Topographic, lithospheric and lithologic controls on the transient landscape evolution after the opening of internally-drained basins. Modelling the North Iberian Neogene drainage.

The opening of internally-drained (endorheic) sedimentary basins often leads to a major drainage change, re-excavation of the basin sedimentary infill, and transient landscape. The timing of such basin openings can be dated only in exceptional cases in which the youngest sedimentary infill remains preserved. For this reason, the processes and timing involved in their transient landscape evolution are poorly known. We explore the role of erodibility, basin geometry and flexural isostasy during the capture of internally-drained basins by means of numerical modelling techniques constrained by recent terrace cosmogenic dating and geomorphological analysis, addressing the issue as to why the Duero and Ebro rivers, draining two Cenozoic sedimentary basins in N Iberia with similar geographical dimensions and drainage histories, have undergone a markedly different erosion evolution leading to distinctly different present morphology. To evaluate how these intrinsic parameters affect the transient landscape evolution, we design a synthetic scenario inspired by those basins. The results show that, once a basin becomes externally drained, its drainage integration and erosion rates are strongly dependent on 1) the basin elevation above the base level; 2) the width of the topographic barrier, 3) its erodibility; and 4) the rigidity of the lithosphere. The results show that transient landscape evolution can last for tens of millions of years even in absence of tectonic activity and changes in base level or climate. Basins isolated by wide and resistant barriers such as the Duero Basin may undergo a multi-million-year time lag between drainage opening and basin-wide incision. In the case of the Duero Basin, this delay may explain the paradoxical time lag between the last lacustrine sedimentation dated at 9.6 Ma and the onset of widespread basin incision variously estimated at 3.5 to 1 Ma.

Evolution of the Cook Ice Cap (Kerguelen Islands) between the last centuries and 2100 ce based on cosmogenic dating and glacio-climatic modelling

The Cook Ice Cap (CIC) on the sub-Antarctic Kerguelen Islands recently experienced extremely negative surface mass balance. Further deglaciation could have important impacts on endemic and invasive fauna and flora. To put this exceptional glacier evolution into a multi-centennial-scale context, we refined the evolution of the CIC over the last millennium, investigated the associated climate conditions and explored its potential evolution by 2100 ce. A glaciological model, constrained by cosmic ray exposure dating of moraines, historical documents and recent direct mass balance observations, was used to simulate the ice-cap extents during different phases of advance and retreat between the last millennium and 2100 ce. Cosmogenic dating suggests glacial advance around the early Little Ice Age (LIA), consistent with findings from other sub-Antarctic studies, and the rather cold and humid conditions brought about by the negative phase of the Southern Annular Mode (SAM). This study contributes to our currently limited understanding of palaeoclimate for the early LIA in the southern Indian Ocean. Glaciological modelling and observations confirm the recent decrease in CIC extent linked to the intensification of the SAM. Although affected by large uncertainties, future simulations suggest a complete disappearance of CIC by the end of the century.

A Multidisciplinary Investigation Into the Eruptive Style, Processes, and Duration of a Cascades Back-Arc Tholeiitic Basalt: A Case Study of the Brushy Butte Flow Field, Northern California, United States

The Cascades back-arc in northern California is dominated by monogenetic tholeiitic basalts that erupted throughout the Pleistocene. Elucidating their eruptive history and processes is important for understanding potential future eruptions here. We focus on the well-exposed monogenetic volcano that emplaced the Brushy Butte flow field, which constructed a ∼150 m tall edifice, has flow lobes up to >10 km long, and in total covers ∼150 km2 with an eruptive volume of 3.5 km3. We use a multidisciplinary approach of field mapping, petrography, geochemistry, paleomagnetism, geochronology, and lidar imagery to unravel the eruptive history and processes that emplaced this flow field. Tholeiitic basalts in northern California have diverse surface morphology and vegetation cover but similar petrographic appearances, which makes them hard to distinguish in the field. Geochemistry and paleomagnetism offer an independent means of distinguishing tholeiitic basalts. Brushy Butte flow field lavas are similar in major-oxide and trace-element abundances but differ from adjacent tholeiitic basalts. This is also apparent in remanent magnetic directions. Additionally, paleomagnetism indicates that the flow field was emplaced during a geologically brief time interval (10–20 years), which 36Cl cosmogenic dating puts at 35.7 ± 1.7 ka. Lidar imagery shows that these flows erupted from at least 28 vents encompassing multiple scoria cones, spatter cones, and craters. Flows can be grouped into four pulses using stratigraphic position and volume. Pulse 1 is the most voluminous, comprising eight eruptions and ∼2.3 km3. Each subsequent pulse started rapidly but decayed quickly, and each successive pulse erupted less lava (i.e., 2.3 km3 for pulse 1, 0.6 km3 for pulse 2, 0.3 km3 for pulse 3, and 0.2 km3 for pulse 4). Many of these flows host well-established lava channels and levees (with channel breakouts) that lead to lava fans, with some flows hosting lava ponds. Similar flow features from tholeiitic eruptions elsewhere demonstrate that these morphologies generally occur over weeks, months, or longer (e.g., Puʻu ʻŌʻō eruption at K–llauea, Hawaiʻi). This multidisciplinary study shows the range of eruptive styles and durations of a Cascades back-arc eruption and illustrates how potential future tholeiitic eruptive activity in the western United States might progress.