First results from GPS measurements on present day alpine kinematics

Journal of Geodynamics 30 (2000) 275±283 First results from GPS measurements on present day alpine kinematics Alessandro Caporali*, Silvana Martin Dipartimento di Geologia, Paleontologia e Geo®sica, UniversitaÁ di Padova, Via Giotto 1, I-35137, Padova, Italy Received 1 December 1998; accepted 15 June 1999 Abstract Present day displacements of a few mm/year result from a preliminary analysis of data from permanent GPS stations positioned along the ¯anks of the Alps. The largest rates occur across the Alps: the Zimmerwald Ð Torino line, in the Western sector, increases its length at a rate of 5.2 mm/year. The line is very nearly accommodated by the Grasse Ð Torino line, which decreases at a very similar rate. No statistically signi®cant displacement is evident in the Eastern sector. Our data thus support the hypothesis that the present day motion of Adria against Europe favours the escape to the West and Southwest of the North-western sectors of Italy. As the analysis of GPS data progresses, it should be possible to constrain by GPS measurements the present location of the Adria rotation pole and better test the `rigid block' assumption. # 2000 Elsevier Science Ltd. All rights reserved. 1. Introduction and geological background The Alps result from a WNW±ESE convergence between Europe and Adria, with a rate of about 1 cm/year between 39 and 6 million year ago (Butler, 1990). The global convergence is partitioned into a N±S convergence and a E±W dextral wrench shear (Lacassin, 1989; Butler, 1990). At present the chain is formed by a 50 km wide central belt bounded by two internal lineaments and by a pair of outward prograding thrust belts (Fig. 1). The internal lineaments are: (1) the frontal Pennine thrust (FPT) to the North and (2) the dextral Periadriatic fault to the South. The FPT transported the Penninic nappes over the European foreland during Oligocene Ð Early Miocene. The FPT and contiguous areas have been re-activated as a * Corresponding author. Fax: +39 49 8272070. E-mail address: alex@geol.unipd.it (A. Caporali). 0264-3707/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 4 - 3 7 0 7 ( 9 9 ) 0 0 0 3 7 - X 276 A. Caporali, S. Martin / Journal of Geodynamics 30 (2000) 275±283 Fig. 1. Network of permanent GPS stations and tectonic map of the Alps, after Dal Piaz et al. (1988), Vialon et al. (1989), Castellarin et al. (1992), Seward and Mancktelow (1994) and Linzer et al. (1995). AA = Aar, GO = Gotthard, MB = Mont Blanc, AR = Aiguilles Rouges, BE = Belledonne, PE = Pelvoux, AG = Argentera., NCA = Northern Calcareous Alps, FPT = Frontal Penninic Thrust, AST = Alpine Sole Thrust. 1: GPS stations, 2: thrusts and normal faults, 3: molasse, 4: external units, 5: Penninic thrusts, 6: Austroalpine basement and cover units, 7: Southalpine and Dinaric basement and cover units. A. Caporali, S. Martin / Journal of Geodynamics 30 (2000) 275±283 277 dextral transpressive fault system since late Neogene between the Rhone valley and the Belledonne massif (Gratier et al., 1989; Hubbard and Mancktelow, 1992) and correspond to the most seismic lineament of the Alps (Eva and Solarino, 1998). Transpression produced the relative uplift in the zone of transcurrent movements, so that deep Mont Blanc, Aiguilles Rouges, Belledonne and Pelvoux Helvetic massifs were exhumed rapidly in the late NeogeneQuaternary time (Seward and Mancktelow, 1994; Lelarge et al., 1991). The Periadriatic fault is completely inactive at present, but it played the role of the main dextral transpressive fault in the Alps during Oligocene Ð Early Miocene times (Schmid et al., 1989). The outward thrust systems include NNW-vergent thrusts prograding over the European foredeep (Alpine sole thrust or AST, Fig. 1) and S-vergent thrusts prograding over the Adriatic foredeep. The dextral transpression on the North side of the Penninic front appears to be related to the development of intraorogenic extensional zones such as the Simplon±Rhone fault zone inside the central Alpine belt, with movements parallel to the orogen responsible for the SWvergent thrusts (Hubbard and Mancktelow, 1992). Transpression probably continues to the NE along the frontal Penninic thrust zone, as suggested by the intraorogenic extensions along the Engadine and Brenner faults. In the Eastern Alps, between the Periadriatic fault and the FPT, several ENE±WSWtrending sinistral faults are present inside the central part of the orogen. The most distinct feature is the Salzachtal-Ennstal-Mariazell-Puchberg (SEMP) fault which extends 400 km towards the Vienna basin. These faults facilitated the lateral extrusion of the central portions of the Eastern Alps towards the Pannonian basin since Oligocene Ð Miocene time (Ratschbacher et al., 1991). They escaped from a zone of continental convergence towards a weakly constrained margin, while the Northern Calcareous Alps (NCA), near the Penninic front, were involved in transpressional contraction. This is indicated by NW-directed thrusting and en-echelon arrays of thrusts and folds laterally displaced by WNW-striking dextral faults (Linzer et al., 1995). However, the present orientation of the dextral faults is due to a clockwise rotation of the NCA during Mid-Miocene (Fodor, 1995). This rotation seems consistent with the post Mid-Miocene clockwise rotation of 