–Pb datings in the Fuping massif provide new insight into the evolution of the eastern part of the Trans-North China Belt (North China Craton), from 2.7 Ga to 1.8 Ga. D1 is responsible for the development of a dome-and-basin structure coeval with crustal melting giving rise to migmatite and Nanying gneissic granites at 2.1 Ga. This dome-and-basin architecture resulted from the interference between a N–S compression of a weak ductile crust and gravity-driven vertical flow, in a high thermal regime. The next events involved flat lying ductile thrusting (D2) and normal faulting (D3) dated at around 1880 Ma and 1830 Ma, respectively. The D2 and D3 events belong to the Trans-North China Orogeny that results in the final amalgamation of the North China Craton. The D1 deformation is considered as evidence for an earlier orogen developed around 2.1 Ga prior to the Trans-North China Orogeny. The change in the deformation style between the 2.1 Ga and 1.8 Ga could be viewed as a consequence of the cooling of the continental crust in the North China Craton.
ISSN: 0191-8141

insu-00309553
https://hal-insu.archives-ouvertes.fr/insu-00309553
https://hal-insu.archives-ouvertes.fr/insu-00309553/document
https://hal-insu.archives-ouvertes.fr/insu-00309553/file/Trap-JStructuralGeology-2008.pdf
DOI : 10.1016/j.jsg.2008.05.001

Éditeur(s) : HAL CCSD Nature Publishing Group
Résumé : International audience
Lateral variations along the Himalayan arc are suggested by an increasing number of studies and carry important information about the orogen’s segmentation. Here we compile the hitherto most complete land gravity dataset in the region which enables the currently highest resolution plausible analysis. To study lateral variations in collisional structure we compute arc-parallel gravity anomalies (APaGA) by subtracting the average arc-perpendicular profile from our dataset; we compute likewise for topography (APaTA). We find no direct correlation between APaGA, APaTA and background seismicity, as suggested in oceanic subduction context. In the Himalayas APaTA mainly reflect relief and erosional effects, whereas APaGA reflect the deep structure of the orogen with clear lateral boundaries. Four segments are outlined and have disparate flexural geometry: NE India, Bhutan, Nepal & India until Dehradun, and NW India. The segment boundaries in the India plate are related to inherited structures, and the boundaries of the Shillong block are highlighted by seismic activity. We find that large earthquakes of the past millennium do not propagate across the segment boundaries defined by APaGA, therefore these seem to set limits for potential rupture of megathrust earthquakes.
ISSN: 2045-2322

hal-01420404
https://hal.archives-ouvertes.fr/hal-01420404
https://hal.archives-ouvertes.fr/hal-01420404/document
https://hal.archives-ouvertes.fr/hal-01420404/file/hetenyi2016.pdf
DOI : 10.1038/srep33866

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : International audience
The Ganos fault is the westernmost segment of the North Anatolian Fault that experienced the Mw = 7.4 earthquake of 9 August 1912. The earthquake revealed 45-km-long of surface ruptures inland, trending N70 E, and 5.5 m of maximum right lateral offset near Güzelköy. The long-term deformation of the fault is clearly expressed by several pull-apart basins and sag ponds, pressure and shutter ridges and offset streams. In parallel with detailed geomorphologic investigations, we measured co-seismic and cumulative displacements along the fault, and selected the Güzelköy site for paleoseismology. A microtopographic survey at the site yields 10.5 0.5 m and 35.4 1.5 m cumulative lateral offsets of stream channels and geomorphologic features. Seven paleoseismic parallel and cross-fault trenches document successive faulting events and provide the timing of past earthquakes on the Ganos fault segment. Radiocarbon dating of successive colluvial wedges in trench T1, and the fresh scarplet above (probably 1912 surface rupture) indicate the occurrence of three faulting events since the 14th century. Parallel trenches (3, 5, 6 and 7) expose paleo-channels and show a cumulative right-lateral offset of 16.5 1.5 m next to the fault, and 21.3 1.5 m total channel deflection. Radiocarbon dating of past channel units and fault scarp-related colluvial deposits imply an average 17 +/ 5 mm/year slip rate and 323 142 years recurrence interval of large earthquakes during the last 1000 years on the Ganos fault. The succession of past faulting events and inferred slip rate west of the Marmara Sea provide more constraint on the long-term faulting behavior in the seismic gap of the North Anatolian Fault and may contribute to a better seismic hazard assessment in the Istanbul region.
ISSN: 1525-2027

hal-00748218
https://hal.archives-ouvertes.fr/hal-00748218
DOI : 10.1029/2011GC003960

