Andrew J. Mason

Penultimate Deglacial Sea-Level Timing from Uranium/Thorium Dating of Tahitian Corals

Early Riser How glacial-interglacial cycles and the long-term variability of sea level depend on the amount of energy received by Earth from the Sun is unclear. Thomas et al. (p. 1186, published online 23 April; see the cover) report results from fossil corals found in Tahiti that indicate that sea level began to rise when insolation at 65° North latitude was near a minimum, not after it had begun to rise, as predicted by the Milankovitch theory. In contrast, the timing of the last deglaciation agrees well with the Milankovitch theory. Thus, glacial cycles do not behave as simply as the Milankovitch theory suggests. Sea levels rose during the penultimate deglaciation while Northern Hemisphere insolation was at a minimum. The timing of sea-level change provides important constraints on the mechanisms driving Earth’s climate between glacial and interglacial states. Fossil corals constrain the timing of past sea level by their suitability for dating and their growth position close to sea level. The coral-derived age for the last deglaciation is consistent with climate change forced by Northern Hemisphere summer insolation (NHI), but the timing of the penultimate deglaciation is more controversial. We found, by means of uranium/thorium dating of fossil corals, that sea level during the penultimate deglaciation had risen to ~85 meters below the present sea level by 137,000 years ago, and that it fluctuated on a millennial time scale during deglaciation. This indicates that the penultimate deglaciation occurred earlier with respect to NHI than the last deglacial, beginning when NHI was at a minimum.

Speleothems Reveal 500,000-year History of Siberian Permafrost

Permafrost Thaw Predictions Permafrost contains twice as much carbon as the atmosphere which could have serious consequences if it were to be released by widespread thawing. Vaks et al. (p. 183, published online 21 February) present a 450,000 year-long record of speleothem growth at selected locations in Siberia, which traces changes in the extent of permafrost over that time period. The authors conclude that conditions only slightly warmer than those of today would cause widespread thawing of continuous permafrost as far north as 60°N. Siberian caves recorded the history of permafrost occurrence during the past 450,000 years. Soils in permafrost regions contain twice as much carbon as the atmosphere, and permafrost has an important influence on the natural and built environment at high northern latitudes. The response of permafrost to warming climate is uncertain and occurs on time scales longer than those assessed by direct observation. We dated periods of speleothem growth in a north-south transect of caves in Siberia to reconstruct the history of permafrost in past climate states. Speleothem growth is restricted to full interglacial conditions in all studied caves. In the northernmost cave (at 60°N), no growth has occurred since Marine Isotopic Stage (MIS) 11. Growth at that time indicates that global climates only slightly warmer than today are sufficient to thaw extensive regions of permafrost.

U–Pb geochronology of Lewisian orthogneisses in the Outer Hebrides, Scotland: implications for the tectonic setting and correlation of the South Harris Complex

The South Harris Complex of the Scottish Outer Hebrides forms a distinctive component of the Lewisian Complex and contains three supposedly co-magmatic meta-igneous bodies, a diorite, a ‘norite’ and an anorthosite, believed to be part of a Palaeoproterozoic arc. New U–Pb zircon ages obtained from these bodies of 1888 ± 2 Ma, 1890 +2/−1 Ma, and 2491 +31/−27 Ma, respectively, demonstrate that the anorthosite cannot be related to the younger calc-alkaline diorite and ‘norite’. The diorite and ‘norite’, however, probably are related products of arc magmatism. The spatial association of the diorite and ‘norite’ with the substantially older anorthosite suggests that the younger intrusions were emplaced into continental crust. Any suture associated with the South Harris Complex remains cryptic, and is not marked by an obvious change in protolith ages in the adjacent tonalitic gneisses.

A re-evaluation of a Laxfordian terrane boundary in the Lewisian Complex of South Harris, NW Scotland

The recent reinterpretation of the Lewisian Complex as a series of amalgamated terranes represents a significant advance in our understanding of these rocks. However, on the Outer Hebrides, a key observation that a supposed post-1675 Ma terrane boundary is based on is flawed. Specifically, on South Harris a group of c. 1675 Ma granite pegmatites has previously been described as occurring only to the NE of, and terminating abruptly within, a supposed terrane-bounding shear zone (the Langavat Shear Zone), which thus post-dates the pegmatites. New mapping indicates that granite pegmatites occur on both sides of, and throughout the Langavat Shear Zone, and that within most of this shear zone the pegmatites are not strongly deformed, inconsistent with them predating this shear zone. Furthermore, new U–Pb geochronological data suggest that the pegmatites SW of the supposed boundary were also intruded around 1675 Ma, similar to those in the NE, and supporting the field evidence that the pegmatites continue through the Langavat Shear Zone without interruption. Two phases of post-pegmatite deformation are identified, but these relate to relatively minor reactivation of the Langavat Shear Zone, not the initiation of this major shear zone, which predates the pegmatites.

