Oldest Rock Unit Research Articles

Integrated Landsat multispectral imagery and airborne geophysical data for enhanced litho-structural mapping: A case study of Adamawa Massif, north-eastern Nigeria

Landsat-7 ETM + satellite imagery integrated with corrected airborne geophysical data (radiometric, gravity, and aeromagnetic) anomalies maps, and field investigations were used to discriminate lithologies and related structures in Mayo-Belwa and Jada-Ganye-Toungo areas (Adamawa Massif), northeastern Nigeria. Field investigations portray the granite-gneisses as the oldest rock units, and have sequentially been invaded by the syn-, late-to post-collisional Older granites. Subsurface linear structures in the area display dominant NE-SW, NW-SE, NNE-SSW, and ENE-WSW trends, reflective of variations in pre-existing structural orientations, and near-surface expressions of reactivated zones of weakness in the basement crust. The geodynamic systems revealed that most elongated lineaments are expressed as axial part of valleys trending NE-SW, ENE and NNW, forming diagonal grid to the NW-SE stresses. Abnormally high total radioactivity gravitates to areas of deflection (alluvium), and late- and post-tectonic granites. The gravity intensity provides a spectrum of anomalous responses including abnormally high positive gravity (volcanic rocks), moderately high positive gravity (granite-gneisses and late-to post-tectonic and syn-tectonic granites), background gravity and low negative gravity (granite-gneisses, alluvium and proluvium). Aeromagnetic anomalies discriminate the Older granites (intense positive anomalies), granite-gneisses, sediments (moderate positive, near background, and moderate-intensive negative anomalies), syn-tectonic granites (moderate positive anomalies), banded-gneiss and amphibolite (intensive negative anomalies), volcanic rocks (intense negative anomaly), and alluvium/proluvium (low positive anomalies). This present study led to the development of up-to-date and accurate litho-structural and geophysical maps of the geologically-rugged and hardly accessible Adamawa Massif, north-eastern Nigeria and further constrain the geochemical (High-K calc-alkaline, peraluminous) and tectonic (syn-to post-collisional) characteristics of typical Pan-African granitoids.

Middle Eocene benthic foraminifera from Qattamiya area, Cairo\u2013Suez district, Egypt: biostratigraphy, paleoecology, and their relation to the Southern and Western Tethyan Provinces

This study deals mainly with the biostratigraphy, paleoecology and paleobiogeogrphy of the Middle Eocene benthic foraminifera from an outcrop exposed at Qattamiya area, Cairo–Suez district, Egypt. This district is covered by relatively thick Eocene sequences composed of limestone intercalated with shales and marl. Stratigraphically, the oldest rock unit exposed in the study area is the Observatory Formation which is followed by the Qurn Formation. The examination of the collected samples led to the recognition of 27 species belonging to 17 genera and 10 families. Three benthic foraminiferal biozones are recognized Elphidium trompi-Quinqueloculina carinata Assemblage Zone, Bulimina jacksonensis Zone, and Quinqueloculina ludwigi Zone. These local biozones are correlated with their equivalents inside Egypt. Therefore, the age is assigned to the Middle Eocene. To detect the paleoecology of the recorded foraminiferal community, some parameters (Foraminiferal abundance, richness, and the percentages of epifaunal and infaunal species) are calculated. Three local paleoecologic ecozones are proposed signifying a hyper-to hyposaline inner neritic environment for the lower part of the Observatory, the middle shelf with low oxygen conditions for the upper part of the Observatory Formation, and hypersaline inner neritic environment for the Qurn Formation. Recently, multivariate analyses are considered as a valuable tool in establishing the paleoecology and paleobiogeographical provinces of the Paleogene microfossils. In this study, a matrix composed of 18 species from six countries (Tunisia, Egypt, Libya, France, Spain and Italy) is suggested for multivariate analyses. This matrix is subjected to the principal component analysis (PCA) and the Q-mode cluster analysis to detect the possible provinces. Consequently, two significant provinces are detected, the Southern Tethyan Province (Tunisia, Libya and Egypt) and the Western Tethyan Province (Spain, Italy, and France). Based on the similarity index, there is a strong similarity between the two provinces, which proposes a marine connection between them during Eocene.

