Subduction Beneath Gibraltar? Recent Studies Provide Answers Marc-André Gutscher
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Subduction beneath Gibraltar? Recent studies provide answers Marc-André Gutscher To cite this version: Marc-André Gutscher. Subduction beneath Gibraltar? Recent studies provide answers. Eos, Transac- tions American Geophysical Union, American Geophysical Union (AGU), 2012, 93 (13), pp.133-134. 10.1029/2012EO130001. insu-00689446 HAL Id: insu-00689446 https://hal-insu.archives-ouvertes.fr/insu-00689446 Submitted on 8 Mar 2021 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Eos, Vol. 93, No. 13, 27 March 2012 VOLUME 93 NUMBER 13 27 MARCH 2012 EOS, TRANSACTIONS, AMERICAN GEOPHYSICAL UNION PAges 133–140 will affect assessments of the long-term Subduction Beneath Gibraltar? natural hazards in this region. Geological Setting Around Gibraltar Recent Studies Provide Answers Explained Through Each Hypothesis PAGES 133–134 The hypothesis of subduction chal- The geodynamic evolution of the west- lenges the widely accepted model that the ern Mediterranean region over the past On 1 November 1755 a powerful earth- southern Iberia region was shaped by con- 30 million years has been characterized by quake shook Portugal, Spain, and north- tinental delamination— a process by which the slow movement of Africa in a north to ern Morocco, with tremors felt across most the bottom layers of thick continental lith- northwest direction toward Eurasia, caus- of northwestern Europe and as far west as osphere detach and sink into the mantle, ing the intervening ocean, the Tethys, to the Azores and as far south as the Cape stretching overlying crust as it sinks—in be slowly consumed as it was subducted Verde islands [Gutscher, 2004]. Known as the wake of a continent-continent collision beneath Africa. Paradoxically, in the the Great Lisbon Earthquake, the event between Africa and Iberia. Geological and boundary region between the two large was followed by a devastating tsunami geophysical studies conducted over the converging plates, there was widespread that swept the nearby Atlantic coast with past 10 years provide new evidence that extension and formation of young oceanic waves 5–15 meters high; tsunami waves weighs in on these two competing hypoth- basins caused by subduction and rollback 1–5 meters high were also observed in eses. Deciding which hypothesis prevails of limited domains of oceanic lithosphere, the Antilles [Gutscher et al., 2006]. Follow- ing the earthquake, huge conflagrations raged for days in the city of Lisbon, where 85% of buildings were leveled. In total, 40,000–60,000 people are estimated to have perished. Modern estimates now consider that the earthquake’s magnitude was between 8.5 and 9.0, making it among the stron- gest earthquakes ever felt. But questions on the earthquake’s exact mechanism con- tinue to perplex researchers. Also perplex- ing are observations of extreme stretch- ing of crust in the western Alboran Sea, an arm of the Mediterranean between Spain and Morocco. Could the processes that stretched the crust there be related to the forces that triggered the 1755 earthquake? A recent hypothesis proposes that the geography and crustal structure in this region have been shaped in large part by the subduction of a narrow strip of oce- anic lithosphere beneath Gibraltar, a pro- cess that possibly affects the area’s tecton- ics and seismicity to this day [Lonergan and White, 1997; Gutscher et al., 2002]. Despite the absence of arc volcanism and shallow- dipping thrust-type earthquakes (two of the characteristic features of active sub- duction), numerous geological observa- Fig. 1. A three-dimensional block diagram of the lithospheric structure of the region surrounding tions provide hints that subduction persists Gibraltar, with shaded hill relief at the surface. The hypothesized plate boundaries, expressed by (albeit at rates of less than 1 centimeter per crustal seismicity and the GPS velocity field, are shown as bold dashed lines. The Rif-Betic front year) and may be capable of generating is shown by red teeth (solid teeth indicate the active portion), and the deformation front of the recurrent great earthquakes like the famous accretionary wedge is shown by green teeth. The asymmetric shape of the oceanic slab at depth 1755 event [Gutscher, 2004; Gutscher et al., is from tomography. Inset shows fast directions for polarized shear waves in the upper mantle 2006, 2009a, 2009b]. (bars represent 0.5- to 1.3-second time shift), indicating a dominant “flow fabric.” This is displayed on a horizontal tomographic slice at 200-kilometer depth, with the blue zone indicating a rapid P wave velocity anomaly (cold, dense lithosphere) and the yellow arrows indicating a proposed BY M.