Segmentation and kinematics of the North America‐Caribbean plate boundary offshore Hispaniola

We explored the submarine portions of the Enriquillo–Plantain Garden Fault zone (EPGFZ) and the Septentrional–Oriente Fault zone (SOFZ) along the Northern Caribbean plate boundary using high‐resolution multibeam echo‐sounding and shallow seismic reflection. The bathymetric data shed light on poorly documented or previously unknown submarine fault zones running over 200 km between Haiti and Jamaica (EPGFZ) and 300 km between the Dominican Republic and Cuba (SOFZ). The primary plate‐boundary structures are a series of strike‐slip fault segments associated with pressure ridges, restraining bends, step overs and dogleg offsets indicating very active tectonics. Several distinct segments 50–100 km long cut across pre‐existing structures inherited from former tectonic regimes or bypass recent morphologies formed under the current strike‐slip regime. Along the most recent trace of the SOFZ, we measured a strike‐slip offset of 16.5 km, which indicates steady activity for the past ~1.8 Ma if its current GPS‐derived motion of 9.8 ± 2 mm a^?1 has remained stable during the entire Quaternary.     

Assessing the effects of catchment‐scale urban green infrastructure retrofits on hydrograph characteristics

Run‐off from impervious surfaces has pervasive and serious consequences for urban streams, but the detrimental effects of urban stormwater can be lessened by disconnecting impervious surfaces and redirecting run‐off to decentralized green infrastructure. This study used a before–after‐control‐impact design, in which streets served as subcatchments, to quantify hydrologic effectiveness of street‐scale investments in green infrastructure, such as street‐connected bioretention cells, rain gardens and rain barrels. On the two residential treatment streets, voluntary participation resulted in 32.2% and 13.5% of parcels having green infrastructure installed over a 2‐year period. Storm sewer discharge was measured before and after green infrastructure implementation, and peak discharge, total run‐off volume and hydrograph lags were analysed. On the street with smaller lots and lower participation, green infrastructure installation succeeded in reducing peak discharge by up to 33% and total storm run‐off by up to 40%. On the street with larger lots and higher participation, there was no significant reduction in peak or total stormflows, but on this street, contemporaneous street repairs may have offset improvements. On the street with smaller lots, lag times increased following the first phase of green infrastructure construction, in which streetside bioretention cells were built with underdrains. In the second phase, lag times did not change further, because bioretention cells were built without underdrains and water was removed from the system, rather than just delayed. We conclude that voluntary green infrastructure retrofits that include treatment of street run‐off can be effective for substantially reducing stormwater but that small differences in design and construction can be important for determining the level of the benefit. Copyright © 2015 John Wiley & Sons, Ltd.     

Oxygen isotope speedometry in granulite facies garnet recording fluid/melt–rock interaction (Sør Rondane Mountains,East Antarctica)

In situ analysis of a garnet porphyroblast from a granulite facies gneiss from Sør Rondane Mountains, East Antarctica, reveals discontinuous step‐wise zoning in phosphorus and large δ¹⁸O variations from the phosphorus‐rich core to the phosphorus‐poor rim. The gradually decreasing profile of oxygen isotope from the core (δ¹⁸O = ~15‰) to the rim (δ¹⁸O = ~11‰) suggests that the ¹⁸O/¹⁶O zoning was originally step‐wise, and modified by diffusion after the garnet rim formation at ~800°C and 0.8 GPa. Fitting of the ¹⁸O/¹⁶O data to the diffusion equation constrains a duration of the high‐T event (~800°C) to c. 0.5–40 Ma after the garnet rim formation. The low δ¹⁸O value of the garnet rim, together with the previously reported low δ¹⁸O values in metacarbonates, indicates regional infiltration, probably along a detachment fault, of low δ¹⁸O fluid/melt possibly derived from meta‐mafic to ultramafic rocks.     

Conductive cooling of spherical bodies with emphasis on the Earth

To explore planetary evolution, we provide conductive cooling profiles that account for planet size, phonon diffusivity and various internal heating scenarios. Our new analytical solution for simple cooling of spheres reveals that heat is removed from only Earth's outermost ~1000 km over geological time. Numerical models with decaying heat production show that any upward concentration of radionuclides causes high temperatures at shallow depths, forcing interior temperatures to increase with time while producing a thermal gradient that forbids lower mantle convection. Hence, differentiation drives upper mantle magmatism and tectonics, leaving a quiescent but hot deep interior, while slowly melting the core.     

High-resolution fluvial records of Holocene environmental changes in the Sahel: the Yamé River at Ounjougou (Mali,West Africa)

The Yamé river, in the Bandiagara Plateau, Dogon Country, Mali, is characterised by extensive alluvial sedimentary records, particularly in the 1 km long Ounjougou reach where Holocene floodplain pockets are inset in the Pleistocene formations. These alluvial records have been investigated via geomorphologic fieldwork and sedimentologic and micromorphologic analyses and are supported by 79 radiocarbon dates. The alluvial deposits of the valley floor correspond to a vertical accretion of 3–10 m. The reconstruction of fluvial style changes provides evidence of four main aggradation periods. From 11,500 to 8760 cal. BP, the alluvial architecture and grain-size parameters indicate a wandering river. This period included phases of pulsed high-energy floods and avulsion related to a northward shift of the summer monsoon to around 14°N after 11,500 cal. BP. From 7800 to 5300 cal. BP, a swampy floodplain environment with standing water pools within a Sudanian savanna/woodland mosaic corresponds to the culmination of the Holocene humid period. From 3800 cal. BP onwards, rhythmic sedimentation attests to an increase in the duration and/or intensity of the dry season, giving a precise date for the local termination of the Holocene Optimum period. During the last two millennia and for the first time during the Holocene, the alluvial formations are progressively restricted whereas the colluvial deposits increase, indicating strong soil erosion and redeposition within the watershed related to an increase in human impact. Four major periods are characterised by incision (I1: ante 11,500, I2: 8760–7800; I3: 6790–6500 cal. BP; I4; 2400–1700 cal. BP) pointing to dramatic changes in fluvial style. They result from high-energy flood flows during dry spells and confirm the capacity of the floodplain pocket in the upstream reach of the Sahelian belt to record rapid Holocene climatic change.