Petrographic Studies of Rocks around Arum and Its Environs, North Central Nigeria

A detailed geological mapping of the area around Arum and environs part of Kurra sheet 189 SW was carried out on the scale of 1: 12, 500. Geologic field mapping and petrographic study (both megascopic and microscopic) were the methodology used. The geologic mapping of the area identified four rock units which are; granite, porphyritic granite, granitic gneiss and Porphyroblastic gneiss. These rock types were distributed such that the granite at the north-eastern part covered about 25%, the north –western portion was occupied by the porphyritic granite which occupied the largest portion of about 30% of the area. The third rock unit is the granitic gneiss which covered only about 20%. The fourth (last) and the oldest rock unit is the Porphyroblastic gneiss covering about 25% of the total area at the south-eastern corner. Megascopic and microscopic study revealed that the rocks in the area comprised of minerals such as; quartz, biotite, muscovite, microcline, feldspar, hornblende, garnet, etc. Structures that were clearly evident in the area included fault, foliation, joints, and veins. Structural analysis showed that their rose diagrams proved a NW-SE, NNE-SSW and NE-SW trends to be dominant.

Post–east African Orogen dilation and magmatism: development of a conjugate-style of dolerite dike swarm in the northwestern Arabian shield

The Al Madinah area bimodal dike swarms underwent detailed field mapping, measurements of dikes’ attitudes, structural analyses, remote sensing data interpretation, and petrology study. The oldest rock unit is a greenish intermediate dioritic unit that varies in its petrologic composition between microdiorite and andesite porphyry which is intruded by alkali granite. The dioritic and alkali granite units have been cut by intermediate to mafic dikes, ranging in texture and composition from micromonzodiorite and micromonzonite to microgabbro, microdolerite, and basalt or basaltic andesite, as revealed by thin-section work. A stereographic projection and rose diagrams of the mafic dikes formed a regional conjugate dike system, following probably a typical andersonian style, consisting of three major sets, namely, the WNW set which trends between N295 and N330, E-W set between N265 and N270, and finally the ENE (N065) set. The WNW set has its own local system, but it is considered as a set in the overall regional conjugate dike system. The WNW and E-W sets are the conjugate pair of the WNW Najd fault system maximum horizontal shearing, where the ENE set reflects the minimum regional horizontal shearing stress of the Najd system. The interpretation of this dike system is a WNW-ESE sinistral faulting and NE-SW dilation that formed Wadi Al-Hamd Ediacaran continental basin, bounded by two Najd faults and Thurwah-Bir Um suture. The study area underwent final thrust faulting that tilted the dikes towards the east. For the consequent magmatisms during the Neoproterozoic period, followed by Cenozoic volcanism, the Al Madinah area is considered a large igneous province.

Geological and petrographical studies around Um Taghir area, Сentral Eastern Desert, Egypt

Um Taghir area is located in the northern extreme boundary of Central Eastern Desert of Egypt at the west of Safaga City. Um Taghir is represented by island arc related rocks and late to post tectonic magmatism. The island arc related rocks are represented by metavolcaniclastic sequences and metagabrroic rocks. Metavolcanoclastic rocks are considered as the older rock units of the study are and intruded by the metagabbro. The late to post tectonic magmatism is represented by (dokhan volcanic, gabbro, tonalite-granodiorite, monzogranite, alkali feldspar granites and different types of dikes). Usually, the gabbroic rock is bearing ilmenite lenses or bands in the bottom of the layered; this is related to magma rich of iron oxides. Petrographically, island arc assemblage is classified in to actinolite hornblende schist and metagabbro that show quite different of their content in plagioclase, hornblende, augite, quartz and biotite. Occasionally, the late to post tectonic magmatism represented by andesite, gabbro, tonalite, granodiorite monzogranite, alkali feldspar granites and different types of dikes. Andesite consists of plagioclase, quartz, alkali feldspar and hornblende. Gabbroic rocks are represented by pyroxene hornblende gabbro and leucogabbro. They show quite different of their content in plagioclase, pyroxene and clear difference in the content of both olivine and hornblende in both of them. While tonalite and granodiorite show quite different of their content in plagioclase, quartz, hornblende, alkali felspar and biotite. On the other hand, monzogranite and alkali feldspar granite, they show plagioclase is varying from oligoclase to albite; K-feldspars, quartz and muscovite are relatively more abundant in the alkali feldspar granite. Finally, the different types of dikes classified into granite, andesite, rhyolite and basalt dikes consist of the different mineral compositions.