-A. GUTSCHER path for return flow behind the slab edge [Diaz et al., 2010]. Eos, Vol. 93, No. 13, 27 March 2012 which triggered rapid movement of inde- delamination? (2) Is subduction still active Is Subduction Still Active Today? pendent blocks and back-arc extension at today? the surface [Faccenna et al., 2004]. Recent geophysical studies on the shallow The western Alboran Sea constitutes Evidence for Subduction structure of the Gulf of Cadiz as well as geo- one of these young basins, with thin crust, detic work on regional kinematics provide high heat flow, and thick sediments (>10 New seismological results offer insights some answers to the question on whether kilometers) deposited over the past 15 mil- on the deep geodynamic processes that subduction is still active. lion years. This basin is entirely enclosed have shaped the Rif-Betic-Alboran region. Multichannel seismic reflection data by the westernmost portion of the Alpine- A study of the dispersion of teleseismic from the Gulf of Cadiz, which imaged an Himalayan fold-and-thrust belt, the Betic body waves traveling through the dense eastward thickening wedge of deformed mountains to the north, and the Rif moun- lithospheric body in the upper mantle con- sediments, show that it overlies a shal- tains in northern Morocco to the south- cludes that it must contain a fairly thin low eastward dipping layer of undeformed west. Together, these arcuate mountain (~10- kilometer- thick) low-velocity layer, sediments [Gutscher et al., 2002; Thiebot belts form the Gibraltar arc (Figure 1). interpreted to be the oceanic crust at the and Gutscher, 2006; Iribarren et al., 2007; Early tomographic studies performed here top of a roughly 100-kilometer slab of sub- Gutscher et al., 2009a, 2009b]. This geome- showed a cold, dense lithospheric body in ducting oceanic lithosphere [Bokelmann try is characteristic of accretionary wedges the upper mantle linked to deep-focus seis- et al., 2011]. These results are not consis- and requires a west directed tectonic push. micity beneath Granada (southern Spain) tent with the delamination model, which Recently published multibeam bathymetric but without a clear connection to scattered requires the descending body to consist of data from the southwestern Iberia region intermediate and shallow seismicity [Seber continental lithosphere. highlight the rough surface and the sharp et al., 1996]. Analyses of the pressure and Seismological studies of shear waves U-shaped boundaries of this accretionary temperature histories of high-grade meta- passing through the upper mantle can complex [Gutscher et al., 2009b] (Figure 1). morphic rocks found in the region indi- reveal the presence of a dominant “fabric” The overall westward directed thrusting cated that they were exhumed rapidly, and due to the preferential alignment of oliv- and transport are expressed by generally this was interpreted to have been caused by ine crystals. Such studies indicate signifi- north-south trending anticlinal folds at the dramatic thinning of the lithosphere and an cantly shorter travel times (arrivals 0.5 to deformation front (just as north-south ori- influx of hot asthenospheric material below 1.3 seconds earlier) for seismic waves polar- ented wrinkles on a tablecloth indicate [Platt and Vissers, 1989]. ized parallel to this dominant fabric (simi- east-west shortening). It is also demon- Together, these observations of a dense lar to the anisotropic fabric of bamboo or strated by the indentation caused by an lithospheric body, apparently detached fiberglass). The fast directions in the upper east-west trending basement high (marked from the surface with hot asthenosphere mantle are aligned parallel to the Rif-Betic “Indenter” in Figure 1), a pattern success- above, seemed to offer a classic exam- arc and veer from north-south, just west of fully reproduced by analog modeling ple for continental delamination [Calvert Gibraltar, to east-west in southern Iberia, [Gutscher et al., 2009a]. et al., 2000]. This extreme crustal thinning just north of the Betics [Diaz et al., 2010] The morphology and bathymetry of the was considered separately from questions (Figure 1 inset). This fabric is consistent accretionary wedge, as well as high-resolu- surrounding the origin of the 1755 earth- with lateral (or toroidal) flow around the tion seismic data of the deformation front quake. The 1755 earthquake was most slab as it retreats to the west. The upper and its lateral boundaries, indicate ongoing commonly explained by movement along mantle flow pattern below the Gibraltar compressional and transpressional defor- faults offshore of southwestern Iberia (e.g., arc is nearly the mirror image of the pat- mation, with likely prolongations onshore Gorringe Bank) near the major fracture tern observed below the Appenine and Cal- in southwestern Spain and northwestern zone separating Africa from Eurasia that abrian arc of southern Italy, where rollback Morocco [Gutscher et al., 2009a, 2009b; connects to the Mid-Atlantic Ridge.