Éditeur(s) : HAL CCSD Geological Society of London
Résumé : International audience
In Eastern China, the North China Block (NCB) has experienced a complex tectonic evolution during Late Palaeozoic to Mesozoic times. The unconformable sedimentary cover, which ranges in age from Neoproterozoic to Permian, underwent two tectonic episodes in Early Triassic and Cretaceous times. An early north–south compressional phase, D1, characterized by north-verging recumbent folds with east–west-trending axes and top-to-the-north ductile shear zones can be observed in a gabbroic pluton and the Neoproterozoic sedimentary host rocks. The available radiometric ages allow us to interpret this event as an Early Triassic back-thrusting related to the final stage of the collision between the North China and South China Blocks. During the LateMesozoic, an extensional event characterized by (1) half-grabens filled by continental terrigeneous red beds and lava flows, (2) low-angle detachment faults, (3) synkinematic granitic plutons, and (4) metamorphic core complexes is widespread in the Eastern Liaoning Peninsula. This extensional D2 event can be subdivided into a ductile deformation and a brittle deformation, corresponding to different expressions of crustal deformation. In every area studied in the Eastern Liaoning Peninsula, the ductile D2 deformation is characterized by a NW–SE-trending stretching lineation with a top-to-the-NWsense of shear.Mica and amphibole fromthe metamorphic country rocks, granodioritic or monzogranitic plutons and their mylonitized margins yield 40Ar/39Ar ages ranging from 130 to 120 Ma. These dates support fast cooling and exhumation rates coeval with the extensional tectonics. The brittle D2 deformation is responsible for the formation of high-angle normal faults marked by low-temperature cataclasites bounding half-grabens. During the Mesozoic, the tectonic regime of Eastern China experienced a significant inversion from compression to extension. The Eastern Liaoning Peninsula massif provides an example of this geodynamic transition.
ISSN: 2041-4927

hal-00134055
https://hal-insu.archives-ouvertes.fr/hal-00134055
DOI : 10.1144/SP280.7

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
ISSN: 0010-7999

hal-01664653
https://hal.archives-ouvertes.fr/hal-01664653
DOI : 10.1007/s00410-017-1394-5

Éditeur(s) : HAL CCSD Wiley-Blackwell
Résumé : International audience
Recent field work and review of radiometric data obtained from Neogene lavas and plutonic rocks exposed in the Eastern Central Patagonian Cordillera (46–48ºS), which overlie subducted segments of the South Chile Ridge, suggest important Late Miocene to Pleistocene morphological changes in relation to base level variations and/or tectonic events. We present new field observations from a region south of Lago General Carrera-Buenos Aires, between the main Cordillera and the Meseta del Lago Buenos Aires, demonstrating that normal faulting controlled valley incisions and occurred during lava emplacement at 5–4 Ma and after 3 Ma. We also show that the 12 Ma basaltic flows of the Meseta del Lago Buenos Aires (~2000 m a.s.l.) have been subjected to deep incision, with younger lavas dated at 1.2 Ma partially filling the valleys. These incisions are thought to reflect progressive eastward tilting of the entire meseta. Our new observations, together with additional features from Central Patagonia, strongly suggest that tectonic events led to a regional widespread morphological change after 5 Ma. The coincidence in time and space between the subduction of segments of the South Chile Ridge at 6 and 3 Ma causing opening of a slab window, and strong base level variations in the studied area, suggests a cause-and-effect relationship. In Central Patagonia, compressional tectonics ended well before extensional events reported here. Causes of uplift and further extension are probably completely disconnected. The uplift is purely tectonic in origin and occurred prior to the subduction of the South Chile Ridge. Extension should be a consequence of this subduction.
ISSN: 0954-4879

insu-00272075
https://hal-insu.archives-ouvertes.fr/insu-00272075
DOI : 10.1111/j.1365-3121.2007.00766.x

