Evolution Of Rift Basins Research Articles

Transition to magma-driven rifting in the South Turkana Basin, Kenya: Part 1

Characterization of the role of magma in driving rift basin evolution is fundamental to our understanding of how extensional plate boundaries develop. Here, we examine patterns of normal faulting, volcanism and magmatism in the South Turkana Basin, Kenya, to test how magmatism has impacted the Quaternary evolution of this system. Utilizing high-resolution, single-channel reflection seismic data and radiocarbon-dated cores, we estimate that shallow Holocene normal faulting accounts for 3.5–5.8 mm a −1 of extension, consistent with estimates farther south in the Suguta Basin. Spatial fault patterns show that extension in the Holocene is concentrated along a c. 20 km wide intra-rift fault network, aligning with the axial volcano, South Island. We suggest that up to 85% of Late Quaternary regional extension is accommodated within an axial magmatic segment rather than on the border fault system responsible for the original basin architecture. Axial extension increases towards South Island, highlighting magmatism as crucial in driving recent shallow upper crustal faulting in the South Turkana Basin. Below this zone of extensional faulting, geochemical evidence supports a developed magma plumbing system that broadly parallels equivalent axial magmatic systems within the Main Ethiopian Rift. Overall, our data support a recent (Mid- to Late Pleistocene) transition to magma-driven axial extension in the Lake Turkana rift. Supplementary material: Supplementary text and the fault, volcanic cone and radiocarbon age data utilized in this study are available at https://doi.org/10.6084/m9.figshare.c.6026624

Source-to-sink evolution of syn-rift alkaline lake sediments in the Lower Permian Fengcheng Formation, Junggar Basin, NW China: Evidence from petrology, detrital zircon geochronology and geochemistry

• Chemical weathering intensity increased upwards in the ∼ 280 Ma Fengcheng Formation. • Detrital material of the FC Formation was supplied by the West Junggar, due to active normal faults. • Syn-rift sediments were deposited in an alkaline lake under arid climatic conditions. Source-to-sink evolution of intracontinental rift basins is an effective tool to evaluate syn -rifting tectonic processes and sediment dispersal. Provenance analysis, using petrological and geochemical analysis and detrital zircons U-Pb dating of the Lower Permian Fengcheng Formation, provides new insights into the provenance of alkaline lake deposits in the Mahu sag, Junggar Basin. Volcanic detritus and detrital feldspar grains are abundant, indicative of proximal sources. The rare earth and trace element characteristics of three sandstone samples are consistent with a dominant source from the Carboniferous – early Permian mafic and felsic rocks in West Junggar. Sandstone geochemistry suggests that chemical weathering intensity increased upwards, probably in response to reduced aridity. The detrital zircon U-Pb age patterns show a prominent Carboniferous peak (340–300 Ma), minor early Permian (∼280 Ma) and Silurian peaks (440–420 Ma). Quantitative unmixing modelling suggests that most detrital zircons were derived laterally from the Central West Junggar (CWJ), and a few zircons were derived axially from the Zharma-Saur and Boshchekul-Chingiz arcs in Northern and Southern West Junggar. The lateral source was more important in the Middle Member than in the Lower Member of the Fengcheng Formation. The average age of the youngest five detrital zircons and the low weathering indices indicate that the maximum slip of the bounding normal fault occurred at 284–?279 Ma. Seismic and hydrothermal activity caused by the normal fault displacement, as well as the increase in the supply of volcanic composition from the lateral CWJ, played an important role in the alkaline lake deposition.

