North-trending rifts in the Himalayas induced by rapid synconvergent exhumation
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SAC files of receiver functions<p>Continental break-up at Rift-Rift-Rift triple junctions commonly represents the &#8220;prequel&#8221; of oceanic basin formation. Currently, the only directly observable example of a Rift-Rift-Rift setting is the Afar triple junction where the African, Arabian and Somalian plates interact to form three rift branches, two of which are experiencing oceanization (the Gulf of Aden and the Red Sea). The younger of the three (the Main Ethiopian Rift) is still undergoing continental extension. We performed analogue and numerical models simulating continental rifting in a Rift-Rift-Rift triple junction setting to investigate the resulting structural pattern and evolution. By adopting a parametrical approach, we modified the ratio between plate velocities, and we performed single-phase (all the three plates move) and two-phase models (with a first phase where only one plate moves and a second phase where all the three plates move). Additionally, the direction of extension was changed to induce orthogonal extension only in one of the three rift branches. Our single-phase models suggest that differential extension velocities in the rift branches determine the localization of the triple junction, which is located closer to the rift branch experiencing slower extension velocities. Furthermore, imposed velocities affect the distribution of deformation and the resulting pattern of faults. The effect of a faster plate is to favour the formation of structures trending orthogonal to dominant velocity vectors, while faults associated with the movement of the slower plates remain subordinate. In contrast, imposing similar velocities in all rift arms leads to the formation of a symmetric fault pattern at the triple junction, where the distribution of deformation is similar in the three rift branches. Two-phase models reveal high-angle faults interacting at the triple junction, confirming that differential extension velocities in the three rift branches strongly affect the fault pattern development and highlighting geometrical similarities with the Afar triple junction.</p>
Triple junction
Half-graben
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Bedrock
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Abstract Emplacement of submarine landslides, or mass‐transport deposits, can radically reshape the physiography of continental margins, and strongly influence subsequent sedimentary processes and dispersal patterns. Typically, progressive healing of the complicated relief generated by the submarine landslide occurs prior to progradation of sedimentary systems. However, subsurface and seabed examples show that submarine channels can incise directly into submarine landslides. Here, the evolution of a unique exhumed example of two adjacent, and partially contemporaneous, submarine channel‐fills is documented. The channels show deep incision (>75 m), and steep lateral margins (up to 70°), cut into a >200 m thick submarine landslide. The stepped basal erosion surface, and multiple terrace surfaces, are mantled by clasts (gravels to cobbles) reflecting periods of bedload‐derived sedimentation, punctuated by phases of downcutting and sediment bypass. The formation of multiple terrace surfaces in a low aspect ratio confinement is consistent with the episodic migration of knickpoints during entrenchment on the dip slope of the underlying submarine landslide. Overlying sandstone‐rich channel‐fills mark a change to aggradation. Laterally stacked channel bodies coincide with steps in the original large‐scale erosion surface, recording widening of the conduit; this is followed by tabular, highly aggradational fill. The upper fill, above a younger erosional surface, shows an abrupt change to partially confined tabular sandstones with normally graded caps, interpreted as lobe fringe deposits, which formed due to down‐dip confinement, followed by prograding lobe deposits. Overlying this, an up‐dip avulsion induced lobe switching and back‐stepping, and subsequent failure of a sandstone body up‐dip led to emplacement of a sandstone‐rich submarine landslide within the conduit. Collectively, this outcrop represents episodic knickpoint‐generated incision, and later infill, of a slope adjusting to equilibrium. The depositional signature of knickpoints is very different from existing models, but is probably reflective of other highly erosional settings undergoing large‐scale slope adjustment.
