ALBERT

Description: New ostracode data from the West African margin indicate that the Outer Basin Sediment Wedge (also termed the pre-salt wedge' and the pre-salt sag basin') is Neocomian to Aptian in age and is contemporaneous with syn-rift deposits developed inboard of the Atlantic hinge zone. Despite the fact that the Outer Basin Sediment Wedge is clearly a syn-rift deposit, it does not exhibit any of the diagnostic characteristics of brittle deformation, such as the existence of normal faults and the faulting and rotation of crustal blocks. Such features are common between the Atlantic and Eastern hinges for the early stages of rifting between West Africa and Brazil, which occurred as a series of extensional phases commencing in the Berriasian and culminating in the Late Aptian. To reconcile the concomitant development of fault-controlled subsidence between the hinges and across the Atlantic hinge zone and sag-basin development seaward of the Atlantic hinge zone requires that: (1) extension seaward of the Atlantic hinge is the result of strain-partitioning between a relatively non-deforming upper crust (i.e. the upper plate) and a ductile-deforming lower crust and lithospheric mantle (i.e. the lower plate) during the second and third rift phases, while (2) between the hinges, early brittle deformation (normal faulting) progresses to ductile deformation in the third rift phase. During the third rift phase, lower plate ductile deformation across the entire region generated regional subsidence both seaward of the Atlantic hinge and between the hinges with little attendant brittle deformation. This extension style produced, directly or indirectly, a sequence of crucial events across the West African margin: (1) the development of the pre-Chela unconformity as lake level dropped in the Early Aptian, exposing the prograding deltas of the Argilles Vertes Formation; (2) the regional development of the Chela unconformity and transgressive lag deposits of the Chela Formation in the Mid-Aptian; (3) the development of regionally extensive, shallow-water, restricted marine conditions across the entire margin (between West Africa and Brazil) immediately prior to evaporite precipitation; and (4) the development of significant post-rift accommodation (deposition of the Late Cretaceous, Paleogene and Neogene formations) in the same region previously characterized by minor syn-rift faulting, repeated dessication cycles (allowing the precipitation of thick evaporites) and negligible erosional truncation of earlier syn-rift units. Previous workers have suggested that the Loeme evaporites were formed as part of the rapid, early post-rift phase of basin subsidence as the region became inundated by sea water across the Walvis Ridge. In this model, it is difficult to develop the restrictive environments required to deposit the thick (〉1 km) evaporites of the Loeme Formation (and the equivalent Ezanga and Ibura evaporites of Gabon and Brazil, respectively) across the entire West African-Brazilian rift system. The existence of shallow-water environments across the entire region is not consistent with water depths determined from the relief of clinoform foresets existing immediately prior to evaporite deposition thus requiring tectonic uplift of the deep-water regions. These evaporites, therefore, appear to be part of the late-stage syn-rift sediment package and the break-up unconformity, if it exists, separates the Loeme evaporites below from the overlying Albian carbonates. A direct consequence of ductile extension is one of increased heat input accompanying the rift stage in those areas dominated by syn-rift sag-basin development. The distribution and amplitude of the heat pulse is governed by the geometry of the mid-crustal weak zone and the distribution and amplitude of the lower plate extension. Seaward of the Atlantic hinge zone, the maximum heat flow is predicted to be in excess of 200 mW m-2, whereas between the hinge zones, the heat flow is significantly less and ranges between 20mW/m2 and 100 mW/m2. Because sediment temperature is a function of thermal conductivity and thickness of sediment overburden, the viability of syn-rift sources and prospectivity of the deep-water West African margin will, to a large degree, depend on the delicate interplay between the cooling of the extended lithosphere and subsequent burial of source rocks as a function of time.