Éditeur(s) : HAL CCSD Seismological Society of America
Résumé : International audience
On 9 October 2007, an unusual sequence of earthquakes began in central British Columbia about 20 km west of the Nazko cone, the most recent (circa 7200 yr) volcanic center in the Anahim volcanic belt. Within 25 hr, eight earthquakes of magnitude 2.3-2.9 occurred in a region where no earthquakes had previously been recorded. During the next three weeks, more than 800 microearthquakes were located (and many more detected), most at a depth of 25-31 km and within a radius of about 5 km. After about two months, almost all activity ceased. The clear P- and S-wave arrivals indicated that these were high-frequency (volcanic-tectonic) earthquakes and the b value of 1.9 that we calculated is anomalous for crustal earthquakes but consistent with volcanic-related events. Analysis of receiver functions at a station immediately above the seismicity indicated a Moho near 30 km depth. Precise relocation of the seismicity using a double-difference method suggested a horizontal migration at the rate of about Graphic, with almost all events within the lowermost crust. Neither harmonic tremor nor long-period events were observed; however, some spasmodic bursts were recorded and determined to be colocated with the earthquake hypocenters. These observations are all very similar to a deep earthquake sequence recorded beneath Lake Tahoe, California, in 2003-2004. Based on these remarkable similarities, we interpret the Nazko sequence as an indication of an injection of magma into the lower crust beneath the Anahim volcanic belt. This magma injection fractures rock, producing high-frequency, volcanic-tectonic earthquakes and spasmodic bursts.
ISSN: 0037-1106

hal-00854504
https://hal.archives-ouvertes.fr/hal-00854504
DOI : 10.1785/0120100013

Éditeur(s) : HAL CCSD Bureau de recherches géologiques et minières
Résumé : Pour calculer la probabilité d'un futur séisme partiellement destructeur...
ISSN: 0292-8477

hal-00854040
https://hal.archives-ouvertes.fr/hal-00854040

Éditeur(s) : HAL CCSD American Geophysical Union (AGU)
Résumé : Structural, metamorphic, and geochronologic work shows that the Ampelos/Dilek nappe of the Cycladic blueschist unit in the eastern Aegean constitutes a wedge of high-pressure rocks extruded during early stages of orogeny. The extrusion wedge formed during the incipient collision of the Anatolian microcontinent with Eurasia when subduction and deep underthrusting ceased and the Ampelos/Dilek nappe was thrust southward over the greenschist-facies Menderes nappes along its lower tectonic contact, the Cycladic-Menderes thrust, effectively cutting out a ∼30- to 40-km-thick section of crust. The upper contact of the Ampelos/Dilek extrusion wedge is the top-to-the-NE Selçuk normal shear zone, along which the Ampelos/Dilek nappe was exhumed by ∼30–40 km. Detailed Rb-Sr and 40Ar/39Ar dating of mylonites demonstrates that both shear zones operated between 42 and 32 Ma. There is no evidence for episodic motion during the ∼10 Myr life span of the shear zones, suggesting that both shear zones operated in a steady, nonepisodic fashion. Our data provide supporting evidence that simultaneous thrust-type and normal sense shearing can accomplish the early exhumation of deep-seated rocks
ISSN: 0278-7407

hal-00405013
https://hal.archives-ouvertes.fr/hal-00405013
DOI : 10.1029/2005TC001872

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Ukrainian shield hosts two Palaeoproterozoic anorthosite-mangerite-charnockite-granite (AMCG) complexes (the Korosten and Korsun-Novomyrhorod complexes) that intruded Palaeoproterozoic continental crust in north-western and central parts of the shield, respectively. We report results of U-Pb zircon and baddeleyite dating of 16 samples from the Korosten plutonic complex (KPC), and 6 samples from the Korsun-Novomyrhorod plutonic complex (KNPC). Fifteen zircon samples from both complexes were also analysed for Hf isotopes. These new, together with previously published data indicate that the formation of the KPC started at c. 1815 Ma and continued until 1743 Ma with two main phases of magma emplacement at 1800–1780 and 1770–1758 Ma. Each of the main phases of magmatic activity included both basic and silicic members. The emplacement history of the KNPC is different from that of the KPC. The vast majority of the KNPC basic and silicic rocks were emplaced between c. 1757 and 1750 Ma; the youngest stages of the complex are represented by monzonites and syenites that were formed between 1748 and 1744 Ma. Both Ukrainian AMCG complexes are closely associated in space and time with mantle-derived mafic and ultramafic dykes. The Hf isotope ratios in the zircons indicate a predominantly crustal source for the initial melts with some input of juvenile Hf from mantle-derived tholeiite melts.The preferred model for the formation of the Ukrainian AMCG complexes involves the emplacement of large volumes of hot mantle-derived tholeiitic magma into the lower crust. This resulted in partial melting of mafic lower-crustal material, mixing of lower crustal and tholeiitic melts, and formation of ferromonzodioritic magmas. Further fractional crystallization of the ferromonzodioritic melts produced the spectrum of basic rocks in the AMCG complexes. Emplacement of the ferromonzodioritic and tholeiitic melts into the middle crust and their partial crystallization caused abundant melting of the ambient crust and formation of the large volumes of granitic rocks present in the complexes.
ISSN: 0301-9268