Submarine landslide
Aggradation
Turbidity current
Mass wasting
Progradation
Passive margin
Terrace (agriculture)
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During the past decade, the Río Grande rift has been recognized as one of the major Cenozoic continental rift systems. The other widely recognized rift systems are the East African rift, the Rhine graben, and Lake Baikal. A series of special publications (Riecker, 1979; Baldridge et al., 1984; Keller, 1986) have drawn the attention of the international scientific community to the Río Grande rift. Early studies suggested this rift might only extend as far south as the Socorro, New Mexico región which is about 300 km north of El Paso/Juárez. However, more recent geological (Seaget and Margan, 1979) and geophysical (Daggett et al., 1986; Sinno et al., 1986; Keller et al., 1988) studies document that the rift can be differentiated from the Basin and Range province and extends for a significant distance into the State of Chihuahua.doi: sin doi
East African Rift
Rift valley
Half-graben
Rift zone
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Rift zone
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Uplifted flanks at intracontinental rifts are supported by flexural isostasy, as shown by the pattern of isostatic residual gravity anomalies associated with them. Models for flexural rift flank uplift differ in their kinematic description of extension, with respect to the asymmetry of rifting and the importance of brittle versus elastic upper crustal deformation. In this paper, I test different kinematic models of continental extension by comparing their predictions of rift flank topography and crustal structure with observations from the Baikal rift (SE Siberia). The rift is characterized by prominent flank topography on both sides of lake Baikal. The flanks reach similar elevations but differ in their structure: the tilt of the footwall flank is away from the basin, whereas the basinward part of the hanging wall flank tilts toward the basin center. Fission track data indicate that very little erosion affected the flanks since rifting started; geomorphological and sedimentological observations suggest that prerift relief was minor. Thus, the present‐day topography reflects rift‐related tectonic uplift. Pure‐shear “necking” and pure‐shear/simple‐shear “detachment” models of extension predict the topographic and Bouguer gravity anomaly patterns observed along a profile across the central Baikal rift equally well. They do not permit to discriminate between different scenarios that have been proposed for the central Baikal rift; that is, half‐graben versus full graben development; rifting at a continuous rate since the Oligocene versus a large increase in extension rate since the Pliocene. The models predict that the kinematics of rifting in Baikal are controlled by a midcrustal (20 km) depth of necking and/or a mid to lower crustal (20–30 km) detachment level; best‐fit elastic thicknesses are in the range 30–50 km. These predictions are in agreement with results from coherence studies of Bouguer gravity and topography, as well as with the rheology of the lithosphere underneath Baikal as inferred from heat flow, seismic refraction and seismological observations. In contrast, a “flexural cantilever” model with low (< 10 km) elastic thickness predicts topographic patterns which are very different from those observed, for a wide range of rifting scenarios. Significant (> 3 km) erosion of the footwall flank is required to fit the topography if a flexural cantilever model is applied; this is incompatible with the fission track data. Thus, the kinematics of extension at deep and narrow intra‐continental rifts such as Baikal appear to be controlled by a strong elastic lithosphere and require significant brittle deformation of the upper crust, as suggested by dynamic models for continental rifting.
Rift zone
Half-graben
Isostasy
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The Oak Ridges Moraine in southern Ontario is a poly- genetic moraine constructed of a number of coalesced deposits of gla- cifluvial and glacilacustrine origin. A detailed study of the facies ar- chitecture has been completed on a series of pit sections extending ; 300 m subparallel to the paleoflow direction. Eight major lithofacies and five facies associations have been described. These data have been interpreted to be upper-flow-regime hyperconcentrated-flood-flow de- posits emplaced under a regime of rapid flow expansion and loss of transport capacity within a plane-wall jet with an associated hydraulic jump. Deposition from the plane-wall jet with jump occurred in three zones of flow transformation: zone of flow establishment, transition zone, and zone of established flow. Massive gravels with unconsolidated sand intraclasts and open-work gravel / gravel-sand couplets were de- posited in the zone of flow establishment by hyperconcentrated and supercritical flows, respectively. Immediately downflow low-angle cross-stratified sand incised by steep-walled scours infilled by diffusely graded sand define the transition zone, the zone of maximum vortex erosion, and the distal limit of deposits emplaced under upper-flow- regime conditions. These strata record rapid bed aggradation from sediment-laden supercritical flows that episodically were scoured by large vortices generated within migrating hydraulic jumps. Strati- graphically upward and downflow strata consist only of lower-flow- regime sedimentary structures. Medium-scale, planar cross-strata and small-scale cross-lamination related to migrating 2-D dunes and cur- rent ripples, respectively, characterize the zone of established flow. The facies and sediment architecture suggest that this fan was deposited during a relatively short period of time (days, weeks) by energetic sed- iment-laden floods.
Hyperconcentrated flow
Hydraulic jump
Debris flow
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Tectonophysics
Rift zone
Rift valley
East African Rift
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Caldera
Alluvial fan
Bedrock
Aggradation
Landform
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