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Analyses of active and past normal fault behaviour in extensional settings provide key insights into regional-scale tectonic processes driven by plate boundary forces. To better understand past tectonic environments in the Tāmaki Makaurau-Auckland region of northern Aotearoa-New Zealand, we examine normal faults with Miocene to Quaternary movements dissecting the Early Miocene Waitematā Group. Structural data were collected from coastal outcrops, including fault geometry and kinematics, and reveal a dominant NE-trending strike direction, indicating NW-SE extension. Faults dissecting coeval and younger Waitākere Group rocks west of the study site exhibit the same NE-striking fabric, and thus may represent the westward continuation of Waitematā Group extensional structures. Furthermore, a NE-trending normal fault fabric is present within the adjacent Taranaki Basin, suggesting that the observed extension in the Waitematā Group was widespread over northern Aotearoa, with some extension still occurring today in southern parts of the Taranaki Basin. We propose that many of the observed normal faults in the Waitematā Group represent the northern portion of the Taranaki Basin fault system, with the Taranaki faults likely representing the younger continuation of the extension recorded in the Waitematā Basin, resulting from the migration of back-arc extension due to roll-back of the Hikurangi subduction margin. [ABSTRACT FROM AUTHOR]

Climate cycles fundamentally control surface processes that affect the distribution of water and sediment, and their associated loads, across the Earth's surface. Here, we use a geodynamic model to examine how water loading can affect mantle melt generation in continental rift settings covered by deep lakes. Our modeling results suggest that lake level fluctuations can modulate the timing and rate of mantle melting. A rapid lake level drop of 800 m has the potential to increase mantle melt volumes by enhancing mantle upwelling beneath the rift, whereas a lake level rise can lead to a reduction of mantle melting. The volume of melt produced driven by lake level fluctuations is also dependent on crustal rheology, extension rate, mantle potential temperature, and lithosphere thickness. Our study identifies the importance of water loading for controlling rift processes, while also demonstrating critical links between changing climate, rift evolution and mantle melting. Plain Language Summary: The break‐up of continents produces subsidence and the formation of rift valleys and where the climate is favorable, rift lakes. Changes in effective moisture in response to climate changes drive water level fluctuations in rift lakes, and their associated loads. But our understanding of the interaction between hydroclimate changes and rift basin evolution remains limited. To address this, we employed a geodynamic model to explore how water loading can influence mantle melt production in continental rift environments. Our model suggests that lake level fluctuations can have a detectable effect on the timing and pace of mantle melting. A lake level drop can increase mantle melt volume by enhancing mantle upwelling underneath the rift, while a lake level rise can lead to a reduction in mantle melting. Additionally, the amount of melt produced by these fluctuations depends on factors such as crustal rheology, extension rate, thermal gradient, and lithosphere thickness. Our findings reveal the significance of water loading in governing rift processes and highlight the potential links between changing climate, rift evolution, and mantle melting. Key Points: Lake level drops of 800 m can enhance decompressive mantle meltingA case study for the Turkana Rift shows a correlation between lake level drops and enhanced volcanism over the last 4 MyrSensitivity of mantle melting to lake loading is controlled by extension rate, mantle potential temperature, and lithosphere thickness [ABSTRACT FROM AUTHOR]

Abstract Magmatism is often invoked as critical for assisting strain focusing during continental rift development, and in driving lithospheric thinning and continental rupture. Accordingly, models of rift basin evolution in the East African Rift System (EARS) have previously focused on magma-rich basins; however, a complete understanding of how magmatism drives rift evolution requires analyses of both magma-rich and magma-poor extensional environments. We investigate a ∼10-million-year history of fault development in the magma-poor Lake Tanganyika Rift of the Western branch of the EARS, utilizing reprocessed legacy and high-resolution commercial seismic reflection datasets. Unlike the magma-rich Eastern branch, fault-strain does not focus into the basin floor along the rift axis within the first ∼10 million years of rift development. Instead, large rift-bounding faults (border faults) accommodate the majority of extensional strain (∼90% of an estimated 11.5 km of extension over the 52 km-wide rift) from rift inception through to present-day rifting. The Western branch of the EARS is also characterized by crustal structure and fault geometries that differ from those observed for successfully rifted magma-poor margins (e.g., listric normal faulting with mid-crustal detachments). In the absence of voluminous magmatism, we suggest that either (1) complete continental rupture cannot be achieved in the Western branch of the EARS, or (2) continental break up in the Western branch will proceed under a model that contrasts with that invoked for both the Eastern branch and magma-poor rifted margins generally. Highlights • First study quantifying changing fault activity during rift evolution in the Western branch of East Africa. • Rift-bounding border faults accommodate ∼90% of extension in the first 10 Myrs of rifting in the Western branch. • Fault-strain allocation in the Western branch contrast models from the Eastern branch and other magma-poor margins. [ABSTRACT FROM AUTHOR]

