On the kinematics and timing of Rodinia breakup: a possible rift–transform junction of Cryogenian age at the southwest cape of Congo Craton (northwest Namibia)

After tilt correction for Ediacaran thick-skinned folding, a pair of Cryogenian half grabens at the autochthonous southwest cape of Congo Craton (CC) in northwest Namibia restore to different orientations. Toekoms sub-basin trended east-northeast, parallel to Northern Zone (NZ) of Damara belt, and was bounded by a normal-sense growth fault (2 290 m throw) dipping 57° toward CC. Soutput sub-basin trended northwest, oblique to NZ and to north-northwest-trending Kaoko Belt. It was bounded by a growth fault (750 m down-dip throw) dipping steeply (~75°) toward CC. Soutput growth fault could be an oblique (splay) fault connecting a Cryogenian rift zone in NZ with a sinistral transform zone in Kaoko Belt. A transform origin for the Kaoko margin accords with its magma-poor abrupt shelf-to-basin change implying mechanical strength, unlike the magma-rich southern margin where a gradual shelf-to-basin change implies a mechanically weak extended margin. A rift−transform junction is kinematically compatible with observed north-northwest−south-southeast Cryogenian crustal stretching within CC. Post-rift subsidence of the CC carbonate platform varies strongly across the south-facing but not the west-facing shelf. A sheared western CC margin differs from existing Kaoko Belt models that posit orthogonal opening with hyper-extended continental crust. Carbonate-dominated sedimentation over southwest CC implies palaeolatitudes ≤35° between 770 and 600 Ma.

Depleted and ultradepleted basalt and picrite in the Davis Strait: Paleocene volcanism associated with a transform continental margin

Volcanic rocks from the Davis Strait were studied to elucidate the tectonomagmatic processes during rifting and the start of seafloor spreading, and the formation of the Ungava transform zone between Canada and Greenland. The rocks are from the wells Hekja O-71, Gjoa G-37, Nukik-2 and Hellefisk-1, and from dredges on the northern Davis Strait High. Ages range from Danian to Thanetian (dinocyst palynozones P2 to P5, 62.5–57.2 Ma). The rocks are predominantly basaltic, but include picrites on the Davis Strait High. Calculated mantle potential temperatures for the Davis Strait High are c. 1500°C, suggesting the volume of magma generated was large; this is consistent with geophysical evidence for magmatic underplating in the region. The rare earth element patterns indicate residual mantle lithologies of spinel peridotite and, together with Sr–Nd isotopes, indicate melting beneath regionally extensive, depleted asthenosphere beneath a lithosphere of thickness similar to, or thinner than, beneath Baffin Island and distinctly thinner than beneath West Greenland. Some sites include basalts with more enriched compositions. Depleted and enriched basalts in the Hellefisk well show contemporaneous melting of depleted and enriched mantle components in the asthenosphere. The Hekja and Davis Strait High basalts and picrites have unique, ultradepleted compositions with (La/Sm)N < 0.5, (Tb/Lu)N < 1 and Nb/Zr = 0.013–0.027. We interpret these compositions as a product of the melting regime within the Ungava transform zone, where the melting column would be steep-sided in cross-section and not triangular as expected at normal spreading ridges. Magmatism along the transform stopped when the tectonic regime changed from transtension to transpression during earliest Eocene time.

Stress variations in space and time within the mantle section of an oceanic transform zone: Evidence for the seismic cycle

The Bogota Peninsula shear zone in New Caledonia (southwest Pacific Ocean) is the exhumed mantle section of an oceanic transform zone. Ductile fabrics in this zone formed at temperatures &gt;820 °C, and differential stresses estimated from microstructures vary spatially and temporally. Along a transform-perpendicular transect, stresses increase toward the high-strain areas. We attribute this stress gradient to an increase in strain rate caused by imposed rather than intrinsic strain localization. Temporal stress variations are indicated by the formation of fine-grained microdeformation zones (MDZs) that truncate and offset coarser grains. We interpret the MDZs to result from zones of brittle deformation caused by earthquake fracture propagation downward in the upper mantle, which are in turn overprinted by ductile deformation at stresses 2–6 times higher (22–81 MPa) than their surrounding steady-state fabrics. We interpret the spatial and temporal variations in microstructures and stresses as reflecting different stages of the seismic cycle in oceanic lithosphere.

Hydrogeochemical changes in trace element concentrations in connection with earthquakes and a volcanic eruption in Iceland

&lt;p&gt;The aim of this study was to identify changes of trace element concentration in groundwater and test for coupling with seismic and volcanic activity in Iceland. Samples used in this study were collected between September 2010 and June 2018 from the HA-01 groundwater well in Hafral&amp;#230;kur (Northern Iceland), south of the Tj&amp;#246;rnes Fracture Zone (oblique transform zone), and near the Lax&amp;#225; and Sk&amp;#225;lfandaflj&amp;#243;t river valleys. The temperature of the groundwater from the HA-01 well is 71&amp;#8211;76 &amp;#176;C, pH is ca. 10.2 (at ~ 25&amp;#186;C), and the dissolved solid content is about 240 ppm, which is typical of low temperature geothermal groundwaters in inland areas of Iceland. The HA-01 well groundwater is also influenced by mixing between old ice age aquifer and younger aquifer groundwater. The same samples were previously analyzed for major element concentrations and isotopic ratios, with results - changes prior to seismic activity - being published in recent papers. The 495 earthquakes (Mw&amp;#8805;4.0, September 2010 to June 2018) considered in this study are from the USGS database. Twenty-two of these earthquakes occurred in the Tj&amp;#246;rnes Fracture Zone with M&lt;sub&gt;w&lt;/sub&gt; between 4.1 and 5.5 whereas the remaining ones with M&lt;sub&gt;w&lt;/sub&gt; between 4 and 5.5 were related to the B&amp;#225;r&amp;#240;arbunga eruption in central Iceland, which began on 29 August 2014 and ended on 27 February 2015. Results of trace element analysis highlight characteristic variations in the temporal series related to the B&amp;#225;r&amp;#240;arbunga eruption (onset in August 2014) and to the 2018 seismic swarm that occurred in the Tj&amp;#246;rnes Fracture Zone. In particular, a marked increase of Li, B, Ga, Mo and Rb and a slight increase of Sr and V were observed prior to and in connection with the onset of the B&amp;#225;r&amp;#240;arbunga eruption. Moreover, our results show a pre-seismic (2018 seismic swarm in the Tj&amp;#246;rnes Fracture Zone) hydrogeochemical variability greater than the background variability. Despite the distance to the B&amp;#225;r&amp;#240;arbunga eruption site, GPS data from northern Iceland show a clear strain changes that are associated with the large dike intrusion that fed the eruption and are possibly correlated with the hydrogeochemical time series. Results from this study in Iceland show that the hydrogeochemical monitoring of volcanic and seismic areas is a promising method in the science of seismic and volcanic precursors.&lt;/p&gt;