Subduction Beneath Gibraltar? Recent Studies Provide Answers Marc-André Gutscher

Subduction beneath Gibraltar? Recent studies provide answers Marc-André Gutscher

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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

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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 stretching 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 oceanic lithosphere beneath Gibraltar, a process that possibly affects the area’s tectonics 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 subduction), 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. subduction is known to occur [Baccheschi Maad et al., 2010; Crutchley et al., 2011]. In However, those who consider that sub- et al., 2007]. The delamination hypothesis addition, a total of 51 active mud volca- duction can explain both observations predicts radial inward flow and fast direc- noes have been identified and sampled in propose that the strip of oceanic lith- tions, inconsistent with these observations the Gulf of Cadiz in the decade following osphere, part of the Tethyan oceanic [Diaz et al., 2010; Bokelmann et al., 2011]. the discovery of the first two in 2001 [see domain that once separated Gondwana Another recent discovery is based on a Gardner, 2001], all located on the accretion- (Africa, India, Australia, and Antarctica) wide-angle seismic survey from offshore ary wedge, highlighting the fluid expulsion from Eurasia, still exists today in the form southern Portugal revealing the presence of a related to sediment compaction and tectonic of a narrow and steeply east dipping slab 7-kilometer-thick oceanic crust in the Gulf of compression within the accretionary wedge of oceanic lithosphere below the Gibral- Cadiz, 100 kilometers south of the coast and [Medialdea et al., 2009]. tar region [Gutscher et al., 2002]. Over the extending beneath the toe of an eastward Finally, geodetic studies of the kinematics past 15 million years, this east-west strip of thickening wedge of deformed sediments [Sal- in this slowly moving region have improved oceanic lithosphere detached itself from lares et al., 2011] (see Figure 1). On the basis of and now reveal an independent Rif-Betic- the adjacent African continent and sank published paleogeographic reconstructions, West Alboran microplate between Iberia vertically into the mantle while remain- these workers interpret the crust to be Jurassic and Africa, moving to the west-southwest ing attached farther west. This caused the in age (about 120–180 million years old) and at 3–5 millimeters per year with respect to slab to bend down and the hinge line to to represent the small westernmost remnant of Africa [Koulali et al., 2011]. This is exactly the roll back to the west, in turn causing east- the once vast Tethys Ocean. Numerous marine type of block motion predicted by an active west extension in the crust of the over- seismic surveys have shown that this crust subduction and rollback geodynamic model. riding plate. This movement also resulted dips to the east [Gutscher et al., 2002; Thie- in a small tectonic block, the “Rif-Betic- bot and Gutscher, 2006; Iribarren et al., 2007; A Convincing Tectonic Scenario Alboran” microplate (between the back- Gutscher et al., 2009a]. It appears contiguous arc basin and the subduction hinge), with the east dipping high P wave velocity On the basis of this review of recent which was drawn to the west in response. body beneath Gibraltar seen by tomography observations, four main predictions of the This two-part hypothesis has provoked [Gutscher et al., 2002; Spakman and Wortel, active east dipping subduction hypothe- heated debate in the scientific commu- 2004]. These observations of oceanic crust in sis can now be confirmed Gutscher[ et al., nity. Testing it requires answers to two basic the Gulf of Cadiz, dipping to the east and con- 2002]. First, east dipping oceanic lithosphere questions: (1) Is the lithosphere beneath nected to a slab of oceanic lithosphere deep has been observed at shallow (crustal) Gibraltar observed by tomography a prod- below Gibraltar, strongly support the subduc- depths and at great (upper mantle) depths. uct of oceanic subduction or of continental tion model. Second, active dewatering occurs in the Eos, Vol. 93, No. 13, 27 March 2012 accretionary wedge. Third, active tectonic Crutchley, G. J., C. Berndt, D. Klaeschen, and abit, J. M. Davila, and N. Amraoui (2011), deformation occurs at the wedge’s boundar- D. G. Masson (2011), Insights into active New GPS constraints on active deformation ies. 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King, Scientifique/University of Brest, Plouzané, France; Res., 105, 10,871–10,898. P. Vernant, R. E. Reilinger, S. McClusky, T. Mour- E-mail: [email protected]