ALBERT

Description: Continental rifts are commonly flanked by zones of high elevation, but the cause of uplift remains controversial. Proposed uplift mechanisms include active and induced asthenospheric upwelling, and isostatically driven lithospheric flexure. Discrimination between these hypotheses requires close constraint of the timing of rift flank uplift and crustal extension. Here, we focus on the well-preserved Neogene Gulf of California rift. The western rift margin is characterized by a prominent east-facing kilometer-scale escarpment, which bounds a west-tilted, topographically asymmetric rift flank. We exploit west-draining canyons incised into the rift flank to constrain the timing of uplift to between ca. 5.6 and 3.2 Ma using 40 Ar/ 39 Ar dating of lavas, which show cut-and-fill relationships to the canyons. Rift flank uplift closely followed the onset of slip on the principal fault of the Loreto rift segment at ca. 8–6 Ma, the age of which we obtain from apatite (U-Th)/He and fission-track thermochronologic analysis of rift escarpment exhumation. Uplift was therefore coeval with lithospheric rupture and the onset of oceanic spreading between ca. 6 and 3 Ma, but post-dates a proposed asthenospheric upwelling event by ~8–10 Ma. The timing of uplift is inconsistent with either active or induced upwelling as uplift mechanisms, and we conclude that rift flank uplift was driven by the flexural response to lithospheric unloading.

Description: The historically active volcanic ocean island of Tristan da Cunha exhibits a complex and dynamic history, with numerous, often compositionally distinct, parasitic centers punctuating the large edifice. To date, the temporal relationship between differing styles of activity has been unclear. We have applied high-precision 40 Ar/ 39 Ar dating to 15 carefully selected samples from Tristan da Cunha to ascertain spatio-temporal relationships of recent volcanism, explore episodicity, and establish if the most recent summit activity post-dated eruptions from parasitic centers lower on the flanks. This has yielded a new suite of reliable Holocene ages, with the youngest dated deposit at 3 ± 1 ka (1). A recent flow at the summit was constrained to 5 ± 1 ka (1), confirming that summit and parasitic activity on the volcano’s flanks overlap in time. The oldest dated deposits were 118 ± 4 ka (1) from a parasitic cone in the southern sector, and 81 ± 10 ka (1) from one of the lowest sub-aerial shield-forming lava flows in the northern sector. Large-scale sector collapse is bracketed between 34 ± 1 ka and 26 ± 5 ka (1) via dating of the youngest headwall lava flow and oldest sub-aerial scarp-filling deposits. No systematic relationship between the new temporal framework, vent location, and eruptive compositions was found. Although magmatic flux has been inferred to be relatively low, Tristan da Cunha is capable of relatively frequent eruptions from a wide variety of vent locations across a broad range of compositions.

Description: Continental rift deposits contain critical clues concerning the evolution of extensional tectonics, yet such evidence is often obscure due to poor geochronology, burial by younger deposits, or later tectonic overprinting. We revisit Corinth rift development, which began as distributed extension created synrift depocenters with rivers flowing into shallow (〈50 m) lakes. Subsequent focused deformation initiated a "Great Deepening" event, evidenced by fan deltas prograding into 300–600-m-deep water. A chronology is provided for the event from 40 Ar/ 39 Ar dating of the Xylocastro ash by single-crystal CO 2 laser fusion, yielding a precise age of 2.550 ± 0.007 Ma (1, full error propagation). Sedimentological data indicate that the ash-bearing sediments were deposited as turbidites and hemipelagites on sublacustrine fans fed from the Mavro fan delta at the faulted south-central rift margin. The ash age and turbidite provenance data enable stratigraphic constraints for an estimate of central rift climax occurring between 3.2 and 3.0 Ma. This is some 0.8–1.0 m.y. earlier than radioisotopic- and magnetostratigraphic-constrained estimates for the eastern Corinth rift. Central rift climax was probably triggered by initial counterclockwise rotation of the Peloponnesus block with respect to central Greece. The rotation pole of this block subsequently migrated to its present position as rift climax moved eastward in an "unzipping" action, with the southern active margin also migrating northward. These events are unlikely to be due to local or regional fault kinematics, but rather to the consequences of deep-seated interactions between the rapidly southward-moving Aegean continental forearc and the slowly northward-subducting African oceanic plate. A possible scenario involves forearc "pushback" with décollement on a low-angle subducting lower plate. This causes acceleration and counterclockwise rotation of Peloponnesus with respect to central Greece and strain localization across the boundary; the Corinth rift.