Geology and Petrographic Studies of Rocks in Anka Sheet 52, Northwestern, Nigeria

Over the years, models of evolution of the schist belts of Nigeria was proposed and classified under ensialic and ensimatic processes of formation. The study area (Anka Sheet 52) lies between latitudes 12 o 00′N and 12°30′N and longitudes 5°30′ E and 6°00′E and covers an area of about 3,086.76 km² (Anka Sheet 52). Field mapping and laboratory studies was considered for this study by taking every detail in the field necessary such as geologic boundaries, structures, physical appearances of rocks and orientation noted in field notebook. Structural characteristics on outcrop was also recorded with the help of photographs and sketches along with text description. Representative rock samples mostly granitoids were prepared into thin for microstructural observations to reconstruct the deformation features and mineralogical assemblages under polarized light microscope. The study area comprises of gneiss of sedimentary protolith and is the oldest rock unit forming a massive sheared and elongated ridge trending north-south for about 1800m with a porphyroblastic texture of gradual lineation of quartz and feldspar in places. Metasediments comprises of quartzite, quartz schist, pellitic schist, phyllite, hornfels and metaconglomerate accounting for more than 35 percent of the study area. Pan-African granites intruded both the gneiss and metasediments and comprises of coarse grained granites, porphyritic granites, diorite and rhyolite to accounts for about 20 percent of the study area. Field and petrological evidences indicates that the Pan-African reworking may have led to the recrystallization of the protolith rocks to form gneiss (paragneiss) resulting from partial melting signifying that metamorphism possibly reach lower amphibolite facies in part of the study area. These rocks were overlain by sedimentary rocks of Cretaceous age. Keywords: Granitoids, Protolith, Metamorphism, Porphyroblastic, Paragneiss DOI : 10.7176/JEES/10-1-06 Publication date: January 31 st 2020

Where does the time go? Assessing the chronostratigraphic fidelity of sedimentary geological outcrops in the Pliocene–Pleistocene Red Crag Formation, eastern England

It is widely understood that Earth's stratigraphic record is an incomplete record of time, but the implications that this has for interpreting sedimentary outcrop have received little attention. Here we consider how time is preserved at outcrop using the Neogene–Quaternary Red Crag Formation, England. The Red Crag Formation hosts sedimentological and ichnological proxies that can be used to assess the time taken to accumulate outcrop expressions of strata, as ancient depositional environments fluctuated between states of deposition, erosion and stasis. We use these to estimate how much time is preserved at outcrop scale and find that every outcrop provides only a vanishingly small window onto unanchored weeks to months within the 600–800 kyr of ‘Crag-time’. Much of the apparently missing time may be accounted for by the parts of the formation at subcrop, rather than outcrop: stratigraphic time has not been lost, but is hidden. The time-completeness of the Red Crag Formation at outcrop appears analogous to that recorded in much older rock units, implying that direct comparison between strata of all ages is valid and that perceived stratigraphic incompleteness is an inconsequential barrier to viewing the outcrop sedimentary-stratigraphic record as a truthful chronicle of Earth history. Supplementary material: Further details of the regional geology and specific information on outcrops are available at https://doi.org/10.6084/m9.figshare.c.4561001

Stratigraphic and Micropaleontological Investigation of Late Paleocene-Eocene Units in the North of Yüğlük Mountain (Bolkar Mountains, Central Taurides)

İnceleme alanı, Yüğlük Dağı’nın (Bolkar Dağları, Orta Toroslar) kuzey kesimlerinde yer alır. Bu araştırma, bölgede yüzeyleyen geç Paleosen-Eosen yaşlı birimlerin çökelme ortamını belirlemek, stratigrafik ve mikropaleontolojik özelliklerini incelemek amacı ile yapılmıştır. Çalışmada, 2 ölçülü kesit alınmış ve toplam 93 örnek incelenmiştir.İnceleme bölgesindeki en yaşlı kaya birimleri Jura-Kretase yaşlı karbonat kayaları ve Geç Kretase yaşlı ofiyolitik melanjdır. Bu birimler, geç Paleosen yaşlı sedimanlarca uyumsuz olarak üzerlenir. Geç Paleosen-Eosen yaşlı birimler, Paleojen örtü kayaçlarının ilk topluluğudur. Bu çalışmada alttan üste doğru düşey geçişler sunan 3 formasyon ayırt edilmiştir. Berçin Formasyonu; akarsu ortamında çökelmiş, masif konglomeralardan oluşur. Tosuntaş Formasyonu; pembe-gri renkli, orta-kalın tabakalı, kavkı kırıntılı, bol bentik foraminiferli, sığ denizel-resifal ortamda çökelmiş kireçtaşları ile temsil edilir. Küçükkoraş Formasyonu; alt kesimleri bol bentik foraminiferli, kırıntılı-kireçtaşı ardalanmasından, üst kesimleri ise kumtaşı-marn ara seviyeleri içeren çamurtaşlarından oluşur. Litolojik özellikleri ve fosil içeriği birimin dış şelf ortamında çökeldiğini işaret etmektedir. Bu birimler, Miyosen yaşlı birimler tarafından açısal uyumsuzlukla üzerlenir.Çalışma alanında yüzlek veren birimlerin stratigrafik konumlarına ve elde edilen mikropaleontolojik bulgulara dayanarak Berçin ve Tosuntaş formasyonlarının geç Paleosen yaşlı; Küçükkoraş formasyonunun ise erken-orta Eosen yaşlı olduğu belirlenmiştir. 