Cenozoic basement-involved rifting of the northern South China Sea margin

The northern South China Sea (SCS) margin developed on a thinning crust comprising basement-involved structures inherited from the post-orogenic extension of the Cathaysia Block. While, the pre-rift structural configuration and its influence on the geometry and development of the Cenozoic rift basins are ambiguous. Based on 3D high-resolution seismic reflection data in the western Pearl River Mouth Basin (PRMB) and onshore fieldwork, we delineated the intrabasement boudinage structure which is constituted by the low-angle E–W-striking Indosinian fold-thrust belts and the steeply dipping NE-striking Yanshanian thrusts. It is concluded that the pre-rift structure played a key role in controlling the architecture and two-phase early rifting of the northern SCS margin. (1) During the Early Eocene syn-rift phase, the Indosinian thrusts were rejuvenated first and controlled the formation of a series of ENE-striking grabens. (2) During the Late Eocene syn-rift phase, both the Indosinian and Yanshanian thrusts were reactivated. The Indosinian thrusts continued to maintain normal faulting and even evolved into the detachment faults. The Yanshanian thrusts were inverted into the NE-striking dextral strike-slip faults, which controlled the formation of the pull-apart basin group en échelon and even the SCS opening. The lithospheric thinning mechanism of the SCS continental margin experienced a major change from the Early Eocene pure shear to the Late Eocene simple shear.

The interactions of volcanism and clastic sedimentation in rift basins: Insights from the Palaeogene‐Neogene Shaleitian uplift and surrounding sub‐basins, Bohai Bay Basin, China

AbstractAlthough volcanism is an important process in the evolution of rift basins, current tectono‐sedimentary models largely neglect its impact on sediment supply, transport pathways, and depositional systems. In this paper, we integrate core, well logs, and 3D seismic data from the Palaeogene‐Neogene Shaleitian (SLT) uplift and surrounding sub‐basins, Bohai Bay Basin, China, to investigate the sedimentology and geomorphology of a volcanic rift basin. Results of this study show that the spatial distribution of extrusive centres was strongly controlled by basement‐involved intra‐basin normal faults. During the early part of the syn‐rift stage, the SLT uplift supplied sediments to transverse fan deltas and braided‐river deltas that fringed the adjacent syn‐rift depocentres. Volcanic deposits mainly occurred as relatively thin lava flow and pyroclastic facies that partially filled fault‐controlled topographic lows, reducing topographic rugosity, and enhanced breaching of basement highs between syn‐rift depocentres. Integration of drainage to the syn‐rift depocentres and development of through‐flowing axial depositional systems was enhanced. During the later part of syn‐rift and in early post‐rift stages, the SLT uplift was progressively inundated, reducing sediment supply to the fringing transverse depositional systems. In contrast, axial braided‐river deltas became the main depositional systems, sourced by large hinterland drainage from the Yanshan fold‐belt to the northwest. Volcanism in the late syn‐rift and early post‐rift occurs as thick lava flow and pyroclastic facies that infill rift topographic lows and locally blocked axial fluvial systems creating isolated lakes. Within hanging wall depocentres, volcanic topographic highs split and diverted axial fluvial and deltaic systems. Furthermore, volcanism supplied large volumes of volcanic sediment to the rift resulting in increased sedimentation rates, and the development of unstable subaerial and subaqueous slopes and deposits, increasing the occurrence of landslides. Based on the observations of this study we update tectono‐sedimentary models for rift basins to include volcanism.

The geometric and temporal evolution of fault‐related folds constrain normal fault growth patterns, Barents Sea, offshore Norway

AbstractExtensional growth folds form ahead of the tips of propagating normal faults. These folds can accommodate a considerable amount of extensional strain and they may control rift geometry. Fold‐related surface deformation may also control the sedimentary evolution of syn‐rift depositional systems. Thus, by examining the stratigraphic record, we can constrain the four‐dimensional evolution of extensional growth folds, which in turn provides a record of fault growth and broader rift history. Here, we use high‐quality 3D seismic reflection and borehole data from the SW Barents Sea, offshore northern Norway to determine the geometric and temporal evolution of extensional growth folds associated with a large, long‐lived, basement‐rooted fault. We show that the fault grew via the linkage of four segments, and that fault growth was associated with the formation of fault‐parallel and fault‐perpendicular folds that accommodated a substantial portion (10%–40%) of the total extensional strain. Several periods of fault‐propagation folding occurred in response to the periodic burial of the fault, with individual folding events (ca. 25 and 32 Myr) lasting a considered part of the ca. 130 Myr rift period. Our study supports previous suggestions that continuous (i.e. folding) as well as discontinuous (i.e. faulting) deformation must be explicitly considered when assessing total strain in an extensional setting. We also show that changes in the architecture of growth strata record alternating periods of folding and faulting and that the margins of rift‐related depocentres may be characterised by basinward‐dipping monoclines as opposed to fault‐bound scarps. Our findings have broader implications for our understanding of the structural, physiographic and tectonostratigraphic evolution of rift basins.

