Paleomagnetic and plate tectonic constraints on the movement of Tibet

The paleomagnetic results from Tibet, north of the Yarlung-Zang bo suture zone, show that Tibet was at about 15°–20°N in Middle Cretaceous time. It then moved south down to 7°–10°N in the Late Cretaceous-Paleogene. The oceanic crust of the Xigaze ophiolites was magnetized at 13°N but thereafter migrated further south. This movement is compared with the relative movement of India and Asia as deduced from magnetic anomalies and paleomagnetism. Experimental models on deformation help us to explain how Tibet moved during the Late Cretaceous under the constraint of the Africa-Arabia indenter and during the Upper Tertiary under the constraint of the Indian indenter.     

Magnetic overprints in granitoids and metamorphic rocks from Limousin (France): evidence for Late Variscan rotations, crustal folding and tilting

Generally, the lack of bedding criteria in basement units hampers the interpretation of paleomagnetic results in terms of geotectonics. Nevertheless, this work demonstrates that successive remagnetizations recorded in Early Carboniferous metamorphic and plutonic units, without clear bedding criteria, can be used to constrain a polyphased tectonic evolution consisting of a regional clockwise rotation, followed by a folding phase, a tilting phase and a second regional clockwise rotation.Metamorphic, ultrabasic, tonalitic and granitic rocks from different parts of Limousin (western French Massif central; 45.5°N/1.25°E), which underwent metamorphism during Devonian–Early Carboniferous or were intruded in the Early–Middle Carboniferous, were sampled in order (a) to identify the magnetic overprinting phases and the related tectono-magmatic events and (b) to constrain the regional and plate tectonic evolution of Limousin. Paleomagnetic results from 32 new and 26 sites investigated previously show that at least 90% of the magnetization isolated in rocks older than 330 Ma are overprints. In agreement with results from adjacent areas of the Variscan belt, the major overprinting phases occurred: (a) in the last stages of the major exhumation phase [332–328 Ma; mean Virtual Geomagnetic Pole (VGP) “Cp”: 37°N/70.5°E], (b) during the post-collisional syn-orogenic extension (325–315 Ma; VGP “B”: 11°N/114°E), (c) in the Latest Carboniferous and Early Permian (VGP “A1”: 27°N/149°E) and (d) in the Late Permian (VGP “A”: 48°N/146°E). The Middle–Late Carboniferous overprints “Cp” and “B” are contemporaneous with emplacement of leucogranitic, crustal derived plutons, and probably result from the hydro-thermal activity related to the magmatism. The drift from “Cp” directions to “B” directions implies that after 330 Ma, Limousin underwent a clockwise rotation by 65°, together with the Central Europe Variscides. The “Bt” components, the VGPs of which deviate from the mean apparent polar wander path (APWP) of the belt, are interpreted as “B” overprints tilted during Late Variscan tectonics, that is, in the time range 325–315 Ma. The first and most important generation of “Bt” overprints was tilted during NW–SE folding associated with NE–SW shortening, updoming and emplacement of leucogranitic plutons. The second generation reveals southeastward tilting due to NE-striking normal faulting. The drift from “B” to “A1” directions implies that Limousin has participated to the second clockwise rotation by 40° of the whole belt in Westphalian times.     

Seismological Evidences for the Multiple Incomplete Crustal Subductions in Himalaya and Southern Tibet