Stratigraphy and Structure of Dhamtaur Area, District Abbottabad, Eastern Hazara, Pakistan

Pre-Cambrian to Paleocene age sedimentary rocks predominantly characterize the fold and thrust belt of eastern Hazara division. The Hazara Slate Formation is the oldest rock unit which represents the Precambrian sequence. The Permian and Triassic sequences are missing. The Jurassic sequence comprises Samana Suk Formation whereas the Cretaceous exposed is Chichali and Lumshiwal formations. The Eocene sequence consists of Nammal and Sakessar formations. The structural fabric of the area is mainly attributed to a series of northwest trending parallel to en echelon anticlines and synclines. Most of these folds are found to be asymmetric and are Northwest facing. Several thrust faults verging both to the north have been mapped that generally dissect the forelimbs of the anticlinal structures. But the following study going to be focus on surface structural features as well as subsurface projections of folds and faults. Study of such structural features has get prime importance in economic geology such as petroleum geology, mining geology and engineering geology. In our study area subsurface projection of folds and faults along the structural transects of the area suggests that these structures have formed as a result of shortening associated with ramping from a regional basal decollement. All the structures clearly demonstrate that the eastern Hazara area has been subjected to compressional deformation/stresses oriented northeast southwest. The repetition of rock units indicates, folding in the area and thrusting of Pre-Cambrian Hazara Formation over younger Paleocene Lockhart Formation, evidence of thrust fault. There are unconformable contacts between Hazara and Samana Suk, Chichali and Lumshiwal, Kawagarh and Hangu and Lockhart formations indicate fluctuation in the environment of deposition. The Bagnotar Fault, Dhamtaur syncline and Thai anticline are the major structural features identified and reported in the study area.

Distinguishing in situ stromatolite biosignatures from silicification and dolomitisation using short wave, visible-near and thermal infrared spectroscopy: A Mars analogue study

The search for life on Mars has relied on infrared spectroscopy from orbit and in situ rovers to detect chemical signals akin to those found within stromatolites on Earth. Carbonate and hydrothermally altered mineral signals were detected on small rock units within the Nili Fossae region, and in small concentrations within the Gusev Crater region, but these minerals were likely a product of dolomitisation rather than an indicator of stromatolitic precipitation. To date, no studies have been undertaken to investigate the effect of weathering processes such as dolomitisation and silicification, which may obscure mineralogical signals that may be associated with stromatolites during in situ spectroscopic detection. This study investigates the capacity that short-wave, visible-near, and thermal infrared spectrometers have in detecting minerals in the oldest known and heavily weathered stromatolites of the Pilbara Craton, Western Australia. A diversity of mineral signals in stromatolitic samples can be distinguished from dolomitisation and silicification in even the oldest rock units, when fresh surfaces are scanned multiple times by a short-wave infrared capable spectrometer, with a minimum spectral resolution of 7nm. It is expected that this approach will significantly inform and direct the interpretation of carbonate signals within the heavily weathered Noachian rock units on Mars.