Impact of basement thrust faults on low-angle normal faults and rift basin evolution: a case study in the Enping sag, Pearl River Basin

Abstract. Reactivation of pre-existing structures and their influence on subsequent rift evolution have been extensively analysed in previous research on rifts that experienced multiple phases of rifting, where pre-existing structures were deemed to affect nucleation, density, strike orientation, and displacement of newly formed normal faults during later rifting stages. However, previous studies paid less attention to the extensional structures superimposing onto an earlier compressional background, leading to a lack of understanding of, e.g. the reactivation and growth pattern of pre-existing thrust faults as low-angle normal faults and the impact of pre-existing thrust faults on newly formed high-angle faults and subsequent rift structures. This study investigating the spatial relationship between intra-basement thrust and rift-related faults in the Enping sag, in the northern South China Sea, indicates that the rift system is built on the previously deformed basement with pervasive thrusting structures and that the low-angle major fault of the study area results from reactivation of intra-basement thrust faults. It also implies that the reactivation mode of basement thrust faults is dependent on the overall strain distribution across rifts, the scale of basement thrust faults, and the strain shadow zone. In addition, reactivated basement thrust faults influence the nucleation, dip, and displacement of nearby new faults, causing them to nucleate at or merge into downwards it, which is representative of the coupled and decoupled growth models of reactivated thrust faults and nearby new faults. This work not only provides insights into the growth pattern of rift-related faults interacting with reactivated low-angle faults but also has broader implications for how basement thrust faults influence rift structures, normal fault evolution, and syn-rift stratigraphy.

Compression‐Induced Anomalous Subsidence in the Extensional Sedimentary Basin: A Numerical Study From the Pearl River Mouth Basin, Northern South China Sea Margin

AbstractCompressional uplift has long been considered as the basic response to basin inversion. However, significant discrepancies are recently observed in the Pearl River Mouth Basin (PRMB), where anomalous subsidence occurs during the Miocene compression phase. Whether the subsidence is another scenario that is associated with the tectonic inversion under certain circumstances remains unknown. Here we conduct systematic numerical modeling to explore the patterns of rift basin evolution from extension to compression. Our results show that two distinct inversion types develop depending on the stretching degree and tectonic quiescence. A less stretched continental crust would facilitate the uplift of basin center accompanied by erosion during shortening; otherwise, the basin center exhibits anomalous subsidence with abundant sediment accumulation. We further demonstrate that these two contrasting types, consistent with the Miocene evolutionary pathways of the eastern and western PRMB, respectively, are strongly conditioned by the along‐strike variation of stretching degree during extension.

Cretaceous basin evolution in northeast Asia: tectonic responses to the paleo-Pacific plate subduction.

ABSTRACTCretaceous rift basin evolution was an important part of the tectonic history of northeast Asia in the late Mesozoic. Three types of rift basins are identified—active, passive and wide rift basins—and they developed in different regions. Passive rift basins in the eastern North China craton are thought to be the consequence of crustal stretching and passive asthenospheric upwelling. Wide rift basins in the eastern Central Asian orogen are assumed to originate from gravitational collapse of the thickened and heated orogenic crust. Active rift basins in the northern North China craton are attributed to uprising of asthenospheric materials along a lithospheric-scale tear fault. Slab tearing of the subducting paleo-Pacific plate is postulated and well explains the spatial distribution of different types of rift basins and the eastward shifting of magmatism in the northern North China craton. The Late Cretaceous witnessed a period of mild deformation and weak magmatism, which was possibly due to kinematic variation of the paleo-Pacific plate.