hal-01622715
https://hal.archives-ouvertes.fr/hal-01622715
DOI : 10.1016/j.precamres.2017.02.009

Éditeur(s) : HAL CCSD Elsevier
Résumé : Grain-size analyses, coupled with end-member modelling, have been performed on the terrigenous fraction of two Leg 184 Ocean Drilling Program sites (1144 and 1146) from the South China Sea. The grain-size distributions over the last 1.8 Ma enable a new interpretation of their connections to sea-level variations and East Asian monsoon strength. Previous investigations in this area have associated grain-size variability with enhanced eolian input during glacial stages. End-member modelling downgrades the importance of this eolian contribution and indicates that the sediments can be described as a mixture of three end-members: fluvial mud inputs, shelf reworking and river mouth migration. Grain-size variations in the Pleistocene section of the cores indicate a multiple-stage evolution: (i) from 1.8 to 1.25 Ma, the downcore grain-size variations are low but show a correspondence between monsoon rainfall intensity and the fine grain-sized fluvial inputs; no link with sea-level variations is noticeable; (ii) from 1.25 to 0.9 Ma, there is an increase (decrease) in the intermediate (fine) end-member (not, vert, similar 100 kyr cycle) that is associated with the onset of a stronger summer monsoon and modest shelf reworking; (iii) from 0.9 to 0 Ma the grain-size record is dominated by global sea-level variations; each glacial stage is associated with extensive shelf reworking and conveyance of coarse particles to the basin.
ISSN: 0033-5894

hal-00404671
https://hal.archives-ouvertes.fr/hal-00404671
DOI : 10.1016/j.yqres.2007.03.004

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The South Liaodong Peninsula massif is the easternmost Mesozoic Metamorphic Core Complex, recognized in Eastern China. It provides a good example of the combination of ductile shearing, synkinematic plutonism and polyphase exhumation. The Jurassic granodioritic plutons, located at the footwall of the detachment normal fault, and dated here at ca 160 Ma, recorded two different phases of cooling. A slow cooling regime of about 3-10°C/my prevailing before 122 Ma, was followed by a significant increase in cooling rate of about 40-55°C/my after that time. By contrast, a single fast cooling path was recorded by the Cretaceous monzogranite situated in the footwall of the detachment normal fault. This result indicates that the Jurassic and Cretaceous plutons recorded different exhumation processes: a Jurassic slow or negligible exhumation and a Cretaceous fast one assisted by normal faulting. These two cooling stages correspond to distinct geodynamic processes during the Jurassic and Cretaceous. Extensional tectonics seems not significant before Early Cretaceous. The second stage, dominated by an extensional regime which develops after ca 120 Ma, is tentatively correlated to the lithosphere removal of the North China Craton.
ISSN: 1367-9120

Droits : http://creativecommons.org/licenses/by-nc-nd/
insu-00528171
https://hal-insu.archives-ouvertes.fr/insu-00528171
https://hal-insu.archives-ouvertes.fr/insu-00528171/document
https://hal-insu.archives-ouvertes.fr/insu-00528171/file/Lin-JAsianEarthSciences-2010.pdf
DOI : 10.1016/j.jseaes.2010.09.007

