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Fundamental features of the geology and tectonic setting of the northeast-propagating Yellowstone hotspot are not explained by a simple deep-mantle plume hypothesis and, within that framework, must be attributed to coincidence or be explained by auxiliary hypotheses. These features include the persistence of basaltic magmatism along the hotspot track, the origin of the hotspot during a regional middle Miocene tectonic reorganization, a similar and coeval zone of northwestward magmatic propagation, the occurrence of both zones of magmatic propagation along a first-order tectonic boundary, and control of the hotspot track by preexisting structures. Seismic imaging provides no evidence for, and several contraindications of, a vertically extensive plume-like structure beneath Yellowstone or a broad trailing plume head beneath the eastern Snake River Plain. The high helium isotope ratios observed at Yellowstone and other hotspots are commonly assumed to arise from the lower mantle, but upper-mantle processes can explain the observations. The available evidence thus renders an upper-mantle origin for the Yellowstone system the preferred model; there is no evidence that the system extends deeper than ϳ200 km, and some evidence that it does not. A model whereby the Yellowstone system reflects feedback between upper-mantle convection and regional lithospheric tectonics is able to explain the observations better than a deep-mantle plume hypothesis.

A free-field recording of the Denali fault earthquake was obtained by the Alyeska Pipeline Service Company 3 km from the surface rupture of the Denali fault. The instrument, part of the monitoring and control system for the trans-Alaska pipeline, was located at Pump Station 10, approximately 85 km east of the epicenter. After correction for the measured instrument response, we recover a seismogram that includes a permanent displacement of 3.0 m. The recorded ground motion has relatively low peak acceleration (0.36 g) and very high peak velocity (180 cm/s). Nonlinear soil response may have reduced the peak acceleration to this 0.36 g value. Accelerations in excess of 0.1 g lasted for 10 s, with the most intense motion occurring during a 1.5-s interval when the rupture passed the site. The low acceleration and high velocity observed near the fault in this earthquake agree with observations from other recent large-magnitude earthquakes. [DOI: 10.1193/1.1778172

INTRODUCTION Colorectal surgery has been associated with a complication rate of 15-20% and mean postoperative inpatient stays of 6-11 days. The principles of enhanced recovery after surgery (ERAS) are well established and have been developed to optimise perioperative care and facilitate discharge. The purpose of this systematic review is to present an updated review of perioperative care in colorectal surgery from the available evidence and ERAS group recommendations. METHODS Systematic searches of the PubMed and Embase ™ databases and the Cochrane library were conducted. A hand search of bibliographies of identified studies was conducted to identify any additional articles missed by the initial search strategy. RESULTS A total of 59 relevant studies were identified. These included six randomised controlled trials and seven clinical controlled trials that fulfilled the inclusion criteria. These studies showed reductions in duration of inpatient stays in the ERAS groups compared with more traditional care as well as reductions in morbidity and mortality rates. CONCLUSIONS Reviewing the data reveals that ERAS protocols have a role in reducing postoperative morbidity and result in an accelerated recovery following colorectal surgery. Similarly, both primary and overall hospital stays are reduced significantly. However, the available evidence suggests that ERAS protocols do not reduce hospital readmissions or mortality. These findings help to confirm that ERAS protocols should now be implemented as the standard approach for perioperative care in colorectal surgery.