Exhumation history of late Mesozoic intrusions in the Tongling–Xuancheng area of the Lower Yangtze region, eastern China: Evidence from zircon (U–Th)/He and apatite fission track thermochronology

The Lower Yangtze region is one of the most important metallogenic belts in eastern China, yet there are few precise low-temperature geochronological data to constrain the evolution of the orefield. We collected samples from 12 late Mesozoic intrusions in the Tongling–Xuancheng area for zircon (U–Th)/He (ZHe) and apatite fission track (AFT) dating. These intrusions are divided into early and late periods on the basis of their emplacement ages. The ZHe and AFT ages of the early intrusions are 144.6–111.0 Ma and 123.1–89.4 Ma, respectively, and those of the late intrusions are 119.9–71.3 Ma and 96.7–66.9 Ma, respectively, with rare outliers. The ZHe and AFT age distributions and the modeled low-temperature thermal histories of these intrusions reveal that the Tongling–Xuancheng area underwent three stages of rapid cooling: during the earliest, middle, and end of the Early Cretaceous. During the earliest Early Cretaceous, rapid cooling resulted from regional compression and in situ cooling of magma. The middle Early Cretaceous phase of cooling occurred in a regional extensional regime characterized by the development of large rift basins and the emplacement of intrusions. The rapid cooling at ~ 100 Ma was concurrent with a global plate reorganization event that resulted in regional basin inversion and the development of unconformities between the Lower and Upper Cretaceous strata in East Asia. Although Cenozoic uplift is not seen in the cooling paths of our samples, regional compression since the Paleogene has uplifted the orebodies to near-surface level, favoring ore exploration and mining.

Insights into evolution of a rift basin: Provenance of the middle Eocene-lower Oligocene strata of the Beibuwan Basin, South China Sea from detrital zircon

Investigating rift basins and their provenance is difficult because their tectonic framework is continuously evolving. The Beibuwan Basin in the South China Sea is a typical rift basin that shares many characteristics of major rift basins worldwide. We present new detrital zircon UPb ages from the Beibuwan basin and, by combining with published data from surrounding drainages, are able to define its potential sources and to investigate its provenance evolution from the middle Eocene-lower Oligocene. Detrital zircon age spectra from distinct structural units and formations of the rift basin reveal spatial-temporal differences in provenance. In the middle-upper Eocene, a similar age pattern with major peaks at 263–253 Ma and 450–416 Ma is preserved across the basin and suggests that sediment was uniformly derived from the Yunkai Massif and Hainan Island. In contrast, during the lower Oligocene the basin displays markedly diverse ages with peaks at 250–243, 416, 756, 967–945, 1096–1084 Ma. The increase of Neoproterozoic zircons in the lower Oligocene stratum suggests that a major change in provenance occurred, most likely associated with the reorganization of the paleo-Pearl River along its central reaches due to a topography reversal associated with the uplift of Tibet and the onset of seafloor spreading in the South China Sea. Our spatial analysis of ages indicates that during the lower Oligocene, the detrital input varied across the basin. Rifting processes in the Beibuwan Basin generated multiple isolated depocenters separated by intra-basinal highs. The basin's architecture greatly affected sediment routing, with structural highs serving as barriers to sediment transport. Sediment provenance in the Beibuwan Basin is primarily controlled by drainage evolution and isolated depocenters associated with an evolving topography, processes which are relevant to the evolution of rift basins globally.

The Unique Geoheritage of the Kachchh (Kutch) Basin, Western India, and its Conservation

