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Serpentinite Emplacement and Deformation in Western Puerto Rico and Their Implications for the Caribbean-North America Plate Boundary Tectonic History

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Serpentinite Emplacement and Deformation in Western Puerto Rico and Their Implications for the Caribbean-North America Plate Boundary Tectonic History SERPENTINITE EMPLACEMENT AND DEFORMATION IN WESTERN PUERTO RICO AND THEIR IMPLICATIONS FOR THE CARIBBEAN-NORTH AMERICA PLATE BOUNDARY TECTONIC HISTORY by Daniel A. Laó-Dávila B.S., University of Puerto Rico, Mayagüez, 2000 M.S., Florida International University, 2002 Submitted to the Graduate Faculty of Arts and Sciences in partial fulfillment of the requirements for the degree of Doctor of Philosophy University of Pittsburgh 2008 UNIVERSITY OF PITTSBURGH FACULTY OF ARTS AND SCIENCES This dissertation was presented by Daniel A. Laó-Dávila It was defended on August 4 2008 and approved by Ian Skilling William Harbert Edward Lidiak Grenville Draper Dissertation Advisor: Thomas H. Anderson ii Copyright © by Daniel A. Laó-Dávila 2008 iii SERPENTINITE EMPLACEMENT AND DEFORMATION IN WESTERN PUERTO RICO AND THEIR IMPLICATIONS FOR THE CARIBBEAN-NORTH AMERICA PLATE BOUNDARY TECTONIC HISTORY Daniel A. Laó-Dávila, PhD University of Pittsburgh, 2008 Serpentinite emplacement in southwestern Puerto Rico indicates a complex plate boundary history between the Caribbean and North America plates. In this study we investigate the kinematics of shear planes within the serpentinite to improve constraints on the tectonic evolution of the region. Shear planes collected within the Monte del Estado and Río Guanajibo serpentinites reveal two predominant groups. One group comprises northwesterly-striking thrust faults and easterly-striking left-lateral faults. A second group comprises northwesterly-striking right-lateral faults and easterly-striking thrust faults. These shear zones reveal two shortening directions that trend NE-SW and N-S. The N-directed shortening is interpreted be older and subsequent stress reactivated the shear planes. The SW-directed shortening is attributed to transpression that caused contraction, uplift, and left-lateral shearing of serpentinite. A subsidiary younger group comprising fewer faults consists of northerly-directed thrusts and northwesterly-directed left-lateral faults and may be related to the last transpressional deformation within Puerto Rico. Thin-section observations show that porphyroclastic peridotite with pyroxenes that are kinked, show deformation lamellae, undulose extinction, and define foliation and lineation. Olivine shows a granuloblastic texture, and many crystals are strain-free and show polygonization indicating recrystallization. These textures indicate high temperature iv deformation that formed prior to serpentinization. Crystal-plastic deformation, recorded by serpentine mylonite and serpentine veins, was followed by brittle faulting. These structures demonstrate that the serpentinite was deformed and uplifted by tectonic stresses at the Caribbean-North America plate boundary zone and not by diapirism as a result of buoyancy differences within the crust. In southwestern Puerto Rico serpentinite emplacement has been described as first by collisional processes and second by diapirism. Structure mapping of the Monte del Estado and Río Guanajibo serpentinite bodies indicate that serpentinite was emplaced by thrusts verging towards the southwest in early Tertiary time. The thrust faults are mostly blind and produced fault-propagation folds in the overlying Late Cretaceous and Tertiary volcano-sedimentary cover. Low-angle thrusts are exposed in places at the southern contact of the Monte del Estado and Río Guanajibo serpentinite bodies and are interpreted to form part of the transpression that occurred in middle Tertiary at the boundary of the Caribbean-North America plates. v TABLE OF CONTENTS ACKNOWLEDGEMENTS........................................................................................................XXI DEDICATION.........................................................................................................................XXIII 1.0 INTRODUCTION..................................................................................................................1 1.1 CARIBBEAN TECTONICS..........................................................................................1 1.1.1 Serpentinites in the Northern Caribbean.............................................................5 1.1.2 Geology of Puerto Rico ......................................................................................7 1.2 OBJECTIVES OF THIS STUDY..................................................................................9 1.3 ULTRAMAFIC ROCKS, SERPENTINIZATION, AND EMPLACEMENT ............10 1.3.1 Peridotite...........................................................................................................10 1.3.2 Serpentinite .......................................................................................................16 1.4 ORGANIZATION OF THIS DISSERTATION..........................................................20 2.0 MICROSTRUCTURES AND TEXTURES OF ULTRAMAFIC ROCKS IN WESTERN PUERTO RICO .............................................................................................................................21 2.1 INTRODUCTION........................................................................................................21 2.2 GEOLOGY OF PUERTO RICO .................................................................................23 2.3 OVERVIEW OF TEXTURES AND MICROSTRUCTURES....................................24 2.3.1 Peridotite textures and microstructures.............................................................24 2.3.1.1 Crystallization .......................................................................................24 vi 2.3.1.2 Deformation textures.............................................................................24 2.3.2 Serpentinite textures and microstructures.........................................................25 2.3.2.1 Textures due to serpentinization............................................................25 2.3.2.2 Deformation textures.............................................................................27 2.3.3 Micro-scale studies of serpentinite in Puerto Rico ...........................................27 2.4 METHODS ..................................................................................................................28 2.5 RESULTS ....................................................................................................................30 2.5.1 Serpentinized Peridotite....................................................................................30 2.5.2 Sheared Serpentinite .........................................................................................35 2.5.3 Sedimentary rocks composed of serpentinite and other lithologies .................36 2.6 DISCUSSION ..............................................................................................................40 2.7 CONCLUSIONS..........................................................................................................44 3.0 KINEMATIC ANALYSIS OF SERPENTINITE STRUCTURES AND THE MANIFESTATION OF TRANSPRESSION IN SOUTHWESTERN PUERTO RICO ..............46 3.1 INTRODUCTION........................................................................................................46 3.2 REGIONAL SETTING................................................................................................48 3.2.1 Puerto Rico .......................................................................................................48 3.2.2 Serpentinite .......................................................................................................53 3.3 METHODS ..................................................................................................................56 3.4 RESULTS ....................................................................................................................57 3.4.1 Serpentinite Textures ........................................................................................57 3.4.2 Shear Zones.......................................................................................................62 3.4.3 Thrust Faulting..................................................................................................62 vii 3.4.4 Left-Lateral Faults ............................................................................................71 3.4.5 Right-lateral faults ............................................................................................71 3.4.6 Normal Faults ...................................................................................................72 3.4.7 Kinematic Significance of Shear Zones............................................................72 3.4.8 Age of structures...............................................................................................76 3.5 DISCUSSION ..............................................................................................................77 3.6 CONCLUSIONS..........................................................................................................82 4.0 PALEOGENE THRUST EMPLACEMENT OF SERPENTINITE: IMPLICATIONS FOR THE TECTONIC HISTORY OF SOUTHWESTERN PUERTO RICO......................................84 4.1 INTRODUCTION........................................................................................................84 4.2 REGIONAL SETTING................................................................................................88 4.2.1 Puerto Rico .......................................................................................................88 4.2.2 Southwest Block
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