Summary We report the results of the highest‐resolution teleseismic tomography study yet performed of the upper mantle beneath Iceland. The experiment used data gathered by the Iceland Hotspot Project, which operated a 35‐station network of continuously recording, digital, broad‐band seismometers over all of Iceland 1996–1998. The structure of the upper mantle was determined using the ACH damped least‐squares method and involved 42 stations, 3159 P‐wave, and 1338 S‐wave arrival times, including the phases P, pP, sP, PP, SP, PcP, PKIKP, pPKIKP, S, sS, SS, SKS and Sdiff. Artefacts, both perceptual and parametric, were minimized by well‐tested smoothing techniques involving layer thinning and offset‐and‐averaging. Resolution is good beneath most of Iceland from ∼60 km depth to a maximum of ∼450 km depth and beneath the Tjornes Fracture Zone and near‐shore parts of the Reykjanes ridge. The results reveal a coherent, negative wave‐speed anomaly with a diameter of 200–250 km and anomalies in P‐wave speed, VP, as strong as −2.7 per cent and in S‐wave speed, VS, as strong as −4.9 per cent. The anomaly extends from the surface to the limit of good resolution at ∼450 km depth. In the upper ∼250 km it is centred beneath the eastern part of the Middle Volcanic Zone, coincident with the centre of the ∼100 mGal Bouguer gravity low over Iceland, and a lower crustal low‐velocity zone identified by receiver functions. This is probably the true centre of the Iceland hotspot. In the upper ∼200 km, the low‐wave‐speed body extends along the Reykjanes ridge but is sharply truncated beneath the Tjornes Fracture Zone. This suggests that material may flow unimpeded along the Reykjanes ridge from beneath Iceland but is blocked beneath the Tjornes Fracture Zone. The magnitudes of the VP, VS and VP/VS anomalies cannot be explained by elevated temperature alone, but favour a model of maximum temperature anomalies < 200 K, along with up to ∼2 per cent of partial melt in the depth range ∼100–300 km beneath east‐central Iceland. The anomalous body is approximately cylindrical in the top 250 km but tabular in shape at greater depth, elongated north–south and generally underlying the spreading plate boundary. Such a morphological change and its relationship to surface rift zones are predicted to occur in convective upwellings driven by basal heating, passive upwelling in response to plate separation and lateral temperature gradients. Although we cannot resolve structure deeper than ∼450 km, and do not detect a bottom to the anomaly, these models suggest that it extends no deeper than the mantle transition zone. Such models thus suggest a shallow origin for the Iceland hotspot rather than a deep mantle plume, and imply that the hotspot has been located on the spreading ridge in the centre of the north Atlantic for its entire history, and is not fixed relative to other Atlantic hotspots. The results are consistent with recent, regional full‐thickness mantle tomography and whole‐mantle tomography images that show a strong, low‐wave‐speed anomaly ben...

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We determine compressional wave velocity and attenuation structures for the upper crust beneath Medicine Lake volcano in northeast California using a high‐resolution active source seismic tomography method. Medicine Lake volcano is a basalt through rhyolite shield volcano of the Cascade Range, lying east of the range axis. The Pg wave from eight explosive sources which has traveled upward through the target volume to a dense array of 140 seismographs provides 1‐ to 2‐km resolution in the upper 5 to 7 km of the crust beneath the volcano. The experiment tests the hypothesis that Cascade Range volcanoes of this type are underlain only by small silicic magma chambers. We image a low‐velocity low‐Q region not larger than a few tens of cubic kilometers in volume beneath the summit caldera, supporting the hypothesis. A shallower high‐velocity high‐density feature, previously known to be present, is imaged for the first time in full plan view; it is east‐west elongate, paralleling a topographic lineament between Medicine Lake volcano and Mount Shasta. This lineament is interpreted to be the result of an old crustal weakness now affecting the emplacement of magma, both on direct ascent from the lower crust and mantle and in migration from the shallow silicic chamber to summit vents. Differences between this high‐velocity feature and the equivalent feature at Newbeny volcano, a volcano in central Oregon resembling Medicine Lake volcano, may partly explain the scarcity of surface hydrothermal features at Medicine Lake volcano. A major low‐velocity low‐Q feature beneath the southeast flank of the volcano, in an area with no Holocene vents, is interpreted as tephra, flows, and sediments from the volcano deeply ponded on the downthrown side of the Gillem fault, a normal fault mapped at the surface north of the volcano. A high‐Q normal‐velocity feature beneath the north rim of the summit caldera may be a small, possibly hot, subsolidus intrusion. A high‐velocity low‐Q region beneath the eastern caldera may be an area of boiling water between the magma chamber and the ponded east flank material. These structural data are useful both for understanding Cascade Range volcanism and for geothermal development in progress on the volcano.

A 3‐D teleseismic tomography image of the upper mantle beneath Iceland of unprecedented resolution reveals a subvertical low wave speed anomaly that is cylindrical in the upper 250 km but tabular below this. Such a morphological transition is expected towards the bottom of a buoyant upwelling. Our observations thus suggest that magmatism at the Iceland hotspot is fed by flow rising from the mantle transition zone. This result contributes to the ongoing debate about whether the upper and lower mantles convect separately or as one. The image also suggests that material flows outwards from Iceland along the Reykjanes Ridge in the upper 200 km, but is blocked in the upper 150 km beneath the Tjornes Fracture Zone. This provides direct observational support for the theory that fracture zones dam lateral flow along ridges.