The Kachchh Basin is a peri-cratonic rift basin in the Gujarat state of Western India which exposes a vast range of geological features which are of great importance for basic and applied research by geoscientists. The fossil-rich stratigraphy of the Kachchh Basin represents the past 200 million years of geological history from the Jurassic Period to the present day, including through the Neogene Period. Both Mesozoic and Cenozoic sequences are highly fossiliferous with a variety of mega, macro, and micro-fossils, including invertebrate, plant, trace, and vertebrate fossils. Crucially, the Kachchh Rift Basin demonstrates a uniquely well exposed, syn to post-rift tectono-sedimentary sequence formed from the Late Triassic to the Early Cretaceous during the break up of Eastern Gondwanaland. The basin is bounded by reactivated primary faults forming tilted horsts and half-grabens. Tilted-up edges of the uplifted fault blocks are represented by uplifted hilly ranges, whereas the intervening subsiding blocks are the geomorphological expression of the low-lying Rann and Banni plains (mudflats, marshes, and grassland). This activity was followed by an inversion cycle initiated in Late Early Cretaceous following the aborting of the rift and is active in the present neotectonic cycle. Thick Deccan Trap lava flows and basic intrusive bodies indicate associated deeper crustal and mantle processes associated with this rift basin evolution. Easily accessed, well-exposed continuous sedimentary sequences with scanty vegetation provide a unique window of opportunity to looking into the geological past of this unique landscape which has preserved the evidence of the evolution of life both in near shore, coastal, and terrestrial environment, as well as a rare opportunity to understand a history of syn/post-rift-fill sedimentation, volcanism, and tectono-geomorphical evolution. More recently the terrain has become a hotspot amongst the planetary scientists as it provides a terrestrial analogue for Lunar and Martian landscapes. These and many other features justify why the unique geological heritage of the Kachchh Basin should be conserved for posterity not only for geoscientists from all over the world but also for local communities and visitors who could learn from its 200 million year history. We therefore propose that the landscapes of the Kachchh Basin should be conserved as a National Geopark in India to achieve such ambitions, and we believe that in this, they have an immense potential to achieve many of the UN’s Sustainable Development Goals. It is proposed to create 9 geo-stations (for administration) and 75 geosites grouped under these stations within the proposed Kachchh Geopark.

Seismic interpretation of the Late Albian-Early Cenomanian Bahariya reservoirs of Burg El Arab oil field for tectonic evaluation: a case study from Western Desert, Egypt

The evolution of different rift basins has been investigated tectonically and stratigraphically for decades using different geological and geophysical techniques based on data collected from outcrops and subsurface sequences. The tectonic subsurface evaluation of the so-called Bahariya sequence of Burg El Arab (BEA) oil field (Western Desert, Egypt) was studied using eleven seismic profiles showing the structural framework of the BEA oil field. The Late Albian-Early Cenomanian Bahariya Formation was dissected by a series of normal fault system, trending in NW-SE and WNW-ESE directions. This system was created due to the Cretaceous-Early Tertiary tectonic deformations related to the Tethyan plate tectonics. These tectonics lead to the presence of a symmetrical anticlinal fold dissected by a set of normal faults that is clearly dipping towards SSE and extending along the direction of NNW-SSE. The results of the structural seismic interpretation of the BEA oil field indicated that there are two main tectonic deformations affecting the subsurface rocks of the Mesozoic: the Jurassic and Early Cretaceous rifting and Late Cretaceous-Early Tertiary shortening deformation. Understanding and interpretation of such structural styles and tectonics are a key issue in hydrocarbon exploration and development in the study area.

From widespread faulting to localised rifting: Evidence from K‐Ar fault gouge dates from the Norwegian North Sea rift shoulder

AbstractAlthough seismic and stratigraphic well information put tight constraints on rift basin evolution, eroded rift shoulders commonly expose polydeformed prerift basement whose deformation history may be difficult to constrain. In this work, we apply K‐Ar dating of fault gouge samples from 18 faults to explore the brittle deformation of the well‐exposed eastern rift margin to the northern North Sea rift. We find evidence of clay gouge formation since the Late Devonian, with distinct Permian and Jurassic fault activity peaks that closely match early stages of the two well‐established North Sea rift phases. A marked decay in fault density away from the rift margin confirms a close relationship between rifting and onshore faulting. The results show that initial rift‐related extension affected a much wider area than the resulting offshore rift. Hence our data support a rift model where strain is initially distributed over a several 100 km wide region, as a prelude to the development of the ~150–200 km wide Permo‐Triassic northern North Sea rift as defined by large marginal faults. Towards the end of the second rift phase, strain localises even more strongly to the 25–50 km wide Viking Graben. Interestingly, a period of early widespread extension is seen for both phases of North Sea rifting and may be a general characteristic of continental rifting. The documented prerift faulting and fracturing of the basement since the Devonian weakened the basement and probably facilitated the widespread initial extension that subsequently localised to form the northern North Sea rift, with further localisation to its relatively narrow central part (Viking Graben).