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Kazdag Massif exposes an extensional metamorphic dome in the Biga Peninsula of northwest Turkey, bounded on both flanks by detachments and/or shear zones. The northern flank is marked by the extensional Alakeci shear zone (ASZ), with poorly known P-T-t path. We therefore focus on metamorphic conditions and temporal history of the ASZ to determine its tectono-metamorphic evolution. ASZ mylonites were derived from both the footwall Kazdag Massif core metamorphic rocks and the hanging wall Cetmi melange lithologies. The mylonitic fabrics in the ASZ depict a top-to-the NNE shearing, parallel to the NNE-plunging stretching lineation and NNW-dipping mylonitic foliation. This geometry implies normal sense movement i.e. north-side down-dip extensional displacement along this flank of the Kazdag Massif. The northward transition from ductile to brittle-ductile shear regime through the ASZ shows that the non-coaxial deformation occurred at decreasing temperatures and degree of metamorphism. The paragenesis in equilibrium of the mylonitic gneisses and schists contains quartz + feldspar + muscovite + biotite + garnet staurolite, with late retrogressive chlorite after biotite and garnet. Calculated thermo-barometric conditions for ASZ rocks yielded pressures between 6.9 and 5.7 kbar and temperatures between 706 and 587 degrees C, which constrain the peak metamorphism. The mylonitic rocks supplied isochron Ar-39/Ar-40 mica ages between 26.97 and 24.19 Ma, which we interpret to date the cooling of the mylonites following the P-T decrease across the ASZ. The metamorphic data support the structural record and strengthen the extensional character of the ASZ. The transition from sillimanite-bearing footwall Kazdag Massif core gneisses in the deeper structural levels to chlorite schists; towards the shallow structural levels at the top of the ASZ, is consistent with an extensional exhumation of the metamorphic pile. These data allow us to determine the peak P-T conditions and the temporal evolution in the northern flank of the Kazdag Massif, where Late Oligocene extensional exhumation was assisted by NNE-directed ductile-brittle ASZ, which had operated from amphibolite to greenschist facies. The ASZ, together with similar time-constrained ductile-brittle activity of SW-directed detachment along the southern flank of the massif, collectively indicates a bivergent mode of extension in the western Kazdag Massif. The latter is likely influenced by magmatic activity spatially and temporally related to these extensional zones. At the regional scale, the bivergent tectono-metamorphic pattern of the Kazdag Massif is similar to those observed on other places of the north-central Aegean domain.
ISSN: 0024-4937

hal-00445246
https://hal.archives-ouvertes.fr/hal-00445246
DOI : 10.1016/j.lithos.2009.02.010

Éditeur(s) : HAL CCSD Geological Society of America
Résumé : If the net flux to the island arc crust is primitive arc basalt, the evolved composition of most arc magmas entails the formation of complementary thick ultramafic keels at the root of the island arc crust. Dunite, wehrlite, and Cr-rich pyroxenite from the Jijal complex, constituting the Moho transition zone of the Kohistan paleo–island arc (northern Pakistan), are often mentioned as an example of high-pressure cumulates formed by intracrustal fractionation of mantle-derived melts, which were later extracted to form the overlying mafic crust. Here we show that calculated liquids for Jijal pyroxenites-wehrlites are strongly rare earth element (REE) depleted and display flat or convex-upward REE patterns. These patterns are typical of boninites and are therefore unlike those of the overlying mafic crust that have higher REE concentrations and are derived from light rare earth element (LREE)–enriched melts similar to island arc basalt. This observation, along with the lower 208Pb/204Pb and 206Pb/204Pb ratios of Jijal pyroxenites-wehrlites relative to gabbros, rejects the hypothesis that gabbros and ultramafic rocks derive from a common melt via crystal fractionation. In the 208Pb/204Pb versus 206Pb/204Pb diagram, ultramafic rocks and gabbros lie on the same positive correlation, suggesting that their sources share a common enriched mantle 2 (EM2) signature but with a major depleted component contribution for the ultramafic rocks. These data are consistent with a scenario whereby the Jijal ultramafic section represents a Moho transition zone formed via melt-rock reaction between subarc mantle and incoming melt isotopically akin to Jijal gabbroic rocks. The lack in the Kohistan arc of cogenetic ultramafic cumulates complementary to the evolved mafic plutonic rocks implies either (1) that a substantial volume of such ultramafic cumulates was delaminated or torn out by subcrustal mantle flow from the base of the arc crust in extraordinarily short time scales (0.10–0.35 cm/yr), or (2) that the net flux to the Kohistan arc crust was more evolved than primitive arc basalt.
ISSN: 0091-7613

hal-00406688
https://hal.archives-ouvertes.fr/hal-00406688
DOI : 10.1130/G23675A.1