168±178 of the Swiss Alps (Kempf et al., 1998), and should be consistent with the Mid-to Late Miocene clockwise rotation of 158 of the Southern Alps (SchoÈnborn, 1992). In the Eastern Alps, convergence due to the indentation of Adria with Europe appears to have been partitioned between the uplift of the Tauern dome in the latest Oligocene Ð Miocene (Grundmann and Morteani, 1985), and the lateral escape of the Austroalpine nappes along faults of the Northern Calcareous Alps. The Miocene evolution of the Eastern Alps, Vienna region and Pannonian domain appears strongly controlled by the progressive development of sinistral shear zones and the dextral displacement along the Easternmost segment of the Periadriatic fault (Linzer et al., 1995). The S-vergent thrust belt includes Southalpine thrusts of di€erent ages with variable geometry. The more internal are rooted between the Periadriatic fault and the Val Trompia and Valsugana thrust belts which were active during Early-Mid Miocene (Castellarin et al., 1992). This continued to the E up to the interference with the older Dinaric front. The CentralEastern part of this Miocene front has been deformed since Late Miocene by the push of the Adriatic indenter and has been forced into an arcuate belt, the Giudicarie±Garda Lake front. This push by the Adriatic region has re-activated the earlier NNE-trending faults of the 278 A. Caporali, S. Martin / Journal of Geodynamics 30 (2000) 275±283 Giudicarie±Garda Lake region and other pre-existing tectonic lines as SE-vergent thrusts (Castellarin et al., 1992). New thrust belts developed at the Northern boundary of the Eastern Po plain Ð the Neogene-Quaternary thrusts of the `Pedemontana' belt (Massari et al., 1993) and deep thrusts (Pieri and Groppi, 1981; Bertotti et al., 1998). The di€erential push exerted by the Adriatic region is responsible for the development of NNW-trending transfer faults which intersect all the thrusts. All these structures are at present seismically active (Slejko et al., 1987). Given the structural pattern of the active faults, of the structures which are suspected to be active and of the inactive structures which could be re-activated, this paper addresses the present-day, large scale kinematics of the Alpine area. By analyzing high quality data from a network of permanent GPS stations along the Alps, we show that the displacements, and hence large scale strain rates, in the past 80 weeks are at most 1±3 mm/year, with the only notable exception of the permanent station Torino, for which we report a Southward motion of 5 mm/ year. 2. The geodetic network of permanent GPS stations To detect displacement ®elds of the order of some mm/year over distances of hundreds of km, the instrumentation is required to collect data uncorrupted by accidental events. The analysis should embody kinematic and dynamic models such that the GPS signal recorded at the stations is reconstructed with systematic errors at or below the level of random errors. Table 1 lists the considered stations, the type of equipment and changes occurred in the time span of the analysis, as documented in the log-sheets that each station makes available to the EUREF Data Centre in Bruxelles. All stations are equipped with state of the art, dual frequency receivers, to calibrate the phase advance due to propagation of the radio signals in the ionosphere. To minimize the e€ect in the GPS data of multiple re¯ections from obstacles Table 1 Permanent GPS stations and equipment used to record the data which were used in this analysis Station Full name name Receiver GRAS GRAZ HFLK Rogue SNR 12 Rogue SNR 8000 Rogue SNR 8C until 19/06/98; Rogue SNR 8000 Trimble 4000 Ssi Trimble 4000 SSi PFAN TORI TREN ZIMM Grasse Graz Hafelekar Antenna Antenna height (m) Log sheet Dorne Margolin T 0.035 Dorne Margolin T 1.964 0.0 until 19/06/98; Dorne Margolin B ÿ0.020 until 19/06/98 (wk 0962): Dorne Margolin T Pfaender Micro pulse L1/l2 0.054 0.000 Torino Compact w/ground plane until 12.07.98 (wk 0966); Dorne Margolin T Trento Trimble 4000 Ssi Compact L1/L2 w/ground plane 0.0625 Zimmerwald Trimble 4000 SSE 4000 ST w/ground plane 0.000 June 1997 June 1996 June 1998 May 1997 September 1998 August 1998 January 1998 A. Caporali, S. Martin / Journal of Geodynamics 30 (2000) 275±283 279 near the stations and ensure that, at each time, the phase data are an estimate of line-of-sight distance to the satellites, the antennas have either a ground plane or a choke ring. The physical location of the centre of phase of each antenna at the two frequencies L1 (1575.42 MHz) and L2 (1227.60 MHz) is modeled for each elevation and azimuth angle using special tables resulting from accurate, antenna-speci®c calibration. The data validation is crucial to ensure consistent quality and a safeguard against systematic errors. Thus the analysis focussed on data from permanent stations only. For sites only occasionally occupied, such screening is often more uncertain. The data have been processed with the Bernese 4.0 software, using mutually consistent, precise ephemeris and Earth Orientation Parameters computed at CODE (Centre for Orbit Determination for Europe) in Bern. For each day, a network adjustment is computed after cycle slip screening and ambiguity resolution. Weekly solutions are constructed by staking seven daily solutions and inverting again the normal equations. Thus