Development of fan deltas alongside intrabasinal structural highs has been overlooked compared to those forming on basin margins. However, these fan deltas may provide important clues regarding the tectonic and climatic controls on deposition during rift development. This paper documents fan delta deposition alongside an intrabasinal structural high within the Early Cretaceous Xiagou Formation of the Jiuquan Basin, China, using subsurface geological and geophysical data. Deposits observed in drill core support fan delta deposition occurring almost exclusively through subaerial and subaqueous gravity flows. Subsurface mapping reveals a consistent decrease in the areal extent of fan deltas from lowstand to highstand system tracts, suggesting that deposition alongside the structural high is sensitive to lake-level changes. The temporal and spatial distribution of the fan deltas display retrogradational stacking patterns, where fan deltas exhibit a decreasing lateral extent up-sequence until fan delta deposition terminated and was replaced by deposition of fine-grained lacustrine deposits. The retrogradational stacking patterns observed alongside the intrabasinal structural high are not observed in fan deltas along the basin margin in the lower parts of the Xiagou Formation. Subsidence profiles also show differential subsidence across the basin during the earliest stages of this formation, likely resulting from border fault movements. These data suggest that non-uniform stacking patterns in the lower parts of the Xiagou Formation reflect basin-scale tectonic movements as the dominant control on synrift deposition patterns. However, later stages of Xiagou Formation deposition were characterized by uniform subsidence across the basin, and uniform retrogradational stacking patterns for fan deltas alongside the intrabasinal structural high and border fault. These observations suggest that basin-scale tectonic movements played a relatively limited role in controlling sediment deposition, and imply a potential change to regional-scale processes affecting fan delta deposition during later synrift stages. Climate change is favored here as the region-scale control on the uniform retrogradational fan delta stacking patterns. This assertion is supported by pollen assemblages, isotope signatures, and organic geochemical analyses, which collectively suggest a change from a humid to semi-arid environment during later synrift stages. We suggest that variations in stacking patterns between different fan delta systems can provide insights into the basin- and regional-scale processes that control rift basin deposition. [ABSTRACT FROM AUTHOR]

Magma flow paths in sill-fed dikes of the Ferrar large igneous province (LIP), contrast with those predicted by classic models of dike transport in LIPs and magmatic rift settings. We examine anisotropy of magnetic susceptibility (AMS) flow paths in dike networks at Terra Cotta Mountain and Mt. Gran, which intruded at paleodepths of ~ 2.5 and ~ 1.5 km. These intrusions (up to 30 m thick) exhibit irregular, interconnected dike-sill geometries and adjoin larger sills (~ 200–300 m thick) at different stratigraphic levels. Both shallowly dipping and sub-vertical magma flow components are interpreted from AMS measurements across individual intrusions, and often match macroscopic flow indicators and variations in dike attitudes. Flow paths suggest that intrusive patterns and magma flow directions depended on varying stress concentrations and rotations during dike and sill propagation, whereas a regional extensional tectonic control was negligible or absent. Unlike giant dike swarms in LIPs elsewhere (e.g., 1270 Ma MacKenzie LIP), dikes of the Ferrar LIP show no regionally consistent vertical or lateral flow patterns, suggesting these intrusion were not responsible for long-distance transport in the province. In the absence of regionally significant, colinear dike swarms, or observed intrusions at crustal depths ≥ 4 km, we suggest that long distance magma transport occurred in sills within Beacon Supergroup sedimentary rocks. This interpretation is consistent with existing geochemical data and thermal constraints, which support lateral magma flow for ~ 3,500 km across the Gondwana supercontinent before freezing. [ABSTRACT FROM AUTHOR]