Geochronological, geochemical, and Nd-Hf isotopic studies of the Qinling Complex, central China: Implications for the evolutionary history of the North Qinling Orogenic Belt

The Qinling Complex of central China is thought to be the oldest rock unit and the inner core of the North Qinling Orogenic Belt (NQOB). Therefore, the Qinling Complex is the key to understanding the pre-Paleozoic evolution of the NQOB. The complex, which consists of metagraywackes and marbles, underwent regional amphibolite-facies metamorphism. In this study, we constrained the formation age of the Qinling Complex to the period between the late Mesoproterozoic and the early Neoproterozoic (ca. 1062–962 Ma), rather than the Paleoproterozoic as previously thought. The LA-ICP-MS data show two major metamorphic ages (ca. 499 and ca. 420–400 Ma) for the Qinling Complex. The former age is consistent with the peak metamorphic age of the high- and ultra-high pressure (HP-UHP) rocks in the Qinling Complex, indicating that both the HP-UHP rocks and their country rocks experienced intensive regional metamorphism during the Ordovician. The latter age may constrain the time of partial melting in the NQOB between the late Silurian and early Devonian. The Qinling Complex is mostly affiliated with subduction–accretion processes along an active continental margin, and should contain detritus deposited in a forearc basin.The available data indicate that the NQOB was an independent micro-continent at least prior to the Neoproterozoic, and included a portion of the Grenville orogenic belt during the period of 1.2–0.8 Ga. The NQOB has experienced a unique geological history, which is obviously different from that of the North China Craton (NCC) and the Yangtze Craton during the Precambrian. The Neoproterozoic granitoids that intruded the Qinling Complex can be interpreted as the products of assembly of the supercontinent Rodinia. The NQOB was separated from Rodinia at ca. 830–740 Ma. Subsequently, the NQOB moved closer to the northern margin of the NCC, and the initial accretion or collision with the NCC occurred from the late Cambrian to the early Ordovician.

Stratigraphy and Structure of Sarda, Manil, Changpur and Naghal Areas, District Kotli, Jammu & Kashmir

Abstract The Chauki, Mandi, Manil colony, Changpur, Khawas and Naghal areas are situated in between the limbs of Hazara Kashmir Syntaxis (HKS). HKS is the part of Himalayan fold and thrust belt that lies in sub-Himalayan domain. Seismically, this is an active zone. Early Miocene to Recent sedimentary rocks are exposed in the area. The stratigraphic units in Kashmir basin are the cover sequence of the Indian plate. These non-marine lithostratigraphic units are molasse deposits formed by the deposition of sediments coming from north carried by the rivers originated from higher Himalayas. Murree Formation of early Miocene age is the oldest rock unit in the studied area. Siwalik Group; Chinji, Nagri, Dhok Pathan and Soan formations of early Miocene to Pliocene and Mirpur Formation of Pleistocene age is exposed. The area is structurally deformed into folds and faults. The Sarda Sarhota syncline, Mandi syncline and Fagosh anticline are major folds in the area. These folds are isoclinal to open in nature, southwest or northeast verging and thrust direction is southwest or northeast. Major reverse faults are Riasi fault and Fagosh fault. The Changpur fault is a normal fault. Primary sedimentary structures present in the area are load cast, ripups and cross bedding. The facing of beds have been marked on the basis of these sedimentary structures.

Geochemistry of a crude oil from a shallow well in Bazian Area, Sulaimani, Kurdistan Region, NE Iraq

A crude oil sample from a shallow well in Bazian area, Sulaimani Governorate, Kurdistan Region was analyzed physically and geochemically in terms of source and its biomarker content. Physical properties of the sample indicate that this crude oil have high boiling point which mean absence of light ends, and middle distillate (i.e. gases, naphtha, kerosene, diesel fuel, etc.) with specific gravity of 0.9572, and API equals to 16.32. Chemical analysis show the sulfur content about 1.7% as well as rich in Fe and Ca. GC/MS analysis indicate to absence of n-alkanes, this supported by the mass chromatogram of m/z85 which show no biomarkers related to n-alkanes in the sample. Only a small portion of tricyclic terpanes (Cheilanthanes) are present. Mass chromatogram of m/z 217 shows some diasteranes, while other steranes are totally absent. No clear aromatic compounds were found in the studied sample. Severe biodegradation was indicated by the shape of chromatogram, absence of n-alkanes, partial degradation of steranes, remaining of diasteranes. The process of water washing may be accompanied with biodegradation as the aromatic components are removed. The presence of tricyclic terpanes usually related to non-terrestrial source material, which possibly the Kolosh Formation and/or older rock units are the source of this heavy oil.

