Arc-continent collisions in the tropics set Earth's climate state

On multimillion-year time scales, Earth has experienced warm ice-free and cold glacial climates, but it is unknown whether transitions between these background climate states were the result of changes in carbon dioxide sources or sinks. Low-latitude arc-continent collisions are hypothesized to drive cooling by exhuming and eroding mafic and ultramafic rocks in the warm, wet tropics, thereby increasing Earth's potential to sequester carbon through chemical weathering. To better constrain global weatherability through time, the paleogeographic position of all major Phanerozoic arc-continent collisions was reconstructed and compared to the latitudinal distribution of ice sheets. This analysis reveals a strong correlation between the extent of glaciation and arc-continent collisions in the tropics. Earth's climate state is set primarily by global weatherability, which changes with the latitudinal distribution of arc-continent collisions.

The petrology and geochemistry of a metabasite belt along the southern margin of Alaska

A 600 km long metabasite belt is exposed at the southern border of the Chugach terrane in southern Alaska, south of the Eocene Chugach Metamorphic Complex (CMC). In this contribution, we present petrologic and geochemical results for parts of this metabasite belt. The metabasites studied are amphibolite grade and their PT conditions are evaluated with hornblende-plagioclase thermometry and the average PT method. From west to east the peak metamorphic conditions calculated are: about 730-793 °C for pressures between 5 and 15 kbar in the westernmost part, about 740-760 °C and 5 kbar in the middle locality and about 640-675 °C and 8 kbar in the easternmost locality. These results are comparable with the metamorphic conditions obtained on metapelite of the CMC for the westernmost and easternmost localities. In contrast, in the central part of the CMC, the metabasites experienced probably lower pressures than the metapelites to the north. Rare earth and trace element patterns of the metabasite belt are comparable with typical altered basalt patterns and reveal MORB and arc-tholeiitic geochemical characteristics. The presence of Ba and U anomalies are interpreted as a result of alteration prior to subduction, the Pb anomaly as a result of an intra-oceanic island arc signature and the Sr anomaly as a result of the interaction of sediments with the metabasites during subduction. We suggest that the association of MORB and arc tholeiitic rocks in the metabasite belt is likely derived from an intra-oceanic island arc which accreted to the Alaskan margin.

What constitutes ‘emplacement’ of an ophiolite?: Mechanisms and relationship to subduction initiation and formation of metamorphic soles

Ophiolites have long been recognized as on-land fragments of fossil oceanic lithosphere, which becomes an ophiolite when incorporated into continental margins through a complex process known as ‘emplacement’. A fundamental problem of ophiolite emplacement is how dense oceanic crust becomes emplaced over less dense material(s) of continental margins or subduction-accretion systems. Subduction of less dense material beneath a future ophiolite is necessary to overcome the adverse density contrast. The relationship of subduction to ophiolite emplacement is a critical link between ophiolites and their role in the development of orogenic belts. Although ophiolite emplacement mechanisms are clearly varied, most existing models and definitions of emplacement concern a specific type of ophiolite (i.e. Oman or Troodos) and do not apply to many of the world’s ophiolites. We have defined four prototype ophiolites based on different emplacement mechanisms: (1) ‘Tethyan’ ophiolites, emplaced over passive continental margins or microcontinents as a result of collisional events; (2) ‘Cordilleran’ ophiolites progressively emplaced over subduction complexes through accretionary processes; (3) ‘ridge-trench intersection’ (RTI) ophiolites emplaced through complex processes resulting from the interaction between a spreading ridge and a subduction zone; (4) the unique Macquarie Island ophiolite, which has been subaerially exposed as a result of a change in plate boundary configuration along a mid-ocean ridge system. Protracted evolutionary history of some ocean basins, and variation along the strike of subduction zones may result in more complicated scenarios in ophiolite emplacement mechanisms. No single definition of emplacement is free of drawbacks; however, we can consider the inception of subduction, thrusting over a continental margin or subduction complex, and subaerial exposure as critical individual stages in ophiolite emplacement.