The structure of upper crustal magma plumbing systems controls the distribution of volcanism and influences tectonic processes. However, delineating the structure and volume of plumbing systems is difficult because (1) active intrusion networks cannot be directly accessed; (2) field outcrops are commonly limited; and (3) geophysical data imaging the subsurface are restricted in areal extent and resolution. This has led to models involving the vertical transfer of magma via dikes, extending from a melt source to overlying reservoirs and eruption sites, being favored in the volcanic literature. However, while there is a wealth of evidence to support the occurrence of dike-dominated systems, we synthesize field- and seismic reflection-based observations and highlight that extensive lateral magma transport (as much as 4100 km) may occur within mafic sill complexes. Most of these mafic sill complexes occur in sedimentary basins (e.g., the Karoo Basin, South Africa), although some intrude crystalline continental crust (e.g., the Yilgarn craton, Australia), and consist of interconnected sills and inclined sheets. Sill complex emplacement is largely controlled by host-rock lithology and structure and the state of stress. We argue that plumbing systems need not be dominated by dikes and that magma can be transported within widespread sill complexes, promoting the development of volcanoes that do not overlie the melt source. However, the extent to which active volcanic systems and rifted margins are underlain by sill complexes remains poorly constrained, despite important implications for elucidating magmatic processes, melt volumes, and melt sources. [ABSTRACT FROM AUTHOR]

Although subparallel swarms of dikes are thought to be the primary feeders to voluminous volcanic eruptions, increasing recognition of volumetrically significant sill complexes suggests that they too play an important role in magma ascent through the shallow crust. However, the extent to which sills and interconnected, sill-fed dikes actually transport magma to the earth's surface in many large igneous provinces (LIP) is presently unclear. By analyzing field relationships and dimensions of intrusions of the Ferrar LIP in South Victoria Land, Antarctica, we show that sill-fed dikes were the likely feeders for voluminous flood basalt eruptions. These intrusions are small but numerous, with cumulative dimensions equivalent to a feeder network 308,000 km long and 1.8 m wide. Due to the tremendous aerial extent of this intrusive network, each individual dike-feeder segment would only be required to actively feed magma for 2 to 3 days on average to erupt the 70,000 km 3 of flood lavas represented by the Kirkpatrick basalts. The Ferrar intrusions form a broadly-distributed array of small, moderately dipping dikes (<2 km long, 1.8 m wide, 56° mean dip) exhibiting almost any orientation. This sill-fed dike network contrasts with dike swarms conventionally depicted to feed flood basalt provinces, and has the appearance of a variably “cracked lid” atop a sill complex. The cracked lid model may apply to a range of shallow feeder systems (<4 km depth) intruding sedimentary basins, where the effects of far-field tectonic stresses are negligible and sill intrusions exert the dominant control on dike orientations. We conclude that sill inflation, and resulting deformation of surrounding host rock, plays a critical role in the ascent of magma in shallow volcanic systems that span the full spectrum of eruptive volumes. [ABSTRACT FROM AUTHOR]

SUMMARY We analyse 10 representative intrusions from two sets of inclined diabase (Ferrar Dolerite) sheets exposed at Allan Hills (South Victoria Land, Antarctica), using petrographic and rock magnetic methods to determine microfabrics and infer magma flow directions. At least one diabase sample was collected at the margins of each intrusion. Magnetite and pyrrhotite contribute to magnetic fabrics of the samples. Thirty-six magnetic fabric directions, inferred from the mutual arrangement of either the magnetic lineation, or the magnetic foliation plane and local macroscopic flow indicators (e.g. horn-shaped apophyses and kinks) at the tips and margins of each intrusion reveal composite (i.e. both lateral and vertical) flow paths recorded along each intrusive segment. Petrographic textures and multiple flow directions inferred at sheet-segment tips reveal that 'passive' injection of magma via hydrofracturing produced the local shallow large igneous province plumbing as a sill-dominated intrusive complex very close to, or intersecting the palaeosurface. This contrasts with 'classic' arrays of either vertically or laterally injected blade-like dykes. [ABSTRACT FROM AUTHOR]