SEISMOLOGICAL EVIDENCES FOR THE MULTIPLE INCOMPLETE CRUSTAL SUBDUCTIONS IN HIMALAYA AND SOUTHERN TIBET     

喜马拉雅西部雅鲁藏布江缝合带地壳尺度的构造叠置

陆陆碰撞过程是板块构造缺失的链条。印度板块与亚洲板块的碰撞造就了喜马拉雅造山带和青藏高原的主体。然而,人们对印度板块在大陆碰撞过程中的行为尚不了解。如大陆碰撞及其碰撞后的大陆俯冲是如何进行的、印度板块是俯冲在青藏高原之下还是回转至板块上部(喜马拉雅造山带内)以及两者比例如何,这些仍是亟待解决的问题。印度板块低角度沿喜马拉雅主逆冲断裂(MHT)俯冲在低喜马拉雅和高喜马拉雅之下已经被反射地震图像很好地揭示。然而,关于MHT如何向北延伸,前人的研究仅获得了分辨率较低的接收函数图像。因而,MHT和雅鲁藏布江缝合带之间印度板块的俯冲行为仍是一个谜。喜马拉雅造山楔增生机制,也就是印度地壳前缘的变形机制,反映出物质被临界锥形逆冲断层作用转移到板块上部,或是以韧性管道流的样式向南溢出。在本次研究中,我们给出在喜马拉雅造山带西部地区横过雅鲁藏布江缝合带的沿东经81.5°展布的高分辨率深地震反射剖面,精细揭示了地壳尺度结构构造。剖面显示,MHT以大约20°的倾斜角度延伸至大约60 km深度,接近埋深为70~75 km的Moho面。越过雅鲁藏布江缝合带运移到北面的印度地壳厚度已经不足15 km。深地震反射剖面还显示中地壳逆冲构造反射发育。我们认为,伴随着印度板块俯冲,地壳尺度的多重构造叠置作用使物质自MHT下部的板块向其上部板块转移,这一过程使印度地壳厚度减薄了,同时加厚了喜马拉雅地壳。     

Paleomagnetic evidence for post-Cretaceous internal deformation of the Chuan Dian Fragment in the Yangtze block: a consequence of indentation of India into Asia

Paleomagnetic samples of Paleocene–Eocene red sandstones were collected at 36 sites from the Jiangdihe-4 and Zhaojiadian formations around the Yongren (26.1°N, 101.7°E) and Dayao areas (25.7°N, 101.3°E). These areas are located in the Chuxiong basin of the Chuan Dian Fragment, southwestern part of the Yangtze block. After stepwise thermal demagnetization, a high-temperature component with unblocking temperature of about 680 °C is isolated from 26 sites. The primary nature of this magnetization is ascertained through positive fold and reversal tests at 95% confidence level. The tilt-corrected mean paleomagnetic directions for the Yongren and Dayao areas are D=17.2°, I=26.6° with α₉₅=5.8° and D=16.5°, I=31.1° with α₉₅=4.8, respectively. Easterly deflected declinations from this study are consistent with those reported from other areas of the Chuxiong basin, indicating its wide presence in the Cretaceous–Eocene formations of the said basin. Comparison with declination values expected from the Cretaceous–Eocene APWP of Eurasia indicates that the magnitude of clockwise rotation systematically increases toward the southeast within the Chuxiong basin as well as in the Chuan Dian Fragment. This trend of the differential tectonic rotation in the Chuan Dian Fragment is consistent with curvature of the Xianshuihe–Xiojiang fault system. Deformation of the Chuxiong basin can fairly be associated with the formation of eastward bulge in the southern part of the Chuan Dian fragment. During southward displacement, the Chuan Dian Fragment was probably subjected to tectonic stresses as a result interaction with the Yangtze and Indochina blocks, which resulted into east–west extension and north–south shortening.     

Multicomponent magnetization in western Dolpo (Tethyan Himalaya, Nepal): tectonic implications

A palaeomagnetic study has been carried out in the Tethyan Himalaya (TH; the northern margin of Greater India). Twenty-six palaeomagnetic sites have been sampled in Triassic low-grade metasediments of western Dolpo. Two remanent components have been identified. A pyrrhotite component, characterized by unblocking temperatures of 270–335 °C, yields an in situ mean direction of D=191.7°, I=−30.9° (k=29.5, α₉₅=5.7°, N=23 sites). The component fails the fold test at the 99% confidence level (k_{in situ}/k_bed=6.9) and is therefore of postfolding origin. For reason of the low metamorphic grade, this pyrrhotite magnetization is believed to be of thermo-chemical origin. Geochronological data and inclination matching indicate an acquisition age around 35 Ma. The second remanence component has higher unblocking temperatures (>400 °C and up to 500–580 °C range) and resides in magnetite. A positive fold test and comparison with expected Triassic palaeomagnetic directions suggest a primary origin.The postfolding character of the pyrrhotite component, and its interpreted age of remanence acquisition, implies that the main Himalayan folding is older than 35 Ma in the western Dolpo area. This study also suggests that the second metamorphic event (Neo-Himalayan) was more significant in the Dolpo area than the first (Eo-Himalayan) one.A clockwise rotation of 10–15° is inferred from the pyrrhotite component, which is compatible with oroclinal bending and/or rotational underthrusting models. This rotation is also supported by the magnetite component, indicating that no rotation of the Tethyan Himalaya relative to India took place before 35 Ma.     