New light on the construction, evolution and correlation of the Langavat Belt (Lewisian Complex), Outer Hebrides, Scotland: field, petrographic and geochronological evidence for an early Proterozoic imbricate zone

The South Harris Complex is a domain of largely Palaeoproterozoic rocks within the late Archaean, tonalite–trondhjemite–granodiorite (TTG)-dominated Lewisian gneisses of the Outer Hebrides, NW Scotland. The complex is distinguished by a high proportion of metasedimentary rocks and distinctive meta-igneous units, in part representing the remnants of a continental volcanic arc. The Langavat Belt defines the NE border zone of the South Harris Complex, separating the latter from the Archaean gneisses further NE, and has been repeatedly interpreted as a discrete supracrustal unit. However, detailed mapping and petrographic analysis reveals that up to 60% of the belt may be composed of highly deformed felsic orthogneiss. The metasedimentary rocks that are present have a late Archaean zircon provenance signature, implying that they are younger than, and possibly derived from the TTG gneisses to the NE. Previous work on zircon populations in the Leverburgh metasediments, towards the southern flank of the South Harris Complex, indicates very different provenance and we repudiate correlation of the two sequences. The disposition of units of metasediment, amphibolite and felsic orthogneiss is not a primary, pre-tectonic feature and is very unlikely to be the result of folding. Although direct evidence in the form of early fabrics has been wiped out by later, penetrative ductile shearing and metamorphic annealing, we propose that the Langavat Belt was assembled as a zone of thrust imbrication during early Proterozoic contraction. After imbrication, ductile shearing occurred in two distinct periods, separated by intrusion of granite pegmatites dated at c. 1660 Ma.

Major early thrusting as a control on the Palaeoproterozoic evolution of the Lewisian Complex: evidence from the Outer Hebrides, NW Scotland

New structural, metamorphic and geochronological data suggest that the Lewisian Complex of the Outer Hebrides can be interpreted as a Proterozoic orogen in which a thrust sheet of juvenile arc material was driven over Archaean gneisses at an early stage. Incubation and loading of the underlying Archaean gneisses triggered prograde metamorphism and extensive ductile deformation with a strong gravitational flattening component combined with top-to-the-WNW transport. Large shear zones developed at this time are of local importance, but are not fundamental structural boundaries. The recognition of early orogenic thrusting followed by extensive ductile deformation shows a close resemblance to the sequence of events recognized within the Nagssugtoqidian orogen, with which the Lewisian Complex is correlated. As in the Nagssugtoqidian orogen, younger penetrative ductile deformation often masks the major structural boundaries formed by older thrusts. Supplementary material: Detailed structural and lithological maps of the Langavat Belt, and geochronological data and analytical methods description are available at www.geolsoc.org.uk/SUP18515.

The Palaeoproterozoic anatomy of the Lewisian Complex, NW Scotland: evidence for two ‘Laxfordian’ tectonothermal cycles

A new structural examination of Palaeoproterozoic high-P granulites on South Harris, NW Scotland, when integrated with previous geochronological, structural and metamorphic studies on key areas of the Lewisian Complex, suggests the existence of two distinct tectonothermal cycles within the Palaeoproterozoic ‘Laxfordian Event’, which on South Harris are separated by a >100 myr hiatus in deformation. The older cycle, from c. 1.91 to 1.85 Ga, records the development of an active continental margin on the Archaean gneisses that dominate the Complex, and the subsequent onset of continent–continent collision; this represents the continuation of the Nagssugtoqidian orogen of Greenland. Evidence for this is concentrated in allochthonous slivers of the former active continental margin displaced during the younger cycle. The younger cycle, around 1.75–1.65 Ga, began with thrust-related crustal thickening that initiated regionally extensive amphibolite-facies metamorphism and ductile deformation, which dominates the preserved ‘Laxfordian’ deformation history. This may be the peripheral expression of the accretion of the Malin block to the SW of the Lewisian, and represents the lateral continuation of the Labradorian–Ketilidian orogen of North America. Supplementary Material: Additional figures are available at http://www.geolsoc.org.uk/SUP18863.