Field observations and structural data from intrusive complexes at Allan Hills and Terra Cotta Mountain, South Victoria Land, Antarctica, demonstrate that interconnected sills and inclined sheets transported magma through the shallow subsurface. These sills and sheets represent the upper-crustal (top 4 km) plumbing system of the 183 Ma Ferrar large igneous province. The sheets are short in length (<1500 m), are moderately inclined (47° and 51° means), and show meter-scale variations in attitude; in places, they intruded bedding planes, resulting in stepped sheet-sill geometries. Sheet geometries and their relationship to the surrounding country rock are consistent with peripheral sheet intrusion under local magmatic stresses arising from roof-lift during sill injection. The sheet intrusions thus reflect the intrusive process itself rather than a far-field tectonic stress regime. The sills and sheets, together with local dolerite masses, formed the intrusive network that supplied magma to the Mawson Formation pyroclastic rocks in various parts of South Victoria Land and, by inference, the Kirkpatrick flood basalt lavas. The predominance of inclined sheets rather than steeply dipping dikes indicates a magmatic environment that is unlike the Jurassic rift arm inferred by previous authors. This could be explained using any of the following three scenarios. (1) The axis of the rift, and hence any rift-hosted dikes, lies beyond the current exposure area. (2) The regionally extensive Ferrar sills may have provided rheologically weak horizons that limited mechanical coupling of the basement rocks and overlying Beacon Supergroup, locally detaching the upper 4 km of the crust from possible synmagmatic basement extension below. (3) The Ferrar large igneous province was emplaced in a neutral tectonic setting. In this scenario, broad-scale distribution of magma through the province was controlled by preexisting structure in the basement, and local intrusion geometries reflect the physical interaction of intruding magma with bedding anisotropy of the Beacon Supergroup. [ABSTRACT FROM AUTHOR]

Lake Taupō (Taupō-nui-a-Tia) infills the composite caldera above an active rhyolitic magmatic system in the central Taupō Volcanic Zone (TVZ). Ground deformation is a key unrest indicator at Taupō volcano. We present a spreadsheet tool, TaupōInflate, to calculate and plot ground deformation from magmatic inflation at depth beneath Taupō caldera. Examples show detection limits for inflating magma bodies and their ascent through the crust beneath Lake Taupō. Source locations where it is challenging to detect even substantial volumes of inflating magma bodies are as large as 20 km3, with volume changes up to 0.01 km3, owing to the restricted station placement around the lake, although a dike propagating from shallow crustal depths towards the surface is likely to be detectable. For a magma overpressure of 10 MPa, the sizes of detectable inflating bodies at depths of 5–8 km using the present monitoring system are larger than the volumes of many past eruptions, illustrating the importance of future improvements to the geodetic network. We discuss the potential for future equipment installation, including lakebed instrumentation that would require approval of local iwi Ngāti Tūwharetoa through the Tūwharetoa Māori Trust Board who oversee the health and wellbeing of Lake Taupō. [ABSTRACT FROM AUTHOR]

Geochemical investigations of volatiles in hydrothermal systems are used to understand heat sources and subsurface processes occurring at volcanic-tectonic settings. This study reports new results of gas chemistry and isotopes (O, H, N, C, and He) of thermal spring samples (T = 36.8–83.5 °C; pH = 8.5–10.3) from the Magadi and Natron basin (MNB) in the East African Rift (EAR). Although a number of thermal springs are shown to ascend along normal faults and feed into major lakes (Magadi, Little Magadi, and Natron), volatile sources and fluxes of these fluids are poorly constrained. CO 2 is the most abundant phase (up to 996.325 mmol/mol), and the N 2 -He-Ar abundances show a mixture of dissolved gases from deep (mantle-derived) and shallow (air/air saturated water) sources. The H 2 -Ar-CH 4 -CO 2 geothermometers indicate that equilibrium temperatures range from ~ 100 to ~ 150 °C. δ 18 O (− 4.4 to − 0.2‰) and δD (− 28.9 to − 3.9‰) values of the MNB thermal waters still lie slightly to the right of the local meteoric water lines, reflecting minor evaporation. Each mixing relationship of N 2 (δ 15 N = − 1.5 to 0.4‰; N 2 / 3 He = 3.92 × 10 6 –1.33 × 10 9 , except for an anomalous biogenic sample (δ 15 N = 5.9‰)) and CO 2 (δ 13 C = − 5.7 to 1.6‰; CO 2 / 3 He = 7.24 × 10 8 –1.81 × 10 11 ) suggests that the predominant mantle component of the MNB volatiles is Subcontinental Lithospheric Mantle (SCLM). However, N 2 is mostly atmospheric, and minor CO 2 is contributed by the limestone end-member. 3 He/ 4 He ratios (0.64–4.00 Ra) also indicate a contribution of SCLM (R/Ra = 6.1 ± 0.9), with radiogenic 4 He derived from a crustal source (R/Ra = 0.02). The MNB 4 He flux rates (3.64 × 10 11 to 3.34 × 10 14 atoms/m 2 s) are significantly greater than the reported mean of global continental flux values (4.18 × 10 10 atoms/m 2 s), implying that magma intrusions could supply mantle 4 He, and related heating and fracturing release crustal 4 He from the Tanzanian craton and Mozambique belt. Total flux values (mol/yr) of 3 He, N 2 , and CO 2 are 8.18, 4.07 × 10 7 , and 5.31 × 10 9 , which are 1.28%, 2.04%, and 0.24% of global fluxes, respectively. Our results suggest that the primary source of magmatic volatiles in the MNB is SCLM, with additional crustal contributions, which is different from the KRV volatiles that have more asthenospheric mantle components. Volatiles from SCLM in magmas stall in the crust to heat and fracture country rock, with accompanying crustal volatile release. These volatile signatures reveal that MORB-type mantle replaces a relatively small volume of SCLM during incipient rifting (< 10 Ma) in the EAR. [ABSTRACT FROM AUTHOR]