Palaeomagnetic data from the precambrian Gwalior traps, Central India

From alternating-field and thermal demagnetization studies on two dolerite “Traps” in the Gwalior Series (Central India), dated at 1830 ±200 m.y., three different palaeomagnetic directions could be distinguished. The characteristic magnetization component, which is considered as the primary magnetization, has a mean direction: D=78°, I=+34.5°, α₉₅=5°, k=369, N=4 (Pole): 155.5°E19°N, dp=3°, dm=5.5°.A comparison of the presented data with other Precambrian and Phanerozoic data from the Indian subcontinent might suggest that the Indian subcontinent underwent a continuous anticlockwise rotational movement during the last 1800 m.y.     

Focal mechanism solution of the Himalayan earthquake of February 14, 1977

Through a closely spaced local network of seismic stations in Himachal Pradesh, India, supplemented by worldwide P-wave first-motion data, the source mechanism of the February 14, 1977 earthquake which occurred very close to the Rawalpindi area in Pakistan has been determined. The fault-plane solution as reported earlier for this event by Seeber and Armbruster (1979) showed thrust faulting. The reliability of their solution has been tested using more P-wave first-motion data from near Indian stations within the epicentral distance of 4°–7°, as well as from distant stations. The inclusion of data from these stations completely changed the type of faulting from thrust to normal type. The new solution parameters have been briefly discussed in relation to the local geological faults/thrusts.     

The Himalayas

After splitting from Gondwanaland, India drifted northwards to collide with the Asian landmass about 40 million years ago. The intervening Tethys ocean was closed by northwards subduction beneath southern Tibet, and the collision created the Himalayan orogenic belt. Continuing northward movement of India at a rate of about 5 cm per year over the last 40 million years has caused it to indent Asia, and the resultant massive shortening is expressed by thrusting of the northern margin of India, by faulting and earthquakes in the Himalayas and China, by rifting and faulting in Tibet, and by the uplift of the Himalayas which is still continuing at rates of up to several millimetres per year.     

Duplex structures in the lewis thrust sheet, crowsnest pass, rocky mountains, Alberta, Canada

The Lewis thrust sheet of the southern Canadian Rocky Mountains contains many spectacular examples of small-scale duplex structures. This paper presents the results of a detailed analysis of such structures found in the Mississippian carbonates of the Banff Formation at Crowsnest Pass, southwestern Alberta.Foreland dipping, hinterland dipping and antiformal stacked duplexes are found in the hangingwall of the Lewis thrust. Out-of-sequence thrusts, back thrusts and folds that push out of the plane of the cross-section, termed lateral lobes, give rise to complex internal geometries. Dominant slip vectors are towards 080–090° but the complex fault geometries have generated significant variations in slip away from this direction. The duplex structures occur as discrete thrust fault-bounded packages with each package having different slip vectors. The panels above and below the duplex structures show consistent slip vectors towards 080–090° whereas the duplexes exhibit a wide scatter of slip vectors from 350–160°. The stacking of duplexes with many horses can be likened to the stacking of many inverted soup bowls, herein termed turtle back structures, and will involve a wide scatter of slip directions, particularly if the horses are of limited lateral extent. Such a stacking mechanism involving out-of-section movement invalidates the assumption of two-dimensional plane strain in the plane of the cross-section that contains the regional tectonic transport direction. Correctly balanced cross-sections cannot be constructed through such stacked duplex structures as described in this paper.     

Remagnetization in the Tennessee salient, Southern Appalachians, USA: Constraints on the timing of deformation

