Faure Walker, J. P.; (2010) Mechanics of continental extension from Quaternary strain fields in the Italian Apennines. Doctoral thesis , UCL (University College London).

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Abstract

Horizontal upper crustal surface strain-rates calculated using slip-vectors from striated faults and offsets of Late Pleistocene-Holocene landforms and sediments are used to investigate the mechanisms responsible for deformation in the Italian Apennines over a variety of length-scales ranging from individual fault segments up to the width of the mountain range. The method used allows strain-rates in any 5km \times 5km grid square or combination of these grid squares to be calculated. This allows comparison of strain-rates from 15 \pm 3 kyrs of slip with those from shorter time periods within polygons that are comparable in size, shape and location with those imposed by geodetic station locations or moment summation calculations. Strain-rates over a time period of 15 \pm 3 kyrs from 5km \times 5km grid squares integrated over an area of 1.28 \times 10^4 km^2 (80km \times 160 km), show the horizontal strain-rate of the Lazio-Abruzzo region of the central Apennines is 1.18^{+0.12} _{-0.04}\times10^{-8}yr^{-1} and -1.83^{+3.80} _{-4.43}\times10^{-10}yr^{-1} parallel and perpendicular to the regional principal strain direction (043^o-223^o \pm 1^o), associated with extension rates of \leq3.1^{+0.7} _{-0.4} mm yr^{-1} if calculated in boxes with a 5km width and 90km length across the Apennines. In Molise-North Campania, the horizontal principal strain-rate calculated over an area of 5\times10^3 km^2 (50km\times100 km) is 2.11^{+1.14} _{-0.16}\times10^{-9} yr^{-1} along the horizontal axis parallel to 039^o - 219^o \pm 3^o, and 0.88^{+2.84}_{-1:30}\times10^{-10} yr^{-1} perpendicular to it, associated with extension rates of \leq0.2^{+0.2} _{-0.1} mm yr^{-1} if calculated in 5km \times 90km transects that cross the Apennines. Within the South Campania-Basilicata region of the southern Apennines of area 8 \pm 103 km2 (50km_160 km), the average horizontal strain-rate over 15 \pm 3 kyrs is 3.70\pm0.26\times10^{-9} yr^{-1} parallel to and 3.65\pm2.05\times10^{-10} mmyr^{-1} perpendicular to the principal strain axis (044^o-224^o\pm2^o), associated with extension rates of \leq0.6\pm0.2mmyr^{-1} if calculated in 5km\times90km transects across the Apennines. The same method is used to calculate strain-rates in Calabria from longerterm offset geological features (\leq 580 ka); the horizontal principal strain-rate calculated over an area of 8\times103 km^2 (40km\times200 km) is 6.71\pm2.13\times10^{-9} yr^{-1} along the horizontal axis parallel to 086^o-226^o\pm3^o, and -8.40\pm5.69\times10^{-10} yr^{-1} perpendicular to it. Strain-rates calculated over 15\pm3 kyrs within 5km\times5km grid squares vary from zero up to 2.34\pm0.54\times10^{-7} yr^{-1}, 3.69\pm1.33\times10^{-8} yr^{-1}, and 1.20\pm0.41\times10^{-7} yr^{-1} in the central Apennines, the Molise-North Campania region, and the southern Apennines, respectively. These strain-rates resolve variations in strain orientations and magnitudes along the strike of individual faults and are used to produce a fault specific earthquake recurrence interval map. In order to study the existence of possible deficits or surpluses of geodetic and earthquake strain in the Apennines, these 15 \pm 3 kyrs multi seismic cycle strain-rates have been compared to short-term strain-rates calculated using geodesy (over 126 yrs, 11 yrs and 5 yrs) and seismic moment summation (over 700 yrs). Regional strain-rates calculated from geodesy and historical earthquakes are greater than those calculated from offset 15 \pm 3 ka landforms and sediments. In detail, 10^{1-2} yr strain-rates are higher than 10^4 yr strain-rates in some small areas (\approx2000 km^2, corresponding to polygons defined by geodesy campaigns and seismic moment summations) with the opposite situation in other areas where seismic moment release rates in large (Ms>6.0) magnitude historical earthquakes have been reported to be as low as zero. This demonstrates (1) the importance of comparing the exact same areas, and (2) that strain-rates vary spatially on the length-scale of individual faults