Faulted anticlines (desert roses) and linear slope deposits in Northwest Iraq: analysis of some morphotectonic features

Distinctive landform of juxtaposition of relatively straight exposures of slope and Quaternary deposits against older rock units in addition to the omission of younger ones are recognized in the Zagros low-folded zone of Mosul area. This phenomenon is attributed to the kinetics and the geometry of basin inversion. Positively inverted faults and their associated folds developed up-growing structures with reverse fault scarps. Those growing structures are passively uplifted above lower blind thrust fault. The up-moving faults were subjected to erosion. The interaction between the rates of erosion and uplift controls the formation of slope and other Quaternary deposits and their distribution. Fast uplift of the reverse faults resulted into formation of cliffs representing the fault scarps and the formation of slope deposits directly next to them. The linear traces of the reverse faults control the straight linear map view distribution of the associated slope deposits. Slower rates of uplift give considerable time to erosion and in consequence the fault scarps are deeply eroded and disappear. This process also resulted into deposition of the other types of Quaternary deposits but still the linear contact between the geological rock units and the Quaternary deposits is existing and easily noticed. The analysis of these features and their interpretation has resulted in a new interpretation of the geological maps of Mosul area and the recognition of many reverse faults that bound the exposed structures. This fault system is used to restore the configuration of the original Late Cretaceous extensional basin.

Sustained low marine sulfate concentrations from the Neoproterozoic to the Cambrian: Insights from carbonates of northwestern Mexico and eastern California

Stratigraphic carbonate-carbon isotope trends are similar for correlative Ediacaran and Cambrian carbonates of Sonora, Mexico, and Death Valley, California. In contrast, the sulfur isotope compositions of both carbonate-associated sulfate (CAS) and pyrite in the two regions exhibit unique trends with high degrees of stratigraphic variability. We have established that the sulfur records are coeval using δ13C chemostratigraphy and biostratigraphic markers, where present. Over short stratigraphic intervals, δ34SCAS variability is consistent with regionally low marine sulfate concentrations during this period. Values of Δ34S (δ34SCAS–δ34Spyr) range from −5.8‰ to +27.1‰ and average ∼+11‰, consistent with limited net fractionation during bacterial sulfate reduction, which is additional evidence for low sulfate concentrations. Modeling based on these regional sulfur isotope trends suggests sustained low sulfate conditions throughout the Neoproterozoic and well into the Cambrian, with concentrations of ∼2mM or lower.When all of the available sulfate proxy data from our work and previously published studies are considered, most Neoproterozoic and Cambrian successions exhibit trends consistent with low seawater sulfate. The persistent and complete disagreement in δ34Ssulfate among multiple basins was briefly interrupted ∼580 million years ago, coincident with the onset of the Wonoka-Shuram carbon isotope anomaly and again near the termination of Series 3 of the Cambrian—characteristics generally unrecognized in older rock units. During these two intervals, similar stratigraphic trends in δ34SCAS are recorded globally, whereas absolute values remain distinct among individual basins. However, these periods of broad isotopic trend agreement coincide with large-magnitude sulfur isotope excursions, which also point to low seawater sulfate concentrations. Therefore, although brief intervals of isotopic homogeneity exist, the Neoproterozoic and Cambrian ocean must have been dominated by low sulfate throughout. Ultimately, the recognition of persistently low sulfate well into the Paleozoic raises questions about the relationships between sulfate concentration in seawater and its primary controls, including ocean oxygenation and its influence on metazoan evolution.

Microstructural finite strain analysis and 40Ar/39Ar evidence for the origin of the Mizil gneiss dome, eastern Arabian Shield, Saudi Arabia

The Mizil antiform is a gneiss-cored culmination situated near the northern end of the Ar Rayn island arc terrane, which is the easternmost exposed tectonic unit of the Arabian Shield. This domal structure has a mantle of metamorphosed volcanosedimentary rocks belonging to the Al-Amar Group, and an igneous interior made up of foliated granodiorite-tonalite with adakitic affinity. The gneissic core has a SHRIMP U–Pb zircon age of 689±10Ma making it the oldest rock unit in the Ar Rayn terrane. An adakite diapir, formed by the melting of the subducted crust of a young marginal basin, and rising through the volcanosedimentary succession of the Ar Rayn island arc is thought to have caused the observed doming. Relatively uniform strain throughout the dome combined with strong vertical shortening and the roughly radial pattern of stretching lineation is consistent with diapirism; the absence of strain localization rules out detachment faulting as a causative mechanism. Amphibolites from the metamorphic envelope have an 40Ar/39Ar age of 615±2Ma; the age gap between core and cover is thought to reflect the resetting of metamorphic ages during the final suturing event, a phenomenon that is often observed throughout the eastern shield. Aeromagnetic anomalies beneath the Phanerozoic sedimentary cover indicate the presence of a collage of accreted terranes east of the Ar Rayn terrane that were probably amalgamated onto the Arabian margin during the latest stages of the closure of the Mozambique ocean; culminant orogeny is believed to have taken place between 620 and 600Ma as these terrane collided with a major continental mass to the east referred to here as the eastern Arabian block (EAB). The Mizil gneiss dome is therefore considered to have formed in a convergent contractional setting rather than being the outcome of extensional post-orogenic collapse.