Description: A seismic compressed high-intensity radar pulse (CHIRP) survey of Pyramid Lake, Nevada, defines fault architecture and distribution within a key sector of the northern Walker Lane belt. More than 500 line-kilometers of high-resolution (decimeter) subsurface imagery, together with dated piston and gravity cores, were used to produce the first comprehensive fault map and attendant slip rates beneath the lake. A reversal of fault polarity is observed beneath Pyramid Lake, where down-to-the-east slip on the dextral Pyramid Lake fault to the south switches to down-to-the-west displacement on the Lake Range fault to the north. Extensional deformation within the northern two thirds of the basin is bounded by the Lake Range fault, which exhibits varying degrees of asymmetric tilting and stratal divergence due to along-strike segmentation. This structural configuration likely results from a combination of changes in slip rate along strike and the splaying of fault segments onshore. The potential splaying of fault segments onshore tends to shift the focus of extension away from the lake. The combination of normal- and oblique-slip faults in the northern basin gives Pyramid Lake its distinctive "fanning open to the north" geometry. The oblique-slip faults in the northwestern region of the lake are short and discontinuous in nature, possibly representing a nascent shear zone. In contrast, the Lake Range fault is long and well defined. Vertical slip rates measured across the Lake Range and other faults provide new estimates on extension across the Pyramid Lake basin. A minimum vertical slip rate of ~1.0 mm/yr is estimated along the Lake Range fault. When combined with fault length, slip rates yield a potential earthquake magnitude range between M6.4 and M7.0. Little to no offset on the Lake Range fault is observed in the sediment rapidly emplaced at the end of Tioga glaciation (12.5–9.5 ka). In contrast, since 9.5 ka, CHIRP imagery provides evidence for three or four major earthquakes, assuming a characteristic offset of 2.5 m per event. Regionally, our CHIRP investigation helps to reveal how strain is partitioned along the boundary between the northeastern edge of the Walker Lane and the northwest Basin and Range Province proper.

Description: The West Tahoe–Dollar Point fault (WTDPF) extends along the western margin of the Lake Tahoe Basin (northern Sierra Nevada, western United States) and is characterized as its most hazardous fault. Fallen Leaf Lake, Cascade Lake, and Emerald Bay are three subbasins of the Lake Tahoe Basin, located south of Lake Tahoe, and provide an opportunity to image primary earthquake deformation along the WTDPF and associated landslide deposits. Here we present results from high-resolution seismic Chirp (compressed high intensity radar pulse) surveys in Fallen Leaf Lake and Cascade Lake, multibeam bathymetry coverage of Fallen Leaf Lake, onshore Lidar (light detection and ranging) data for the southern Lake Tahoe Basin, and radiocarbon dates from piston cores in Fallen Leaf Lake and Emerald Bay. Slide deposits imaged beneath Fallen Leaf Lake appear to be synchronous with slides in Lake Tahoe, Emerald Bay, and Cascade Lake. The temporal correlation of slides between multiple basins suggests triggering by earthquakes on the WTDPF system. If this correlation is correct, we postulate a recurrence interval of ~3–4 k.y. for large earthquakes on the Fallen Leaf Lake segment of the WTDPF, and the time since the most recent event (~4.5 k.y. ago) exceeds this recurrence time. In addition, Chirp data beneath Cascade Lake image strands of the WTDPF offsetting the lake floor as much as ~7.5 m. The Cascade Lake data combined with onshore Lidar allow us to map the WTDPF continuously between Fallen Leaf Lake and Cascade Lake. This improved mapping of the WTDPF reveals the fault geometry and architecture south of Lake Tahoe and improves the geohazard assessment of the region.