The Appalachian fold–thrust belt is characterized by a sinuous trace in map-view, creating a series of salients and recesses. The kinematic evolution of these arcuate features remains a controversial topic in orogenesis. Primary magnetizations from clastic red beds in the Pennsylvania salient show Pennsylvanian rotations that account for about half of the curvature, while Kiaman-aged (Permian) remagnetizations display no relative rotation between the limbs. The more southern Tennessee salient shows a maximum change in regional strike from ~ 65° in Virginia to ~ 10° in northern Georgia. Paleomagnetic results from thirty-two sites in the Middle to Upper Ordovician Chickamauga Group limestones and twenty sites from the Middle Cambrian Rome Formation red beds were analyzed to constrain the relative age of magnetization as well as the nature of curvature in the Tennessee salient. Results from three sites of the Silurian Red Mountain Formation were added to an existing dataset in order to determine whether the southern limb had rotated.After thermal demagnetization, all three sample suites display a down and southeasterly direction, albeit carried by different magnetic minerals. The syn-tilting direction of the Chickamauga limestones lies on the Pennsylvanian segment of the North American apparent polar wander path (APWP), indicating that deformation was about half completed by the Late Pennsylvanian. The Rome and Red Mountain Formations were also remagnetized during the Pennsylvanian. Both the Chickamauga limestones and Rome red beds fail to show a correlation between strike and declination along the salient, suggesting either that the salient was a primary, non-rotational feature or that secondary curvature occurred prior to remagnetization, as it did in Pennsylvania. Moreover, remagnetized directions from the Red Mountain sites show no statistical difference between the southern limb of the salient and the more northeasterly trending portion of the fold–thrust belt in Alabama. Thus, all of the studied units in the Tennessee salient are remagnetized and show no evidence for rotation. This confirms that remagnetization was widespread in the southern Appalachians and that any potential orogenic rotation must have occurred prior to the Late Pennsylvanian.     

Paleomagnetic versus GPS determined tectonic rotation around eastern Himalayan syntaxis in East Asia

Northward indentation of the Indian Plate has brought about significant tectonic deformation into East Asia. A record of long-term tectonic deformation in this area for the past 50 M yr, particularly the vertical axis rotation, is available through paleomagnetic data. In order to depict rotational deformation in this area with respect to Eurasia, we compiled reliable paleomagnetic data sets from 79 localities distributed around eastern Himalayan syntaxis in East Asia. This record delineates that a zone affected by clockwise rotational deformation extends from the southern tip of the Chuan Dian Fragment to as far as the northwestern part of the Indochina Peninsula. A limited zone that experienced a significant amount of clockwise rotation after an initial India–Asia collision is now located at 23.5°N, 101°E, far away from an area (27.5°N, 95.5°E) where an intense rotational motion has been viewed by a snapshot of GPS measurements. This discrepancy in clockwise rotated positions is attributed to southeastward extrusion of the tectonic blocks within East Asia as a result of ongoing indentation of the Indian Plate. A quantitative comparison between the GPS and paleomagnetically determined clockwise rotation further suggests that following an initial India–Asia collision the crust at 30°N, 94°E paleoposition was subjected to southeastward displacement together with clockwise rotation, which eventually reached to present-day position of 23.5°N, 101°E, implying a crustal displacement of about 1000 km during the past 50 M yr.     

青藏高原现今构造变形特征与GPS速度场

文章以青藏高原的GPS观测数据为基础 ,结合活动地质构造资料 ,研究了青藏高原的现今构造变形状态和机制 ,并探讨青藏高原现今构造变形所反映的大陆内部动力学过程。GPS观测的速度矢量揭示了青藏高原整体向北和向东运动的趋势 ,平行于印度和欧亚板块碰撞方向上的地壳缩短量约是 38mm/a ,而青藏高原周边主要断裂带的滑动速率均在 10mm/a以下。大约 90 %的印度与欧亚板块相对运动量被青藏高原的地壳缩短所吸收和调节。GPS速度矢量由南向北逐渐向东偏转 ,向东的分量也增加 ,形成了以羌塘地块北部 (或玛尼—玉树—鲜水河断裂 )和祁连山中部为中心的两个地壳物质向东流动带。青藏高原的向东挤出实际上是地壳物质在印度板块推挤下和周边刚性地块阻挡下围绕东构造结发生的顺时针旋转。     

Metamorphism and Isotopic Evolution of Granulites of Southern India: Reference to Neoproterozoic Crustal Evolution

