Silicic caldera volcanoes are often associated with hydrothermal systems economically important for electricity generation and localization of ore deposits. Despite their potential importance, the poor exposure that is typical in caldera settings has limited the number of detailed studies of the relationship between caldera structures and fluid flow. We use field mapping, outcrop scale scanline transects, and petrographic analyses to characterize fault rocks, alteration, and veins in the well-exposed 22.9 Ma Lake City caldera fossil hydrothermal system. The caldera margin consists of relatively straight segments linked by more structurally complex intersections; these structural intricacies produce a zone of deformation that can reach >300 m wide. Structural analyses show that the wide (up to ~60 m) fault core of the ring fault contains abundant subparallel veins, with orientations similar to that of the caldera margin. Smaller displacement faults inside the caldera generally have narrow (<1 m), hydrothermally cemented fault cores with more variably oriented veins in the surrounding damage zone. These findings at Lake City illustrate that fluid flow is controlled by lithology and the location and displacement of faults, e.g., ring fault versus intracaldera fault. Fault connectivity is another key control. We propose a conceptual model where fluid flow in caldera-hosted settings is influenced by: (1) the presence of favorable lithologies (proximity to magmatic intrusions and/or the presence of permeable lithologies), (2) a high density of faults and fractures, and (3) favorable orientations of faults and fractures that promote the formation of discontinuity intersections.
The city of Auckland sits on an active volcanic field, which comprises approximately 50 volcanoes that have erupted over the last ~250 kyr. The most recent eruption, Rangitoto, occurred ~750 years ago. Many of the volcanic centres in the Auckland Volcanic Field (AVF) had initial phreatomagmatic eruptions, with pyroclastic density currents that left behind surge deposits. Because the AVF encompasses much of the Auckland urban area, there is little physical exposure. Accurate source locations and runout distances of the surge deposits are poorly known. Consequently, it is difficult to model the hazard and risk associated with the initial explosive phases of a future AVF eruption.As part of the project for DEtermining VOlcanic Risk in Auckland (DEVORA), ground penetrating radar (GPR) imaging was completed in association with coastal cliff mapping, calibration with quarry exposures, and limited other outcrops at the Crater Hill, Maungataketake and Waitomokia volcanic centres. The GPR images of the surge deposits could help to estimate pyroclastic current directions, the volcanic surge origins, and the proximal and distal deposit geometries for the purposes of modelling the flow dynamics and the consequent risk. Coastal cliff exposures indicated that even subtle features in the GPR profiles could be correlated with geological features, lending greater confidence that interpretations of the profiles will provide crucial information when outcrops and cliff exposures are not available. Keywords-surge deposits; volcanic; pyroclastic; hazard; risk , ,1752'8&7,21 $XFNODQG 1HZ =HDODQG ¶V ODUJHVW FLW\ LV EXLOW RQ DQ DFWLYH YROFDQLF ILHOG ZLWK DERXW NQRZQ YROFDQLF FHQWUHV RYHU WKH ODVW ND )LJ DOPRVW DOO RI ZKLFK DUH PRQRJHQHWLF >@ >@ >@ >@ 7KXV WKHUH LV D QHHG WR EHWWHU FRQVWUDLQ DQG HYDOXDWH WKH SRWHQWLDO ULVNV DQG KD]DUGV IURP WKH QH[W YROFDQLF HUXSWLRQ )LJXUH 7KH $XFNODQG 9ROFDQLF )LHOG KDV DOPRVW NQRZQ YROFDQLF FHQWUHV 7KUHH FHQWUHV ZHUH LQYHVWLJDWHG DV SDUW RI WKH '(925$ SURMHFW WR HYDOXDWH YROFDQLF ULVN LQ $XFNODQG 0RGLIHG IURP >@ DV FLWHG LQ >@ 653
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