Abstract. Clinoforms are the building blocks of prograding stratigraphic sequences. These sigmoid-shaped surfaces can be found forming today on modem deltas. Sedimentation rate profiles over the clinoform surface of these deltas show low rates of sediment accumulation on both topset and bottomset regions, with a maximum accumulation rate on the upper foreset region. We present a model for the formation of clinoforms that relies on the interpretation of modem clinoform sedimentation as a result of the distribution of shear stresses at the mouth of a river. Model clinoform surfaces are generated using an equation for the conservation of suspended sediment concentration, together with a conservation of fluid equation for simple time-averaged flow velocity fields. In the model, suspended sediment is advected horizontally into a basin, and gravitational settling of sediment particles is counteracted by vertical turbulent diffusion. In shallow water, shear stresses are too large to allow deposition, and sediment bypasses the topset region. With increasing water depth, near-bed shear stresses decrease, and sediment is allowed to deposit at the foreset region, with gradually decreasing rates toward deeper water. This sedimentation pattern leads to progradation of the clinoform surfaces through time. The clinoform surfaces produced by the model capture the fundamental morphological characteristics of natural clinoforms. These include the gradual slope rollover at the topset and bottomset, steeper foreset slopes with increased grain size, and an increase in foreset slope through time as clinoforms prograde into deeper water. Because the parameters controlling the model clinoforms have a direct relation to physical quantities that can be measured in natural systems, the model is an important step toward unraveling the physical processes associated with these deposits.

[1] Oblique rifting began synchronously along the length of the Gulf of California at 6 Ma, yet there is no evidence for the existence of oceanic crust or a spreading transform fault system in the northern Gulf. Instead, multichannel seismic data show a broad shallow depression, $70 Â 200 km, marked by active distributed deformation and six $10-km-wide segmented basins lacking well-defined transform faults. We present detailed images of faulting and magmatism based on the high resolution and quality of these data. The northern Gulf crust contains a dense (up to 18 faults in 5 km) complex network of mainly oblique-normal faults, with small offsets, dips of 60-80°and strikes of N-N30°E. Faults with seafloor offsets of tens of meters bound the Lower and two Upper Delfín Basins. These subparallel basins developed along splays from a transtensional zone at the NW end of the Ballenas Transform Fault. Twelve volcanic knolls were identified and are associated with the strands or horsetails from this zone. A structural connection between the two Upper Delfín Basins is evident in the switching of the center of extension along axis. Sonobuoy refraction data suggest that the basement consists of mixed igneous sedimentary material, atypical of mid-ocean ridges. On the basis of the near-surface manifestations of active faulting and magmatism, seafloor spreading will likely first occur in the Lower Delfín Basin. We suggest the transition to seafloor spreading is delayed by the lack of strain-partitioned and focused deformation as a consequence of shear in a broad zone beneath a thick sediment cover.

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The late Tertiary subsidence history of the southern Levant continental margin, situated in the southeastern Mediterranean Sea, was quantitatively analyzed. Paleodepth reconstruction across the margin off Ashdod suggests the existence of a deep basin in pre‐Messinian time which resembles the present one. This implies that, the deposition of the evaporites in the study area during the Messinian desiccation of the Mediterranean Sea occurred in a deep basin. The path of the tectonic subsidence of the basement since early Tertiary is generally smooth as expected from the nature of the thermal subsidence. The unconformity beneath the Messinian indicates erosion of 50–200 m at the coastal plain. In the Pliocene, the tectonic subsidence in the coastal plain and shelf area diverts from the expected thermal path and increases from 250 m to 450 m, respectively. In the Quaternary the rate of tectonic subsidence nearly resumed the predicted thermal subsidence. Sedimentation and subsidence rates decrease but are still higher than those of the pre‐Messinian. We suggest that the evolution of the southern Levant margin is most probably influenced by three main causes: (1) the Messinian event in late Miocene, (2) the deposition of large volumes of Nile derived sediments since the Pliocene, and (3) the flexural response of the lithosphere to the load from the Nile delta and/or from the uplift of the Judea Mountains (the western shoulder of the Dead Sea Transform). We interpret the latter to be the cause of the anomalous subsidence of the southern Levant margin during the Pliocene.