Granulites are important component of high-grade metamorphic rocks reflecting intense conditions observed for crustal rocks in terms of temperature, and pressure. This review paper demonstrates how these high-grade granulites are critical to understanding the evolution of the lower continental crust with special reference to southern India. Geothermobarometric traverse across different granulite blocks in southern India shows wide ranging P-T conditions of metamorphism (700–1000 ° C, and 5–10 kbar). The sapphirine-, orthopyroxene-sillimanite, and spinel -bearing quartz-deficient granulites recognised from parts of southern granulite terrain (Ganguvarpatti, Kiranur, and Palani hill ranges etc.) show oriented sillimanite aggregates pseudomorph after course twinned kyanite, staurolite + kyanite assemblages, and corroded blebs of gedrite within orthopyroxene, suggesting a prograde stage of a clockwise P-T evolution. Evidence of ITD history comes from the textures in which an early Mg-rich garnet (X_Mg 52–60) with orthopyroxene (up to 10% Al₂O₃) involving sillimanite breakdown forming variety of symplectites having combinations of orthopyroxene, sapphirine, cordierite, and spinel. These spectacular reaction textures, and mineralogic sensors from the high-grade rocks establish a prograde clockwise P-T-t path with notable decompressive history (ITD) in the southern granulite terrain. The inferred P-T-t paths have been further integrated with the recent geochronological, and isotopic data to constrain the timing, and duration of metamorphism, emplacement of the magmatic protolith for characterising the evolution of the granulites, and their bearing on the geodynamic implications. Based on the emerging evidence for Neoproterozoic tectonothermal imprints in the southern granulite terrain, history of the assembly of dispersed fragments is also addressed within the East Gondwana framework.     

Geological Setting and Controls on the Development of Graphite, Sillimanite and Phosphate Mineralization within the Jiamusi Massif: An Exotic Fragment of Gondwanaland Located in North-Eastern China?

China is the world's biggest producer and exporter of graphite and also possesses the world's largest reserves of crystalline graphite. One of the main production areas is in northeastern China around Jixi, in Heilongjiang Province, where graphite, sillimanite and phosphate form in original sedimentary horizons within the Mashan Group. The Mashan Group is composed of khondalitic metasediments and tectonically interleaved orthogneisses, all metamorphosed to granulite facies. The peak conditions have been calculated at 800–850°C at 5.3–6.2 kb and the P–T–t path defines a tight clockwise loop involving simultaneous decrease in temperature and pressure, falling to 410–490°C at less than 3 kb. This metamorphic event is interpreted as resulting from continent–continent collision, with emplacement of mantle–derived mafic magma and possible delamination at the base of the crust. Metamorphism has been precisely dated at 500 Ma, using U–Pb zircon SHRIMP techniques. It appears likely that this terrane formed part of an originally more extensive orogenic belt of Late Pan–African age. Rocks of similar age are recorded from east Antarctica, Western Australia, India and Sri Lanka, suggesting that the Mashan Group was most likely juxtaposed with these areas within Gondwanaland. The mechanism by which the Mashan Group arrived at its present position between the Siberian and North China Cratons is poorly constrained, but it appears likely that, along with the North China, South China and Tarim Blocks, it underwent a northward drift from an initial location off northern Australia to finally dock with the Siberian Craton in the Late Permian to Late Jurassic.     

Morphological characteristics and emplacement mechanism of the seamounts in the Central Indian Ocean Basin

The morphotectonic features of the Central Indian Ocean Basin (CIOB) provide information regarding the development of the basin. Multibeam mapping of the CIOB reveals presence of abundant isolated seamounts and seamount chains sub-parallel to each other and major fracture zones along 73° E, 79° E and 75°45′ E. Morphological analyses were carried out for 200 seamounts that occur either as isolated edifies or along eight sub-parallel chains. The identified eight parallel seamount chains that trend almost north–south and reflecting the absolute motion of the Indian plate, probably originated from the ancient propagative fractures. Inspite of the differences in their height, the seamounts of these eight chains are morphologically correlatable. In the study area the seamounts are clustered north and south of 12° S latitude. Interestingly, in the area north of 12° S (area II: 9°–12° S) the seamounts are distinctly smaller (≤ 400 m height) whereas, the area south of 12° S (area I: 12°–15° S) has a mixed population of seamounts. The normalized abundance of the CIOB seamount is 976 seamounts/10⁶ km² but on a finer scale this value varies from 500 to 1600 seamounts/10⁶ km², which is less than the seamount concentrations of the Pacific and Atlantic oceans (9000 to 16,000 seamounts/10⁶ km²). Three categories of seamounts are present in the CIOB e.g. (1) single-peaked (2) multi-peaked and (3) composite. The study indicate that single-peaked seamounts are dominant (89%) while multi-peaked is less (8%) and composite ones are rare (3%) in the CIOB.The progressive northward movement of the Indian continent caused collision between India and Asia at around 62 Ma ago. A majority of the near-axis originated seamounts in the CIOB seemed to have formed as a consequence of the temporally widespread (Cretaceous  65 Ma to late Eocene < 49 Ma) collision between India and Eurasia. The regional stress patterns in the Indian plate vary N to NE in the continent and N to NW in Indian Ocean areas. The combined effect of the regional stress patterns maintained the orientation of the seamount chains and the local stress regime helped in the upwelling of magma and formation of seamounts. The low heat flow, morphological features and geochemical signature indicate that the morphotectonic structures formed contemporaneously with the oceanic crust.     

