This paper explores the suitability and advantages of combining the trenching technique with geophysical surveys [ground penetrating radar (GPR) and electrical resistivity tomography (ERT)] for sinkhole characterization in a mantled karst area. The approach is applied to two active sinkholes concealed by anthropogenic deposits and formed by contrasting subsidence mechanisms; collapse and sagging. The ERT section acquired across the collapse sinkhole images the clayey fill of the depression as an obvious low resistivity area, showing the approximate location of the sinkhole edges. Spatially dense GPR surveys provide information on the position of the boundaries of the concealed subsidence structures and their three‐dimensional (3D) internal geometry, revealing the dominant subsidence mechanism. We illustrate the impact of several factors on the quality of the GPR data such as sinkhole size, nominal frequency of the antennas, antenna shielding, and the presence of backfilled excavations and above‐surface objects. Trenches provided detailed information on the subsurface structure of the sinkhole, subsidence magnitude, partitioning of the strain, and the position of the sinkhole edges, especially when they are deep enough and excavated across the central sector and perpendicular to the boundaries. The stratigraphic and structural relationships observed in the trench were then used to infer the spatial evolution of the sinkholes (e.g. enlargement), their kinematic behavior (episodic versus progressive), and to differentiate discrete subsidence events and their associated magnitude. Numerical dates were used to estimate average subsidence rates and the recurrence of subsidence events. Such integrated data sets may be used as an objective basis to forecast the future behavior of potentially damaging sinkholes and to assess the associated hazard and risk. Copyright © 2013 John Wiley & Sons, Ltd.

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The active lateral spread of the Peracalç Range (Spanish Pyrenees) has developed on a Cretaceous limestone sequence around 250 m thick, underlain by tectonically thickened (~2.5 km) Triassic halite-bearing evaporites and clays. Outward expansion of the Triassic sequence by ductile deformation and probably halokinesis toward the debuttressed and unloaded front of the range has been accommodated in the overlying cap rock through the development of a striking horst and graben morphostructure. Fault scarps show anomalously high height to length ratios (aspect ratio; H max /L) compared to the values reported for tectonic faults. This retrogressive gravitational deformation has aborted a paleodrainage, expressed as wind gaps, hanging valleys, and defeated streams. The signifi cant vertical displacement component in this rock spread is attributed to subsidence caused by interstratal evaporite dissolution, as supported by the dissolution-induced collapse and graben structures mapped at the foot of the range. To our knowledge, the rock spread of Peracalç, covering around 4.5 km 2 and with a minimum volume of 0.9 km 3 , is the largest documented landslide of the Pyrenees. The excavation of trenches and the acquisition of electrical resistivity tomography profi les provided information on the thickness and subsurface structure of the graben fi lls, the age of the lateral spread (older than 45 ka), an unexpected episodic kinematic behavior of the gravitational faults, and the timing of deformation events, including slumping of lake deposits.

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This work analyses a suite of relict tufa mounds generated by artesian karstic springs in Isona area (Spanish Pyrenees). Geological and geophysical data (seismics and vertical electrical resistivity soundings) indicate that the location of the discharge area in which the spring mounds formed was controlled by (1) a bulge in the axial zone of the anticline that affects the Areny-Montsec aquifer, with the consequent thinning of the overlying confining unit and (2) N-S and E-W trending extensional faults. These uncommon meteogene mounds occur in two stepped morphostratigraphic units that constitute the caprock of a mesa 9 km 2 in area. The upper tufa complex is 47 m thick and has yielded several U/Th dates of >350 ka. The obtained U/Th ages for the mounds of lower tufa complex, 10 m thick, range from >350 to 214 ± 11 ka. The sedimentological analysis of the scarce exposures and electrical resistivity profiles show the same morphological and depositional components as those described in geothermal springs: (1) Cylindrical vents; (2) Pools fed by the vents and dammed by annular tufa barriers; (3) Tufa barriers (rimstones) constructed by overflowing waters through vertical accretion and progradation. These rimstones may have overhanging upflow sides. (4) Slope tufa facies with terracettes and microgours.