and on a timescale between 10^{1-2} yrs and 10^4 yrs in the Apennines. The results are used to discuss temporal earthquake clustering and the natural variability of the seismic cycle. Spatial variations in upper crustal strain-rate measured across exposed fault scarps since 15\pm3 ka are also used to discuss the regional deformation related to plate boundary and sub-crustal forces, specifically, whether mantle upwelling and uplift contribute to forces associated with the active extension in the Italian Apennines. Strain-rates calculated in 5km \times 90km boxes across the Apennines are compared with data on cumulative upper-crustal strain, topography, free-air gravity and SKS splitting delay times that are a proxy for strain in the mantle. High extension-rates across the Apennines since 15 \pm 3 ka (0.4-3.1mm yr^{-1}) occur in the southern Apennines and central Apennines where values for finite extensional strains that have developed since 2-3Ma are highest (2-7km cumulative throw), and where mean topography from SRTM data (Shuttle Radar Topography Mission) is > 600m; the intervening area of Molise-North Campania with < 600m topography has extension-rates < 0.4mm yr^{-1} and lower values for finite extensional strains (< 2km cumulative throw). These two areas with high upper-crustal extension-rates overlie mantle that has relatively-long spatially-interpolated SKS delay times (1.2-1.8 seconds) and relatively-high free-air gravity values of 140-160 mGals; the intervening area of lower extension-rates has relatively-low spatially-interpolated SKS delay times of 0.8-1.2 seconds and relatively-low free-air gravity values of 120 mGals. These correlations suggest that at the regional length-scale, a sub-crustal process, that is, dynamic support of the topography by mantle upwelling, controls the present-day upper-crustal strain-rate field in the Apennines and the geography of seismic hazard in the region. At a smaller length-scale, in order to investigate the relationship between the throws and 3D orientation of breaching faults crossing relay zones, kinematic data, throw-rates and total throws have been measured for an active normal fault in the Italian Apennines that displays a relay zone at its centre. The c.0.8km long breaching fault, investigated in detail, dips at 67^o\pm5^o and strikes obliquely to c.2-3km long faults outside the relay zone which dip at 61^o\pm5^o.Total throws of pre-rift limestone define a throw profile with a double maximum (370\pm50 m; 360\pm50 m) separated by an area of lower throw (100\pm50 m) where the breaching fault is growing. Throw-rates implied by offsets across bedrock scarps of Late Pleistocene-Holocene landforms (15 \pm 3 ka) are higher across the breaching fault (0.67\pm0.13mm yr^{-1}) than for locations of throw maxima on the neighbouring faults (0.38\pm0.07mm yr^{-1}; 0.55\pm0.11mm yr^{-1}). The deficit in total throw will be removed in 0.68-1.0 Myrs if these deformation rates continue. To investigate why the highest throw-rates occur in the location with lowest total throw, horizontal strain-rate tensors were calculated in 1km \times 2km boxes. It is shown that the oblique strike and relatively-high dip of the breaching fault mean that it must have a relatively-high throw-rate in order for it to have a horizontal strain-rate concomitant with its position at the centre of the overall fault. It is shown that whether throw minima at locations of fault linkage are preserved during progressive fault slip depends on the 3D orientation of the breaching fault. The above is used to discuss the longevity of throw deficits and multiple throw maxima along faults in relation to seismic hazard and landscape evolution. Overall, this thesis shows that calculation of horizontal strain-rates using the method developed herein, supported by collection of field data from active faults, can provide new insights into regional mechanisms of continental extension, seismic hazard, the seismic cycle, and fault growth; it provides a test of the hypothesis that earthquake recurrence is spatially random, providing evidence that instead, earthquake recurrence shows a spatial pattern that is controlled by fault evolution and sub-crustal processes.

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