Petrogenesis of Late Cretaceous lava flows from a Ceno-Tethyan island arc: The Raskoh arc, Balochistan, Pakistan

The Raskoh arc is about 250km long, 40km wide and trends in an ENE direction. The oldest rock unit in the Raskoh arc is an accretionary complex (Early to Late Jurassic), which is followed in age by Kuchakki Volcanic Group, the most wide spread unit of the Raskoh arc. The Volcanic Group is mainly composed of basaltic to andesitic lava flows and volcaniclastics, including agglomerate, volcanic conglomerate, breccia and tuff, with subordinate shale, sandstone, limestone and chert. The flows generally form 3–15m thick lenticular bodies but rarely reach up to 300m. They are mainly basaltic–andesites with minor basalts and andesites. The main textures exhibited by these rocks are hypocrystalline porphyritic, subcumulophyric and intergranular. The phenocrysts comprise mainly plagioclase (An30–54 in Nok Chah and An56–64 in Bunap). They are embedded in a micro-cryptocrystalline groundmass having the same minerals. Apatite, magnetite, titanomagnetite and hematite occur as accessory minerals.Major, trace and rare earth elements suggest that the volcanics are oceanic island arc tholeiites. Their low Mg # (42–56) and higher FeO (total)/MgO (1.24–2.67) ratios indicate that the parent magma of these rocks was not directly derived from a mantle source but fractionated in an upper level magma chamber. The trace element patterns show enrichment in LILE and depletion in HFSE relative to N-MORB. Their primordial mantle-normalized trace element patterns show marked negative Nb anomalies with positive spikes on K, Ba and Sr which confirm their island arc signatures. Slightly depleted LREE to flat chondrite normalized REE patterns further support this interpretation. The Zr versus Zr/Y and Cr versus Y studies show that their parent magma was generated by 20–30% melting of a depleted mantle source. The trace elements ratios including Zr/Y (1.73–3.10), Ti/Zr (81.59–101.83), Ti/V (12.39–30.34), La/YbN (0.74–2.69), Ta/Yb (0.02–0.05) and Th/Yb (0.11–0.75) of the volcanics are more consistent with oceanic island arcs rather than continental margin arcs. It is suggested that the Raskoh arc is an oceanic island arc which formed due to the intra-oceanic convergence in the Ceno-Tethys during the Late Cretaceous rather than constructed on the southern continental margin of the Afghan block, as claimed by previous workers. It is further suggested that the Semail, Zagros, Chagai–Raskoh, Muslim Bagh, and Waziristan island arcs were developed in a single but segmented Cretaceous Ceno-Tethyan convergence zone.

Regional trends in active diapirism revealed by mountain range‐scale InSAR time series

We construct an orogen‐wide InSAR time series over the Zagros Mountains and eastern Markan Subduction Zone in southern Iran from 19 Envisat tracks spanning 2003–2010. We observe active salt diapirism of the infra‐Cambrian Hormuz Salt at 20 locations and identify several diapirs that are not moving. All active diapirs reach the surface within the Asmari Limestone or older rock units while we do not observe any active diapirism occurring within younger exposures, reflecting a complex vertical accumulation of the basal salt and reactivation of diapirs by erosion. Diapir reactivation of this sort may be a critical factor affecting the feasibility of sequestering nuclear waste and CO2 in evaporite sequences over geologic time scales. The distribution of active diapirism is indifferent to increases in mean elevation, suggesting a relatively uniform thickness of the Zagros Fold Belt above the basal salt with steps in topography driven by basement thickening. We do not observe aseismic uplift of anticlines because rates are likely below the detection threshold of our time series. This suggests that uplift is accommodated at small rates (<2–3 mm/yr) over numerous structures instead of focused at the deformation front.