Description: Gravity-flow deposits recovered in a suite of sediment cores in Lake Tahoe were examined to determine if the event deposits were triggered by strong shaking from earthquakes on active faults within and in close proximity to the Lake Tahoe Basin. The acoustic character and distribution of individual lacustrine deposits as well as potential source regions were constrained by high-resolution seismic Chirp reflection and multibeam bathymetric data. Between 14 and 17 Holocene event deposits have been identified in Lake Tahoe, and examination of their source areas suggests they originated from different initiation points along the steep margin, with some being synchronous around the basin, as opposed to flood-related deposits. Lithologic characteristics, magnetic susceptibility, carbon and nitrogen isotopic signatures, opal content, and 14 C dating indicate that these event deposits are reworked lacustrine material. Radiocarbon dates indicate that the emplacement of these event deposit sediments correlates well with the late Holocene paleoseismic earthquake record developed for the Tahoe Basin. When taken alone, the causality of these events may appear ambiguous, but when the evidence is examined comprehensively, it suggests that strong shaking may likely have been the primary trigger for many of the event deposits observed in the lake throughout the Holocene. For example, four event deposits are assigned to Tahoe Basin faults. The most recent earthquakes occurred on the Incline Village fault (between 630 and 120 cal. yr B.P.); the southern segment of the West Tahoe fault (between 4510 and 4070 cal. yr B.P.); on the central and northern segments of the West Tahoe fault (5600–5330 cal. yr B.P.); and on the West Tahoe fault (between 7890 and 7190 cal. yr B.P.). The oldest of the four associated Tahoe Basin events coincides with the beginning of an extended period when Lake Tahoe was likely not spilling or spilling intermittently, and this suggests that active faulting and footwall uplift cut off the outlet at this time, exaggerating drought conditions downstream. Likewise, the event between 5600 and 5330 cal. yr B.P. on the West Tahoe fault may have exaggerated a smaller drought reflected downstream in Pyramid Lake. This event may also be the most recent event (MRE) on the largest segment of the West Tahoe fault. If correct, the period since the last rupture is approximately twice the estimated average recurrence interval for the Rubicon segment of the West Tahoe fault. A more complete Holocene record of strong shaking greatly extends the paleoseismic record in the region and indicates a combined recurrence interval of between 750 and 800 yr for all faults in the region.

Description: A multichannel seismic (MCS) experiment spanning 600 km across the Alarcón Rise and its conjugate rifted margins in the southern Gulf of California (western North America) provides insight into the spatial and temporal evolution of extension between Baja California and the mainland (Mexico). Stratigraphic analysis of multiple rift basins within the Alarcón spreading corridor indicates an initial stage of oblique extension starting ca. 14–12 Ma. This initial phase of extension was characterized by the formation of several large basins in the center of the gulf and on the southeast margin with negligible synrift sedimentation. A second phase of oblique extension, likely synchronous with large-scale basin opening in the central and northern Gulf of California, began ca. 8–5 Ma and was characterized by the formation of smaller half-grabens distributed across the conjugate margins that contain both synrift and postrift deposits. A key feature imaged within the MCS data is a highly reflective, ropey layer at the top of basement, interpreted to be either volcanic rocks from the 25–12 Ma Comondú Group, and/or early rifting volcanic rocks that are between 11 and 9 Ma, or younger. This volcanic layer is extensively faulted, suggesting that it predates the episode of early extension. Upper crustal extension appears to be equally distributed across conjugate margins, forming a symmetrical continental rift. Two styles of rifted basin are observed; older basins (estimated as 14–11 Ma using sedimentation rates) show distributed extension with extensive basement faulting. In contrast, the younger basins (likely post–6 Ma) are asymmetrical with synrift deposits thickening into the basin-bounding faults. The northeast-southwest geomorphic expression of the Tamayo bank and trough and other features provides additional evidence that northwest-southeast oblique extension began ca. 12 Ma. These new spatial and temporal constraints, when combined with a crustal thickness profile obtained across the entire Alarcón corridor, suggest that significant northwest-southeast oblique extension within the Gulf of California started well before 6 Ma, in contrast to earlier models.

Description: A period of cooling about 13,000 years ago interrupted about 2,000 years of deglacial warming. Known as the Younger Dryas (YD), the event is thought to have resulted from a slowdown of the Atlantic meridional overturning circulation in response to a sudden flood of Laurentide Ice Sheet meltwater that reached the Nordic Seas. Oxygen isotope evidence for a local source of meltwater to the open western North Atlantic from the Gulf of St Lawrence has been lacking. Here we report that the eastern Beaufort Sea contains the long-sought signal of 18O-depleted water. Beginning at ~12.94 ± 0.15 thousand years ago, oxygen isotopes in the planktonic foraminifera from two sediment cores as well as sediment and seismic data indicate a flood of meltwater, ice and sediment to the Arctic via the Mackenzie River that lasted about 700 years. The minimum in the oxygen isotope ratios lasted ~130 years. We suggest that the floodwater travelled north along the Canadian Archipelago and then through the Fram Strait to the Nordic Seas, where freshening and freezing near sites of deep-water formation would have suppressed convection and caused the YD cooling by reducing the meridional overturning. © 2018, The Author(s).