Tectono-sedimentary development of multiphase rift basins: An example of the Lufeng Depression

Tectono-sedimentary development of multiphase rift basins: An example of the Lufeng Depression

Multiphase activation of the boundary fault system of the eastern Dampier subbasin, Northwest Shelf of Australia

Investigating the timing, structural style, and development of multiphase extensional fault systems is essential for understanding rift basin evolution and for assessment of structural trap integrity. Borehole-controlled interpretation and analysis of two-dimensional and three-dimensional seismic data sets from the eastern Dampier subbasin indicate that a northeast-trending basement weakness zone was subjected to west-northwest–east-southeast oblique extension in the latest Triassic–late Middle Jurassic, resulting in systematic segmentation of the Rosemary fault system (RFS). Temporal stress change during Cretaceous north-south extension produced complex fault systems along the RFS, including (1) east-west–trending isolated faults with maximum displacement close to their center; (2) east-west–trending abutting faults, which initially nucleated as isolated faults, later abutted against the main structure, showing large displacement accruement on the composite fault; and (3) northeast-southwest–trending splay faults characterized by systematic left-stepping segmentation, with maximum displacement occurring at the point where the splay faults deviate from the main structure. In the Miocene, the RFS was locally reactivated by northwest-southeast compression in the northeastern part of the fault system, developing a compressive fault-propagation fold at the upper tip of the inverted extensional fault. This study suggests that the style of basin boundary fault reactivation depends largely on preexisting structures and temporal stress changes. The fault reactivation style is also a significant factor in influencing basin architecture, sediment distribution, fault linkage processes, and petroleum basin prospectivity.

Rift migration and transition during multiphase rifting: Insights from the proximal domain, northern South China Sea rifted margin

Multiphase rifting with tectonic quiescence intervals is one of the most distinguishing features during the evolution of rift basins and rifted margins, which always produce oceanward rift migration. However, this law seems to be challenged in the proximal domain of the northern South China Sea (SCS) rifted margin. Based on high-quality 2D/3D seismic reflection data and borehole data from the Zhu 1 Depression located in the northern SCS, we examine the temporal and spatial variations of rifting. The regional unconformity, cyclicity of faulting, and rotation in extension direction indicate that the Zhu 1 Depression experienced continuous two-phase rifting with four stages but without distinct tectonic quiescence intervals. In Stage 1 of Syn-rift 1, rifts were distributed across the whole depression. They began to migrate towards the continent in stage 2 of Syn-rift 1, which continued until stage 1 of Syn-rift 2, and then migrated to the east. Rifting was also accompanied by a clockwise rotation of the strikes of fault systems, which were ~N065°E and ~N080°E in stage 1 and stage 2 of Syn-rift 1, respectively, and then rotated to ~ S085°E during Syn-rift 2. We suggest that the interactions between the pre-existing basement fabrics and the regional stress fields rotating clockwise can account for this rotation and the eastward rift migration. The rift migration towards the continent is supposed to be caused by the emplacement of the high-velocity lower crust (HVLC) underneath the Dongsha Uplift, which may initiate in the Eocene. The distinctive multidirectional, multiphase, and multi-mechanical rift migration reflects the uniqueness of the SCS rifted margin. This study helps us understand the complicated rifting process that interacts with pre-existing fabrics, regional stress field, and the emplacement of high-velocity lower crust.

