JOURNALOF GEOPHYSICAL RESEARCH, VOL. 100, NO. B7, PAGES 12,357-12,373, JULY 10, 1995 Shapeof the subductedRivera and Cocosplates in southern Mexico: Seismic and tectonicimplications Mario Pardo and Germdo Sufirez Insfitutode Geoffsica,Universidad Nacional Aut6noma de M6xico Abstract.The geometry of thesubducted Rivera and Cocos plates beneath the North American platein southernMexico was determined based on the accurately located hypocenters oflocal and te!eseismicearthquakes. The hypocenters ofthe teleseisms were relocated, and the focal depths of 21 eventswere constrainedusing a bodywave inversion scheme. The suductionin southern Mexicomay be approximated asa subhorizontalslabbounded atthe edges by the steep subduction geometryof theCocos plate beneath the Caribbean plate to the east and of theRivera plate beneath NorthAmerica to thewest. The dip of theinterplate contact geometry is constantto a depthof 30 kin,and lateral changes in thedip of thesubducted plate are only observed once it isdecoupled fromthe overriding plate. On thebasis of theseismicity, the focal mechanisms, and the geometry ofthe downgoing slab, southern Mexico may be segmented into four regions ß(1) theJalisco regionto thewest, where the Rivera plate subducts at a steepangle that resembles the geometry of theCocos plate beneath the Caribbean plate in CentralAmerica; (2) theMichoacan region, where thedip angleof theCocos plate decreases gradually toward the southeast, (3) theGuerrero-Oaxac.a region,bounded approximately by theonshore projection of theOrozco and O'Gorman fracture zones,where the subducted slab is almostsubhorizontal and underplates the upper continental plate for about250 kin, and (4) the southernOaxaca and Chiapas region, in southeasternMexico, wherethe dip of the subductiongradually increases to a steepersubduction in CentralAmerica. Thesedrastic changes in dip do not appearto takeplace on tearfaults, suggesting that smooth contortionsaccommodate these changes in geometry.The inferred80 and 100 km depthcontours of the subducted slab lie beneath the southernfront of the Trans-Mexican Volcanic Belt, suggestingit is directlyrelated to the subduction.Thus the observed nonparallelism with the MiddleAmerican Trench is apparentlydue to thechanging geometry of theRivera and Cocos platesbeneath the North American plate in southernMexico, and not to zones of weakness in the crustof the North Americanplate as someauthors have suggested. Introduction takes place on a fold. They also suggestedthat to the southeast,the changein dip may be related to differencesin The seismicity and tectonics of southern Mexico are the age of oceanic lithosphere on either side of the characterizedby the subductionof the oceanicCocos and Tehuantepecridge. Riveraplates beneath the North Americanlithosphere along Until recently, the acceptedview was that the Cocos plate the Middle American Trench. Several studies have shown that subductsbeneath central Mexico at a constantand relatively this subductionzone exhibits lateral variations in the dip of shallowangle of ~12 to 15ø [Stoiberand Carr, 1973;Hanus the subductedoceanic lithosphere[e.g., Molnar and Sykes, and Vanek, 1978; Havskov et al., 1982; Burbach et al., 19.84; 1969; Stoiber and Carr, 1973; Dean and Drake, 1978; Nixon, Bevis and lsacks, !984; Singh and Mortera, 1991]. Using 1982; Beyis and lsacks, 1984; Burbach et aL, 1984; Ponce et data from a permanentlocal network, Sutirez et al. [1990] al., 1992]. A steep subductiongeometry in southeastern definedthe geometryof the subductedplate in more detail and Mexico [Ponce et al., 1992] and in the Jaliscoregion of showed that in central Mexico the slab does not subduct at a westernMexico [Pardoand Sudrez,1993] bound a slabdipping steadyshallow angle. Instead,after an initial dip of-15 ø, the apparentlyat a very shallowangle in centralMexico. slab becomesalmost subhorizontal and underplatesthe North On the basis of teleseismic data, Burbach et aI. [19134] American upper lithosphere. This plate geometry is concludedthat the subductedCocos plate can be dividedinto reminiscent of that found in central Peru and central Chile, thesethree major regions dipping at differentangles. Beyis where the Nazca plate also subductshorizontally beneath the andlsacks [1984], using a hypocentraltrend surface analysis, SouthAmerican upper lithosphere. findno evidenceof tearfaulting separating these segments of Perhapsone of the more striking tectonic features in thesubducted lithosphere •[nd" suggested that the transition southern Mexico is the location of the Trans-Mexican Volcanic Belt. The Trans-Mexican Volcanic Belt crosses southern Mexico from east to west at about I9øN. The distributionof the volcanicarc is oblique(-16 ø) relativeto the 1Nowat Departamento deGeoffsica, Universidad