Continental rifts normally initiate within previously deformed lithosphere and thus their evolution and architecture can be largely controlled by inherited weak zones in the pre-rift crust. Here, we quantify the role of the strength and obliquity of pre-existing crustal-scale weak zones in the evolution of continental rift systems. We use a 3D numerical geodynamic model to assess strain localization, associated fault development, and rift segmentation during the early stages of tectonic extension. We find that both the strength and obliquity of the weak zones significantly influence the patterns of strain localization. A pre-existing very weak zone with low obliquity can promote the development of continuous and long-lived border faults parallel to the rift axis. Conversely, a comparatively strong weak zone with high obliquity leads to a staggered en-echelon rift geometry that lacks rectilinear laterally persistent strain localization. Furthermore, we find that rift obliquity and weak zone strength may modulate rift fault length, throw, and azimuth. These results provide new and compelling insights into the structure and evolution of natural active rifts that develop within orogenic basement terranes. • 3D geodynamic models elucidate evolution of early-stage rifting with pre-existing weak zone. • Strength of inherited weak zones modulates early-stage rift architecture. • Fault statistic analysis quantifies the impacts of weak zone strength and obliquity. [ABSTRACT FROM AUTHOR]

Active continental rifts are ideal sites for understanding the break-up of continents, and long-lived rift lake environments are known as important reservoirs for endemic communities and biodiversity. The sedimentary fill of the Lake Tanganyika Rift records a long history of continental extension and variable tropical climate, that is unparalleled in its duration and fidelity. Recently acquired, state-of-the-art 2D seismic reflection data, together with reprocessed legacy data, are used to evaluate the evolution and distribution of sedimentary facies over the rift lake. Using seismic stratigraphic analysis, we reconstruct past depositional environments and the paleogeography of the lake and assess how tectonic-driven subsidence and hydroclimate variability modified the lake basin. We identify six syn-rift seismic units that overly the acoustic basement and identify depositional units beneath the syn-rift sequence that suggest episodes of pre-rift sedimentation. Based upon the seismic facies analysis, the earliest seismic stratigraphic unit is interpreted as deposited in an early-stage rift system of low-relief, that was dominated by alluvial, fluvial and shallow lacustrine conditions. Subsequent units exhibit attributes of a lacustrine environment of much greater water depth, enhanced catchment relief and accommodation, consistent with a more mature rift. In Seismic units 2–5, we observe extensive deltaic deposits and deep-water fans, and locally canyons, channels, channel-levee complexes, turbidites, slumps and other mass flow deposits. In the latter part of its history, erosional surfaces and abundant lowstand delta facies are observed, indicating the rift experienced dramatic hydroclimate cycles. We assess the relative timing of key features of the rift, including the emergence of major structures and rift segment boundaries, development of major drainages and linkages to upstream rift lakes. The evolving sedimentary facies of the rift illustrate a shallow- to-deep progression of rift valley environments and the more limited littoral habitats that influenced the evolution of its unique endemic organisms. • The study presents the basin evolution of the Lake Tanganyika Rift, East Africa. • We use seismic stratigraphic approach to reconstruct past depositional environments. • We assess the influence of paleogeography to the habitats for aquatic organisms. • Fluvial and shallow lakes dominated the early stage of the rifting. • During later stages of rift development, extension increased causing dramatic relief. • Hydroclimate variations promoted large shifts in lake level and sedimentary facies. [ABSTRACT FROM AUTHOR]

The article discusses the structure of upper crustal magma plumbing systems that controls the distribution of volcanism and influences tectonic processes. It discusses the models involving the vertical transfer of magma via dykes, extending from a melt source to overlying reservoirs and eruption sites. The article mentions that plumbing systems need not be dominated by dykes and magma can be transported within widespread sill-complexes, promoting the development of volcanoes.