Extent and Cessation of the Mid‐Cretaceous Hikurangi Plateau Underthrusting: Impact on Global Plate Tectonics and the Submarine Chatham Rise

AbstractSubduction of oceanic plateaux are events that have occurred infrequently in Earth's history. Although rare, these events are thought to considerably influence regional tectonics and global plate motions. In the mid‐Cretaceous the oceanic Hikurangi Plateau collided with the formerly active East Gondwana margin: An event which falls into the same of a sudden change from subduction to extensional processes, including graben formation, development rift basin development, and exhumation of metamorphic core complexes in the Zealandia continent as well as a global‐scale plate reorganization event. In this study, we use recently acquired seismic refraction and gravity data along one profile across the submarine Chatham Rise, which represent a former accretionary wedge of the East Gondwana subduction zone. We demonstrate that the southward extent of the subducted Hikurangi Plateau in the lower crust beneath the submarine Chatham Rise along our profile is only ~150. This is ~150 km less than the previously suggested extent. Furthermore, we interpret that a slice of the subducted Phoenix Plate remains attached to the southern edge of the Hikurangi Plateau. We suggest that cessation of subduction in response to the Hikurangi Plateau jamming as well as the rollback and detachment of the Phoenix Plate slab played an important role in the changing tectonic forces across Zealandia in mid‐Cretaceous. Moreover, we suggest that the subduction cessation along the Hikurangi Plateau segment led to the fragmentation of the Gondwana subduction zone and Phoenix Plate, which significantly influenced and potentially prolonged the global mid‐Cretaceous plate reorganization event.

Detrital-zircon provenance of a Torridonian fluvial-aeolian sandstone: The 1.2 Ga Meall Dearg Formation, Stoer Group (Scotland)

The Meall Dearg Formation is a 1.2 Ga fluvial-aeolian sandstone exposed in the northwestern Highlands of Scotland, and represents one of the least studied rock units in the Torridonian succession, especially in terms of sedimentary provenance. Three hundred and nine detrital zircon grains were analysed from four samples of the Meall Dearg Formation using sensitive high-resolution ion microprobe, allowing for its provenance to be resolved at the terrane scale. Ages range from c. 3,100 to c. 1,750 Ma, with dominant proportions between c. 2,900 and c. 2,650 Ma, and subordinate fractions of c. 2,500 Ma and c. 1,900 Ma. U-Pb protolith-zircon ages from eight previous studies across the Lewisian Gneiss Complex were compiled and compared to detrital-age distributions of the Meall Dearg Formation. Strong similarities in age trends and palaeocurrent data corroborates the hypothesis of sedimentary provenance from the Lewisian Gneiss Complex. Detritus was prevalently derived from the Rhiconich and Gruinard terranes, and subordinately from the Assynt, Gairloch (including the Loch Maree Group and Ard Gneiss), and Ialltaig terranes. Age distributions from the fluvial and aeolian samples show near-matching distributions, indicating sediment transfer between adjacent depositional environments, and possibly derivation from similar source regions. The Meall Dearg Formation, part of the Stoer Group, is postulated to record the overfilling stage of a rift basin near the eastern flank of the Columbia Supercontinent at c. 1.2 Ga. Critical comparisons can be drawn between the Stoer Group and Gardar Rift of southern Greenland, based on their age, palaeogeography, and patterns of rift-basin development.

Destruction of the North China Craton and its influence on surface geology and terrestrial biotas