Tectonic implications of a paleomagnetic study of the Sarmiento Ophiolitic Complex, southern Chile

A paleomagnetic study was carried out on the Late Jurassic Sarmiento Ophiolitic Complex (SOC) exposed in the Magallanes fold and thrust belt in the southern Patagonian Andes (southern Chile). This complex, mainly consisting of a thick succession of pillow-lavas, sheeted dikes and gabbros, is a seafloor remnant of the Late Jurassic to Early Cretaceous Rocas Verdes basin that developed along the south-western margin of South America. Stepwise thermal and alternating field demagnetization permitted the isolation of a post-folding characteristic remanence, apparently carried by fine grain (SD?) magnetite, both in the pillow-lavas and dikes. The mean “in situ” direction for the SOC is Dec: 286.9°, Inc: − 58.5°, α95: 6.9°, N: 11 (sites).Rock magnetic properties, petrography and whole-rock K–Ar ages in the same rocks are interpreted as evidence of correlation between remanence acquisition and a greenschist facies metamorphic overprint that must have occurred during latest stages or after closure and tectonic inversion of the basin in the Late Cretaceous.The mean remanence direction is anomalous relative to the expected Late Cretaceous direction from stable South America. Particularly, a declination anomaly over 50° is suggestively similar to paleomagnetically interpreted counter clockwise rotations found in thrust slices of the Jurassic El Quemado Fm. located over 100 km north of the study area in Argentina. Nevertheless, a significant ccw rotation of the whole SOC is difficult to reconcile with geologic evidence and paleogeographic models that suggest a narrow back-arc basin sub-parallel to the continental margin. A rigid-body 30° westward tilting of the SOC block around a horizontal axis trending NNW, is considered a much simpler explanation, being consistent with geologic evidence. This may have occurred as a consequence of inverse reactivation of old normal faults, which limit both the SOC exposures and the Cordillera Sarmiento to the East. The age of tilting is unknown but it must postdate remanence acquisition in the Late Cretaceous. Two major orogenic events of the southern Patagonian Andes, in the Eocene (ca. 42 Ma) and Middle Miocene (ca. 12 Ma), respectively, could have caused the proposed tilting.     

P–T–t conditions of high-grade metamorphic rocks of the Garzon Massif, Andean basement, SE Colombia

The metamorphic evolution of the Garzón Massif, Colombia, is established on the basis of the textural, goethermobarometric, and geochronological relationships of the metamorphic minerals. The geothermobarometric data define a clockwise, nearly isothermal decompression path (ITD) for rocks from Las Margaritas migmatites, constrained by four P–T areas: 780–826 °C and 6.3–8.0 kbar, 760–820 °C and 8.0–8.8 kbar, 680–755 °C and 6.6–9.0 kbar, and 630 °C and 4 kbar. For the a garnet-bearing charnockitic gneiss from the Vergel granulites, the path is counterclockwise, constrained by geothermobarometric data of 5.3–6.2 kbar and 700–780 °C and 6.2–7.2 kbar and 685–740 °C. The clockwise ITD path represents a loop followed by the orogen during the transitional granulite–amphibolite metamorphic conditions, probably associated with a subduction process followed by a collisional tectonic event. This subduction framework produced continental crust thickening between 1148 and 1034 Ma and later collision with another continental block approximately 1000 Ma ago. The orogenic exhumation occurred with moderate uplift rate. The counterclockwise trajectory and two metamorphic events suggest a vertical displacement between the Vergel granulites and Las Margaritas migmatites units, because there is no isotopic difference that indicates the existence of different terranes. The data confirm that the metamorphic evolution for this domain was more dynamic than previously believed and includes: (1) metamorphic processes with the generation of new crust with a possible mixture of old material and (2) metamorphic recycling of continental crust. These geological processes characterize a complex Mesoproterozoic orogenic event that shares certain features with the Grenvillian basement rocks participating in the formation of Rodinia.     