each estimate of a baseline length represents a weighted average of daily estimates over one week, with weights computed from the daily least squares solutions. To estimate the large scale deformation which has occurred along the Alps during the time span of the analysis, we consider the rate of change of baselines, as these are scalars, thus independent of the reference system. The time series of the baselines are described in Fig. 2 and an interpretation of the deformation of this regional scale geodetic network is presented in Fig. 3. It should be noted that all the rates are in all cases smaller than 10 mm/year, thus small by standards of large scale plate kinematics. We consider as statistically signi®cant deformations of at least 2 mm/year, that is a strain rate of 3  10ÿ16 sÿ1 for a typical baseline of 200 km. To interpret the data in Fig. 2, we must consider that a negative or positive slope (in mm/year) represents a length decrease or, respectively, a length increase of the baseline. The largest rates occur across the Alps: the Zimmerwald (Bern) Ð Torino line, on the Western sector, increases its length at a rate of 5.2 mm/year. This increase seems to be entirely accommodated by a corresponding decrease of the Grasse±Torino line, which is closely aligned with the Zimmerwald Ð Torino line. Also the Torino±Pfaender line shows a length increase, but at a more modest rate of 3.1 mm/year. Thus Torino seems to move S±SW. On the Eastern sector, the Hafelekar (Innsbruck)±Trento and Graz±Trento lines are stable, but the baselines from Trento to the NW, i.e. to Zimmerwald and Pfaender, decrease by 3 mm/year, just above the threshold of uncertainty. The lines with a strong E±W component (e.g., Trento±Torino, or Zimmerwald±Pfaender, Pfaender±Hafelekar) show variations at or below the threshold of noise. The overall picture (Fig. 3) arising from this analysis suggests a counterclockwise rotation of the Trento±Torino line relative to the somewhat more stable stations North of the Alps. If this trend is con®rmed by an analysis covering a longer time span, a dextral strike slip fault on the Northern side of the Alps, and a transpressional fault somewhere in the Northern Apennines or Central Po Plain,would be required to accommodate this rotation. 3. Discussion and conclusions The deformation of the geodetic network observed in the Alps results from a limited number 280 A. Caporali, S. Martin / Journal of Geodynamics 30 (2000) 275±283 Fig. 2. Observed rates of change of the length of baselines in Fig. 1. Each point corresponds to one week of data. Acronyms for GPS stations in Fig. 1: ZIMM = Zimmerwald (Bern), TORI = Torino, GRAS = Grasse, PFAN = Pfaender, TREN = Trento, HFLK = Hafelekar (Innsbruck). Positive slope corresponds to length increase; negative slope corresponds to length decrease. Vertical units in meters. A. Caporali, S. Martin / Journal of Geodynamics 30 (2000) 275±283 281 Fig. 3. A kinematic interpretation of the deformation of the geodetic network during the time interval of the analysis. of stations and time span, and must therefore be interpreted conservatively. However, the stability of the solution, in the sense of root mean square repeatability of the baseline estimates in Fig. 2, justi®es a discussion of the constraints implied by these data on models of current deformation on a large scale. With this caveat, we interpret the observed motion as due to a rigid counterclockwise rotation of the Italian peninsula relative to a stable continental Europe. This interpretation would then qualitatively support earlier proposals (e.g. Castellarin et al., 1992; Renner and Slejko, 1994) based on geologic and structural data. According to Regenauer-Lieb and Petit (1997), the Adriatic Sea and the Eastern half of the Italian peninsula should correspond to the present-day Adriatic indenter (McKenzie, 1972). Further support to the hypothesis of present-day rotation of the Italian peninsula is o€ered by paleomagnetic data which suggest a counterclockwise rotation of Adria of 158 with respect to Africa during the Tertiary (Anderson, 1987), and also by the rejuvenation of the Neogene foredeep basins at the boundary of the Po plain to the East (Massari, 1990). At present Africa is moving relative to Eurasia with anticlockwise rotation rates increasing from 4 to 7 mm/year from Morocco to Tunisia. This increase, due to the increased distance 282 A. Caporali, S. Martin / Journal of Geodynamics 30 (2000) 275±283 from the Euler pole of rotation of Africa relative to Eurasia, predicts a relative northward motion of the Africa plate of 7±8 mm/year in the Ionian and Libyan Seas (Kahle et al., 1998). Accordingly, the movement of Africa should constrain a displacement of the Adriatic region (Adria) towards N±NW. The large scale deformation which is implied by the observed temporal change of the baselines appears to be in accordance with this hypothesis, but the limited time span of the GPS data relative to the geological time scale of the processes should be emphasized. In conclusion our data, although preliminary, provide support for the hypothesis that the present-day compression of the Adriatic indenter against Europe at its Northern boundary and beneath the Po plain facilitates the global escape to the West (and Southwest) of the Northwestern regions of Italy. 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