The Gulf of Suez and Red Sea rifts are currently bordered by large asymmetric uplifts that are undergoing erosion. We find that the amount and timing of erosion vary systematically along the strike of the margin and have been controlled by variations in the prerift stratigraphy. The prerift strata are composed of cliff‐forming Eocene‐Cretaceous carbonates overlying the easily eroded Cretacous‐Cambrian “Nubian” sandstone. This lithologic succession promotes scarp retreat of the sedimentary section, followed by dissection of the underlying basement. The prerift section thins from over 2000 m at the northern end of the rift to less than 400 m at its junction with the Red Sea. Thus, at the northern part of the Gulf of Suez, the Nubian sandstone is minimally exposed, and the carbonates form a scarp at the rift border fault. Farther south, undercutting of the carbonates by erosion of the sandstone has resulted in scarp retreat. The escarpment cuts diagonally away from the border fault and is over 100 km inland from the border fault at the southernmost Gulf of Suez. The amount of retreat varies inversely with the sediment thickness. Exposure and erosion of basement are initiated by the retreat of the escarpment, and the depth of erosion, as indicated by fission track ages, increases with distance from the escarpment. These observations are explained by a model in which erosion along the Gulf of Suez is initiated as rift flank uplift becomes sufficiently large to expose the friable sandstones. Undercutting of the escarpment and exhumation of basement has been propagating northward and westward for at least 20 m.y. The average rate of scarp retreat has been 6 km/m.y. and the along‐strike propagation of the erosion has been 12 km/m.y. The diachronous erosion of the the rift flanks at the Gulf of Suez highlights the importance of distinguishing between the timing of uplift and of erosion. Both thermochronometric and stratigraphic data primarily indicate the timing of erosion, which may differ significantly from the timing of the uplift that initiates it. They must be interpreted carefully to avoid erroneous conclusions about rift tectonics.

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The stratigraphic record of Cenozoic uplift and denudation of the Himalayas is distributed across its peripheral foreland basins, as well as in the sediments of the Ganges–Brahmaputra Delta (GBD) and the Bengal–Nicobar Fan (BNF). Recent interrogation of Miocene–Quaternary sediments of the GBD and BNF advance our knowledge of Himalayan sediment dispersal and its relationship to regional tectonics and climate, but these studies are limited to IODP boreholes from the BNF (IODP 354 and 362, 2015-16) and Quaternary sediment cores from the GBD (NSF-PIRE: Life on a tectonically active delta, 2010-18). We examine a complementary yet understudied stratigraphic record of the Miocene–Pliocene ancestral Brahmaputra Delta in outcrops of the Indo-Burman Ranges fold–thrust belt (IBR) of eastern India. We present detailed lithofacies assemblages of Neogene delta plain (Tipam Group) and intertidal to upper-shelf (Surma Group) deposits of the IBR based on two ∼ 500 m stratigraphic sections. New detrital-apatite fission-track (dAFT) and (U-Th)/He (dAHe) dates from the Surma Group in the IBR help to constrain maximum depositional ages (MDA), thermal histories, and sediment accumulation rates. Three fluvial facies (F1–F3) and four shallow marine to intertidal facies (M1–M4) are delineated based on analog depositional environments of the Holocene–modern GBD. Unreset dAFT and dAHe ages constrain MDA to ∼ 9–11 Ma for the Surma Group, which is bracketed by intensification of turbidite deposition on the eastern BNF (∼ 13.5–6.8 Ma). Two dAHe samples yielded younger (∼ 3 Ma) reset ages that we interpret to record cooling from denudation following burial resetting due to a thicker (∼ 2.2–3.2 km) accumulation of sediments near the depocenter. Thermal modeling of the dAFT and dAHe results using QTQt and HeFTy suggest that late Miocene marginal marine sediment accumulation rates may have ranged from ∼ 0.9 to 1.1 mm/yr near the center of the paleodelta. Thermal modeling results imply postdepositional cooling beginning at ∼ 8–6.5 Ma, interpreted to record onset of exhumation associated with the advancing IBR fold belt. The timing of post-burial exhumation of the IBR strata is consistent with previously published constraints for the avulsion of the paleo-Brahmaputra to the west and a westward shift of turbidite deposition on the BNF that started at ∼ 6.8 Ma. Our results contextualize tectonic controls on basin history, creating a pathway for future investigations into autogenic and climatic drivers of behavior of fluvial systems that can be extracted from the stratigraphic record.

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