deChile, Santiago. Middle American Trench (Figure 1), in an unusal geometry which is not parallel to the subductionzone. The Trans- Copyright1995 by theAmerican Geophysical Union. MexicanVolcanic Belt is ~100 km wide andis cut by a Paperhumber 95JB00919. sequenceof grabensthat are obliquerelative to the general 0148-0227/95/95JB.,00919505.00 trend of the arc [Nixon, 1982]. ' 12,357 12,358 PARDO AND SUAREZ: SUBDUCTION OF THE RIVERA AND COCOS PLATES -115" - 110 ø - 105" - 1 O0 ø -95 ø -90 ø -85 ø -80 ø 25 25 ø NORTH AMERICA CARIBE 20" 20 ø PACIFIC 15' 15" -'OCOS.' 10' 10 ø -115' -110 ø -105 ø -100 ø -95 ø -90 ø -85 ø -80 ø Figure 1. General tectonic setting of the study area. Five lithosphericplates arc shown: Pacific, Rivera, Coco.,,, North A•nerica, and Caribbean. Relative convergence rates (cm/yr) between the {•ceanic and continental plates are indicated by open arrows, and absolute motions of the upper plates relative to a hot spot rra•ne of reference ,ire shown by shaded arrows. The abrcviations are EPR, East Pacific Rise; TFZ, Tamayo Fracture Zone; RFZ, Rivera FractureZone; OFZ, Orozco Fracture Zone; OGFZ, O'Gorman Fracture Zone; TR, TchuantcpecRidge; MAT, Middle AmericanTrench; JB, JaliscoBlock; CG, Colima Graben;EG, E1 Gordo Graben. Solid triangles indicate Quaternaryvolcanism, and the crossedarea indicates the Trans- Mexican Volcanic Belt (TMVB). The volcanic arc erupted since the Miocene and mostly Volcanic Belt is presentedhere. The availability o1'data from during the Plioccne and Quaternary. A diversity of structures permanentand temporary local networksand the occurrenceof such as large strato-volcanoes, monogenetic cineritic cones, recentearthquakes recorded at tcleseismicdistances which had shield volcanoes, and several calderas are found on the not been studied previously otter tin excellent opportunityto volcanic belt. The chemical and petrologicai characteristics understand this region ol' complex subduction geometry in .of the Trans-Mexican Volcanic Belt indicate that, in general, greater detail. the volcanic sequences that form this belt are c,lic-alkaline, The purposeof this study is to determinethe geometryof although some localized zones of alealine volcanism exist the subducted Cocos and Rivera plate, based on all the along the Trans-Mexican Volcanic Belt, especially in western available and reliable hypocenter locations of earthquakes Mexico [e.g., Luhr et al., 1985; Allan et al., 1991]. The recorded with local and teleseismic networks, and to studythe radiometric dating of volcanic rocks and the broad nature of regional distribution of stresseswith reported and newly the Trans-Mexican Volcanic Belt suggest the migration of determined focal mechanisms. Together, these data are volcanism toward the trench sihce 5.3 Ma [Delgado et al., analyzedto determinean integralmodel of the subductionin 1993]. southernMexico and its implicationson the seismicityand A number of contrasting hypotheseshave been proposedto tectonics of' the area. account for the nonparallelism of the Trans-Mexican Volcanic Belt relative to the Middle American Trench and its relation to Tectonic Setting the subductedoceanic plate. These hypothesescan be divided This studyincludes the subductionof the Rivera andCocos into two groups: (1) those which favor a direct association plates in southernMexico, west of the intersectionof the between subduction and volcanism [e.g., Molnar and Sykes, 1969; Demant and Robin, 1975; Nixon, 1982; Burbach et al., Tehuantepecridge with the Middle American Trench (Figure 1). Accordingto Mammerickxand Klitgord [1982], the tectonic 1984; Suc•rezand Singh, 1986], and (2) those which suggest that the Trans-Mexican Volcanic Belt bears no direct tectonic historyof this regionfor the last 25 m.y. hassuffered at least three major plate reorganizations.The old Farallonplate relation with the subduction along the Middle American evolved first to the Guadalupe plate, which was later Trench and explain its origin as resulting from zones of weakness within the crust of southern M6xico, inherited from segmentedinto the actual Rivera and Cocosplates. The positionof thespreading centers moved east, from about 12.5 earlier episodes of deformation [e.g., Mooser, 1972; Gastil to 11 Ma, to the Mathematician seamounts. Later, between and Jensky, 1973; Shurbet and Cebull, 1973; Johnson and 6.3 to 3.5 Ma, it shifted to its actual position of the East Harrison, 1989]. Pacific Rise (EPR). An integral study that takes into account the geometry of the subducted Rivera and Cocos plates beneath the North Rivera Plate American lithosphere (NW of 94øW), the stressdistribution in the subducted slab, and the relation between the subduction Theexistence of thesmall Rivera plate was first suggested process and the anomalous