The Pk evaluation metric, initially proposed by Beeferman, Berger, and Lafferty (1997), is becoming the standard measure for assessing text segmentation algorithms. However, a theoretical analysis of the metric finds several problems: the metric penalizes false negatives more heavily than false positives, overpenalizes near misses, and is affected by variation in segment size distribution. We propose a simple modification to the Pk metric that remedies these problems. This new metric—called Window Diff—moves a fixed-sized window across the text and penalizes the algorithm whenever the number of boundaries within the window does not match the true number of boundaries for that window of text.

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We review the structure and evolution of a number of Riphean-Phanerozoic rifts and extensional basins within the territory of the former Soviet Union (FSU). Horst-and-graben formation in strong crustal and subcrustal lithosphere layers can explain the multi-trough character of rift systems observed in the Russian platform, the Vilyuy rift, the West Siberian rift system, the Pechora-Kolva rift system and the Laptev Sea rift. Many features in the evolution of these rifted basins are incompatible with predictions of classical stretching models. Basin subsidence often occurs in the absence of any noticeable stretching and over time scales much longer than predicted by models of thermal subsidence. Other observations include a time gap between rifting and the onset of post-rift basin subsidence of tens to hundreds Ma and a correlation in timing of subsidence phases of rifted basins and platforms with opening and closure events of adjacent ocean basins. These observations point to an important role for mechanisms such as eclogite formation within or beneath the lithosphere as well as intraplate compression and stress-induced lithospheric deflection.

Abstract. Near-vertical incidence reflection seismic data acquired in the Tagil Volcanic Arc (Middle Urals) show the upper crust to be highly reflective. Two intersecting seismic lines located near the ongoing -5400 m deep SG4 borehole show that the main reflectivity strikes approximately N-S and dips -350-55 ø to the east. Prominent reflections intercept the borehole at -1000, -1500, 2800-2900, -3400, and between -4000 and 5400 m, which correspond to intervals of low velocity/low density/low resistivity. The surface projections of these reflections lie parallel to the strike of magnetic anomaly trends. Multioffset vertical seismic profile (VSP) data acquired in the SG4 borehole show a seismic response dominated by P to S reflected converted waves from the moderately east dipping reflectivity and from a set of very steep east dipping reflectors not imaged by the surface data. Modeling of the VSP data constrains the depth at which reflectors intercept the borehole and suggests that the P to S conversions are best explained by low-velocity porous intervals rather than higher-velocity mafic material. The most prominent east dipping reflection on the surface seismic data is only imaged on VSP shots that sample the crust closer to the E-W seismic line. This discrepancy between the VSP and the surface seismic data is attributed to rapid lateral changes in the physical properties of the reflector. Surface and borehole data suggest that the low-velocity/low-density/low-resistivity intervals are the most important source of reflectivity in the SG4 borehole area, although lithological contrasts may also play a role. Drill cores from the these zones contain hydrothermal alteration minerals indicating interaction with fluids. Tectonic criteria suggest that they might represent imbricated fracture zones often bounding different lithologies and/or intrusions. Some of them might also represent high-porosity lava flows or pyroclastic units, common in island arc environments.