Intraplate mountain building in response to continent–continent collision—the Ancestral Rocky Mountains (North America) and inferences drawn from the Tien Shan (Central Asia)

The intraplate Ancestral Rocky Mountains of western North America extend from British Columbia, Canada, to Chihuahua, Mexico, and formed during Early Carboniferous through Early Permian time in response to continent–continent collision of Laurentia with Gondwana—the conjoined masses of Africa and South America, including Yucatán and Florida. Uplifts and flanking basins also formed within the Laurentian Midcontinent. On the Gondwanan continent, well inboard from the marginal fold belts, a counterpart structural array developed during the same period. Intraplate deformation began when full collisional plate coupling had been achieved along the continental margin; the intervening ocean had been closed and subduction had ceased—that is, the distinction between upper versus lower plates became moot. Ancestral Rockies deformation was not accompanied by volcanism. Basement shear zones that formed during Mesoproterozoic rifting of Laurentia were reactivated and exerted significant control on the locations, orientations, and modes of displacement on late Paleozoic faults.Ancestral Rocky Mountain uplifts extend as far south as Chihuahua and west Texas (28° to 33°N, 102° to 109°W) and include the Florida-Moyotes, Placer de Guadalupe–Carrizalillo, Ojinaga–Tascotal and Hueco Mountain blocks, as well as the Diablo and Central Basin Platforms. All are cored with Laurentian Proterozoic crystalline basement rocks and host correlative Paleozoic stratigraphic successions. Pre-late Paleozoic deformational, thermal, and metamorphic histories are similar as well. Southern Ancestral Rocky Mountain structures terminate along a line that trends approximately N 40°E (present coordinates), a common orientation for Mesoproterozoic extensional structures throughout southern to central North America.Continuing Tien Shan intraplate deformation (Central Asia) has created an analogous array of uplifts and basins in response to the collision of India with Eurasia, beginning in late Miocene time when full coupling of the colliding plates had occurred. As in the Laurentia–Gondwana case, structures of similar magnitude and spacing to those in Eurasia have developed in the Indian plate. Within the present orogen two ancient suture zones have been reactivated—the early Paleozoic Terskey zone and the late Paleozoic Turkestan suture between the Siberian and East Gondwanan cratons. Inverted Proterozoic to early Paleozoic rift structures and passive-margin deposits are exposed north of the Terskey zone. In the Alay and Tarim complexes, Vendian to mid-Carboniferous passive-margin strata and the subjacent Proterozoic crystalline basement have been uplifted. Data on Tien Shan uplifts, basins, structural arrays, and deformation rates guide paleotectonic interpretations of ancient intraplate mountain belts. Similarly, exhumed deep crustal shear zones in the Ancestral Rockies offer insight into partitioning and reorientation of strain during contemporary intraplate deformation.     

Seminar on geodynamics of the Himalayan region: Harsh. K. Gupta (Editor). National Geophysical Research Institute, India, 1973, 221 p., US $ 8.00

For a detailed palaeomagnetic research on Upper Permian red beds in the Wardha Valley (Central India) 265 samples from 47 sites at 6 localities were investigated.The samples from 3 localities (17 sites) appeared to be completely remagnetized during Early Tertiary times by the vast Deccan Trap flood basalts effusions. The samples from 22 sites of the other three localities (results from 8 sites rejected) could become cleaned from hard secondary Deccan Trap components by detailed thermal demagnetization.The resulting primary magnetization component reveals a mean direction (regardless of polarity, 7 sites normal, 15 sites reversed): D = 101.5°, I = +58.5°, α₉₅ = 6.5°, N = 3. This mean direction corresponds to a pole position at 129° W 4° N (dp = 7°, dm = 9.5°). This pole position fits well with other acceptable Late Permian—Early Triassic pole positions for the Indian subcontinent. From these acceptable results, a mean Permo-Triassic pole for the Indian subcontinent was computed at: 125° W 6°N. This Indian Permo-Triassic pole position, when compared with data from other Gondwanaland continents, suggests the hypothesis of an early movement between India and Africa before Permo-Triassic times.The partial or total remagnetization of some Indian red beds, mainly of Gondwana age, during Deccan Trap times is explained as acquisition of viscous Partial Thermoremanent Magnetization. This mechanism was advanced by Briden (1965), Chamalaun (1964) and Irving and Opdyke (1965).