An Active Composite Pull-apart Basin Within the Central Part of the North Anatolian Fault System: the Merzifon-Suluova Basin, Turkey

The North Anatolian Fault System (NAFS) that separates the Eurasian plate in the north from the Anatolian microplate in the south is an intracontinental transform plate boundary. Its course makes a northward convex arch-shaped pattern by flexure in its central part between Ladik in the east and Kargı in the west. A number of strike-slip basins of dissimilar type and age occur within the NAFS. One of the spatially large basins is the E-W-trending Merzifon-Suluova basin (MS basin), about 55 km long and 22 km wide, located on the southern inner side of the northerly-convex section of the NAFS. The MS basin has two infills separated from each other by an angular unconformity. The older and folded one is exposed along the fault-controlled margins of the basin, and dominantly consists of a Miocene fluvio-lacustrine sedimentary sequence. The younger, nearly horizontal basin infill (neotectonic infill) consists mainly of Plio-Quaternary conglomerates and sandstone-mudstone alternations of fan-apron deposits, alluvial fan deposits and recent basin floor sediments. The two basin infills have an angular unconformity between them and the deformed pattern of the older infill reveals the superimposed nature of the MS basin. The MS basin is controlled by a series of strike-slip fault zones along its margins. These are the E-W-trending Merzifon dextral fault zone along its northern margin, the E-W-trending Sarıbuğday dextral fault zone along its southern margin and the NW-trending Suluova normal fault zone along its eastern margin. The basin is cut by the E-W-trending Uzunyazı dextral fault zone, which runs parallel to the northern and southern bounding fault zones and displays a well-developed overlapping relay pattern by forming a positive flower structure. The faults of the zone cut Quaternary neotectonic infill and tectonically juxtapose the fill with older rock units. The central faults are seismically more active than the bounding faults, and are therefore relatively younger faults. The early-formed rhomboidal basin is subdivided by these E-W-trending younger faults into several coalescing sub-basins, converting it into a composite pull-apart basin. The total cumulative post-Pliocene dextral offset along the southern bounding faults is about 12.6 km.

Paleoproterozoic evolution of the eastern Alxa Block, westernmost North China: Evidence from in situ zircon U–Pb dating and Hf–O isotopes

The Alxa Block is the westernmost part of the North China Craton (NCC). In the past, it has been considered to be part of the Archean NCC. However, formation and evolution of this block remain poorly understood, and this has hampered a broader understanding of the NCC. In this study we analyzed the in situ zircon U–Pb age and Hf–O isotopes for the two oldest rock units in the eastern Alxa Block, namely the Bayanwulashan and Diebusige Complexes. The Bayanwulashan Complex consists mainly of metamorphic rocks with mafic and felsic protoliths. SIMS U–Pb zircon dating results indicate that the primary magmatic ages of the mafic and felsic igneous rocks are ca. 2.34Ga and ca. 2.32–2.30Ga, respectively, and both sets of rocks were overprinted by two metamorphic events at ca. 1.89Ga and ca. 1.79Ga. Geochemical and zircon Hf–O isotopic data suggest that the ca. 2.34Ga amphibolites within the Bayanwulashan Complex are characteristically high in TiO2 (up to 3.2%), Zr (up to 394ppm) and Ti/V (>30), resembling the basaltic rocks formed within continental rifts. The ca. 2.32–2.30Ga felsic rocks were generated by remelting of dominant meta-igneous rocks that have zircon Hf model ages of 2.92–2.81Ga. The Diebusige Complex is composed of amphibolites, mafic gneisses and paragneisses. It was intruded by granites dated at ca. 1.97–1.98Ga, and subjected to high grade metamorphic events at ca. 1.89Ga and ca. 1.79Ga. Re-examination of U–Pb ages for detrital and metamorphic zircons indicate that the depositional ages of protoliths of the Diebusige paragneisses may be considered to be between ca. 2.45 and 2.0Ga. These U–Pb zircon age data do not support the existence of exposed Archean rocks in the eastern Alxa Block, although the possibility of Archean rocks at deeper crustal levels cannot be ruled out entirely.The eastern Alxa Block contrasts to the neighboring Yinshan Block and Khondlite Belt, as well as the Trans-North China Orogen, in terms of the timing and evolution of magmatism and metamorphism. Thus, we conclude that the Alxa Block is a separated Paleoproterozoic terrane from the Western Block of the NCC, rather than the western extension of the Yinshan Block or part of the Khondalite Belt, as previously proposed.