The North China Craton (NCC), a major geological element of East Asia, had hehaved as a stable craton untile late Mesozoic. The NCC experiencd multiple contraction and extension in late Mesozoic, evidened by three phases of shortening and two phases of stretching. The three shortening phases occurred in the late Middle Jurassic, earliest Early Cretaceous, and latest Early Cretaceous, respectively. Crustal stretching happened in Late Jurassic and Early Cretaceous times, and was marked by large-scale magmatism and development of rift basins and metamorphic core complexes. Widespread vigorous magmatism and extensive rifting indicate the loss of the NCC stability in the Early Cretaceous. Alternating crustal contraction and extension of the NCC are attributed to the change in subduction angles of the paleo-Pacific plate, with low-angle subduction resulting in shortening and high-angle subduction leading to stretching. However, magmatism, deformation and lithospheric thinning are merely the various expressions of the NCC destruction. It is shown that deep thermal processes must have strongly altered the properties of overlying lithosphere, resulting in a change from the continental into oceanic mantle. It was the transformation of physico-chemical properties of the lithospheric mantle that destabilized the NCC. Also noticeable are occurrences of the Yanliao and Jehol biotas that evolved in the Middle-Late Jurassic and Early Cretaceous, respectively, and just coincided with the initial and peak stages of the NCC destruction. Flourishing and vanishing of the biotas appeared closely related to volcanic intensity and rift basin development in space and time, implicating a profound control of deep dynamic processes on surface paleogeography and ecosystems. Implicitly, life should have been evolving in accord with surrounding environments. Volcanism might have exerted a strong influence on prosperity and disappearance of the Yanliao and Jehol biotas, with ash fallout leading to perfect preservation of a variety of precious fossils. Also noteworthy is that biotas thrived in the period when rift basins were developing, whereas biotas died out when basins were inverted. It is still a challenge to explore the linkage of terrestrial life evolution and tectonic processes. Recent developments in geodynamic studies reveal that subduction of the western paleo-Pacific plate might have produced a unique mantle profile in late Mesozoic, with the stagnant plates in mantle transition zone being sandwiched between the lower mantle and the big mantle wedge. Extensive dehydration and decarbonization of the stagnant plates resulted in melting and metasomatism of the upper and lower parts of mantle transition zone. These processes should have played a crucial role in altering the nature of lithospheric mantle and in triggering large-scale magmatism in the crust. A big mantle wedge is thought to have developed beneath the East Asia continent during the Early Cretaceous as a result of the slab rollback and the stagnation in the mantle transition zone. The combined tectonic processes must have exerted a profound influence on both the NCC destruction and Yanliao/Jehol biotal evolution. Interdisciplinary researches will help us understand the complicated interaction of deep and surface processes and comprehend the past, present and future of our habitable planet.

Provenance and paleogeography of the Jurassic Northwestern Qaidam Basin (NW China): Evidence from sedimentary records and detrital zircon geochronology

The Qaidam Basin is the largest Meso–Cenozoic petroliferous basin on the northeastern Tibetan Plateau and was situated at the northern margin of the Eastern Tethys during the Mesozoic. Jurassic strata are the main source rocks and reservoirs in the northern and western segments of this basin. However, due to exceptionally thick overlying Cenozoic strata, intensive post-depositional modification and limited deep exploration data, several crucial problems regarding this basin remain controversial, including provenance supply, original sedimentary extent, and overall properties in the Jurassic period. This hinders further understanding of regional geological evolution and hydrocarbon exploration. This paper focuses on comparison of residual Jurassic strata in the northwestern Qaidam Basin, the adjacent Saishiteng Mountains and the Suganhu area to the north based on sedimentology and provenance analyses. Detrital zircon geochronology was employed to examine the provenance of Jurassic sediments in different areas and to reconstruct their dispersal patterns. Results show that the Lower Jurassic deposits are characterized by near provenance and rapid facies shift. Fan deltas, braided river deltas, and lacustrine deposits are commonly developed. The Lower Jurassic source area was mainly located in the south Altyn Tagh Mountains. Sedimentary extent and depocenters in the Middle and Late Jurassic transferred northwards to the current piedmont of the Saishiteng and South Qilian mountains. The residual Jurassic in the Saishiteng Mountains is similar and comparable with coeval strata in the Suganhu area as well as the inner Qaidam Basin. Detrital zircon ages indicate multiple provenances in the Middle Jurassic, of which the southern Qilian Mountains are the most important. In the Middle Jurassic, deposition occurred across large areas of the Saishiteng Mountains which belonged to the same basin as the inner Qaidam Basin and the Suganhu areas. A series of small-scale continental rift basins developed in the western part of northwestern Qaidam in the Early Jurassic and then developed into a larger depression from the late Early Jurassic to the Late Jurassic. Post-collision extension was conducive to the development of rift basins in the Early Jurassic. Subduction of the northern Mongolian–Okhotsk and southern Central Tethys Ocean interactively led to an extensional tectonic setting and the development of a depression basin in late Early to Middle Jurassic. The continuous northward compression of the Qiangtang Block mainly resulted in a compressive tectonic setting in the Late Jurassic.