Forearc response to subduction of the Cocos Ridge, Panama-Costa Rica JEFF CORRIGAN Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78713 PAUL MANN Institute for Geophysics, University of Texas at Austin, 8701 Mopac Boulevard, Austin, Texas 78759 JAMES C. INGLE, JR. Department of Geology, Stanford University, Stanford, California 94305 ABSTRACT Stratigraphic, paleontologie, and structural data from two Panama-Costa Rica forearc peninsulas located landward of the aseismic Cocos Ridge along the southernmost Middle America Trench document rapid Pliocene subsidence and basin infilling followed by Quaternary deformation and uplift. On the Burica Peninsula, an approximately 3,000-m- thick, Pliocene-Pleistocene clastic sedimen- tary sequence, characterized by fine-grained turbidite deposits and volumetrically minor coarser-grained channel-fill, slump, and debris-flow deposits, bears depth-diagnostic foraminifera that document rapid Pliocene subsidence and deposition on a south-dipping paleoslope. New paleobathymetric and age estimates from foraminifera in these rocks in- dicate shallowing of depositional paleodepths from 2,000+ to 1,200 m during the late Plio- cene. Present-day exposure of these rocks in- dicates an average Quaternary uplift rate of ~ 1 mm/yr. The turbidite section on the Bur- ica Peninsula is interpreted to represent a trench-slope deposit and is correlated to a similar, marine sedimentary section on the ad- jacent Osa Peninsula of Costa Rica. Structures developed within Pliocene-Pleis- tocene strata of the outer-forearc Burica and Osa Peninsulas record minor (<5%) subhori- zontal, margin-perpendicular shortening and margin-perpendicular extension. Bedding ori- entations on the Osa and Burica Peninsulas generally record arcward tilting by 10°-30° of these outer-forearc blocks. On the Bur- ica Peninsula, the Medial fault zone, a north- striking, high-angle, right-lateral strike-slip (?) fault, has truncated and reoriented the Pliocene-Pleistocene sedimentary sequence by fault-bend (forced) folding. A linear, margin-parallel fault, herein named the "Ballena-Celmira fault zone," bounds the arcward side of the Osa and Burica Peninsu- las and has accommodated vertical displace- ments between the outer forearc (Burica-Osa area) and inner forearc (Terraba area) throughout the Tertiary. Structural and seis- micity observations from the forearc region suggest that Neogene deformation of the lead- ing edge of the arc in this area reflects the effects of isostatic uplift, and possibly in- creased interplate shear stresses, as a result of subduction of the Cocos Ridge beneath this part of the arc since 1 m.y. ago. INTRODUCTION Studies of magnetic anomalies in the eastern  Pacific suggest that the aseismic Cocos Ridge, a  200- to 300-km-wide hot-spot trace that stands  2.0 to 2.5 km shallower than the surrounding  sea floor, encountered the Middle America  Trench (MAT) approximately 1 m.y. ago  (Lonsdale and Klitgord, 1978; Fig. 1). Local var- iation in MAT morphology, forearc structure,  arc volcanism, and seismicity has been attrib- uted by previous workers to the subduction of  the Cocos Ridge (Table 1).  The purpose of this paper is to document the  style and magnitude of forearc deformation  landward of the Cocos Ridge, based on strati- graphic and structural mapping in Neogene sed- imentary rocks in western Panama and Costa  Rica and associated biostratigraphic and paleo- bathymetric studies. We present new structural  and stratigraphic data from the Burica and Osa  Peninsulas and review the structure and stratig- raphy of the surrounding forearc region land- ward of the MAT-Cocos Ridge intersection  (Fig. 1). New data presented in this paper rep- resent the results of 4 months of structural and  stratigraphic mapping of the Burica Peninsula of  Panama-Costa Rica at a scale of 1:25,000; 3  days of reconnaissance field work on the Osa  Peninsula, Costa Rica; and foraminiferal analy- sis of samples collected by University of Texas at  Austin and Stanford University field parties on  the Burica Peninsula. Analyses of sedimentary  facies, paleoflow indicators, sandstone petrog- raphy, and foraminiferal paleobathymetric data  from the Burica and Osa Peninsulas are used to  constrain environments of deposition and the  subsidence/uplift history of this part of the  Panama-Costa Rica forearc. Observations of  local and regional structures affecting upper  Neogene sedimentary rocks, in conjunction  with previously published earthquake seismicity  studies, allow us to better characterize forearc  deformation associated with Cocos Ridge  subduction.  The Universal Transverse Mercator (UTM)  grid location system is used throughout this  paper to locate outcrops mentioned in the text.  The study area is within the southeastern 17  PKV and northeastern 17PKU 100,000 m grid  squares of Inter-America Geodetic Survey and  Army Map Service 1:50,000 topographic sheets  3640 III and 3640 IV (series 762, edition 2- IGNTG, 1984). For brevity, we use only the last  six digits of the grid reference (G.R.) to specify  localities. These topographic sheets are available  through Instituto Geográfico "Tommy Guardia"  in Panama City, Panama.  TABLE 1. PROPOSED EFFECTS OF COCOS RIDGE  SUBDUCTION ON THE MIDDLE AMERICA ARC, FOREARC,  AND TRENCH OF COSTA RICA AND WESTERN PANAMA  Reference  1. Gap in active volcanic chain of  Costa Rica  2. Extremely rapid Pleistocene uplift  of forearc area of Costa Rica  (25 mm/yr)  3. Folding and tilting of forearc  sedimentary rocks of Costa Rica  4. Increased seismic coupling between  forearc and underthrusting Cocos plate  results in more frequent, large, thrust- type earthquakes  5. Anomalously shallow Benioff Zone  beneath Costa Rica  6. Decrease in subduction zone seismicity  beneath Costa Rica  McGeary and others (1985)  Alt and others (1980)  Hey wood and Silver (1983)  Adamek and othere (1987)  Burbach and others (1984)  Vogt and others (1976)  Geological Society of America Bulletin, v. 102, p. 628-652, 17 figs., 2 tables, May 1990. 628