Martin Meschede

A method of discriminating between different types of mid-ocean ridge basalts and continental tholeiites with the Nb1bZr1bY diagram

Abstract Analyses of more than 1800 basaltic rocks from different geotectonic settings define a new discrimination diagram using the immobile trace elements Nb, Zr and Y. The diagram distinguishes between different types of mid-ocean ridge basalts (“MORB”, including “N-type MORB”, from normal mid-ocean ridges and “P-type MORB”, from plume-influenced regions), within-plate tholeiites (WPT), and within-plate alkali basalts (WPA). Volcanic arc basalts (VAB) plot within both WPT and N-type MORB fields and thus cannot be distinguished from these types. Ancient continental tholeiites which plot in the conventional Ti1bZr1bY diagram within the MORB/VAB field are clearly distinguished from N-type MORB.

A PLATE-TECTONIC MODEL FOR THE MESOZOIC AND EARLY CENOZOIC HISTORY OF THE CARIBBEAN PLATE

Abstract We present a model in which the Caribbean plate is an intra-American feature formed along the Caribbean spreading center as opposed to the current model that considers the Caribbean plate as a far-travelled crustal segment that formed in the Pacific region. Paleomagnetic data, which cover an age range from Jurassic through Paleocene, indicate the ophiolite complexes in Costa Rica and Panama formed in an equatorial position. The ophiolites did not fundamentally change their position relative to South America since their origin. Basaltic rocks of the lower part of the ophiolites are of mid-ocean ridge type suggesting formation at a spreading-center. They are interpreted to have formed as part of the proto-Caribbean crust at a spreading axis in an intra-American position during Jurassic and Early Cretaceous times. The upper part of the ophiolites is mainly built up by island arc and intraplate basalts. The island arc basalts evolved at the Central American landbridge which started in the Middle Cretaceous. The intraplate basalts are related to the Caribbean plateau basalt which thickened and stiffened the Caribbean crust in Middle Cretaceous to probably Campanian times. Changes in relative plate motions along the Middle American convergent plate margin are reflected by orientations and time sequences of paleostress tensors calculated from fault-slip data in southern Mexico and Costa Rica. Kinematic indicators from mylonite zones display sinistral movement along secondary shear zones within the southern Mexican crustal complexes and along the northern boundary of the Chortis block in the Motagua–Polochic fault system. This reflects the eastward drift of the Chortis block since the beginning of the Cenozoic. At this time the Chortis block became part of the Caribbean plate which moved more than 1000 km towards the east relative to the North and South American plates during the Cenozoic.

Tectonic erosion and consequent collapse of the Pacific margin of Costa Rica: Combined implications from ODP Leg 170, seismic offshore data, and regional geology of the Nicoya Peninsula

The convergent margin off the Pacific coast of the Nicoya Peninsula of Costa Rica exhibits evidence for subduction erosion caused by the underthrusting Cocos plate. Critical evidence for efficacy of this process was recovered at the Ocean Drilling Program (ODP) drilling Site 1042 (Leg 170), positioned ∼7 km landward of the Middle America trench axis off the Nicoya Peninsula. The primary drilling objective at this site was to identify the age and origin of a regionally extensive and prominent seismic discontinuity, the so-called base-of-slope sediment (BOSS) horizon or surface. The BOSS horizon, which can be traced landward from near the trench to the Nicoya coastal area and parallel to it for hundreds of kilometers, separates a low-velocity (∼ 2.0–2.5 km s−1) sequence of slope sediment, from an underlying sequence of much higher-velocity (>4–4.5 km s−1) rock. Site 1042 reached the acoustically defined BOSS horizon at a below sea level depth of ∼ 3900 m and yielded a carbonate-cemented calcarenitic breccia of early-middle Miocene age. Sedimentological, geochemical, paleontological, and cement paragenesis data document that the breccia accumulated in a shallow water depositional environment. On the basis of coastal exposures, the BOSS horizon, as a margin-wide geologic interface, can be temporally and lithostratigraphically correlated to a regional angular unconformity. This unconformity, known as the Mal Pais unconformity, separates Neogene and younger shelf-to-littoral beds from the underlying mafic units of the Mesozoic Nicoya Complex and Cretaceous and early Tertiary sedimentary sequences. At Site 1042 it is inferred that tectonism caused the vertical subsidence of the early Neogene breccia from a shallow to a deep water setting. The Mal Pais unconformity of the BOSS horizon thus connects the rock fabric of the outermost part of margin to that of coastal Nicoya and implies that in the early Neogene the Nicoya shelf extended seaward to near the present trench axis. This circumstance requires that the early Neogene trench axis was at least 50 km seaward of where it is now located. The long-term effects of subduction erosion, similar to those described for the scientifically drilled Japan, Tonga, and Peru margins, best account for offshore and onshore evidence for a post-Paleogene history of crustal thinning and landward trench migration of Costa Ricas Pacific margin. During the past 16–17 Myr the calculated mass removal and landward migration rates are 34–36 km³ Myr−1 km−1 of margin, and 3 km Myr−1, respectively. These values are similar to those found for other Pacific margins dominated by nonaccretionary subduction zone processes.

Origin of the Central American ophiolites: Evidence from paleomagnetic results

The ophiolite complexes exposed along the Pacific coast of Costa Rica and western Panama evolved from Jurassic ocean floor to the island arc of the Tertiary to the present. We attribute Late Cretaceous volcanism to the Caribbean sill event that thickened the oceanic crust to form an oceanic plateau. Tectonostratigraphic units formed by Late Cretaceous nappe thrusting and block rotation. Rotated blocks, as revealed by paleomagnetic data, are covered by neoautochthonous sedimentary and volcanic rocks. Collision with the Cocos Ridge in the late Tertiary fragmented and rotated the ophiolite complexes south of the Costa Rica Fracture Zone. The oceanic basement formed in the Jurassic Period in an equatorial position. Its latitudinal drift path as deduced from paleomagnetic inclination data matches that of the South American continent and differs from that of the Farallon/Phoenix plates. We favor formation of the ophiolite complexes in a Caribbean (inter-American) position as opposed to a distant origin in the Pacific region.

Geodynamic evolution of southern Costa Rica related to low-angle subduction of the Cocos Ridge: constraints from thermochronology

Abstract The Late Tertiary shallow subduction of the Cocos ridge under the Caribbean plate controlled the evolution of the Cordillera de Talamanca in southeast Costa Rica, which is a mountain range that consists mainly of granitoids formed in a volcanic arc setting. Fission track thermochronology using zircon and apatite, as well as 40Ar–39Ar and Rb–Sr age data of amphibole and biotite in granitoid rocks constrain the thermal history of the Cordillera de Talamanca and the age of onset of subduction of the Cocos ridge. Shallow intrusion of granitoid melts resulted in fast and isobaric cooling. A weighted mean zircon fission track age (13 analyses) and Rb–Sr biotite ages of about 10 Ma suggest rapid cooling and give minimum ages for granitoid emplacement. In some cases 40Ar–39Ar and Rb–Sr apparent ages of amphibole and biotite are younger than the zircon fission track ages, which can be attributed to partial resetting by hydrothermal alteration. Apatite fission track ages range from 4.8 to 1.7 Ma but show no correlation with the 3090-m elevation span over which they were sampled. The apatite ages seem to indicate rapid exhumation caused by tectonic and isostatic processes. The combination of the apatite fission track ages with subduction parameters of the Cocos plate such as subduction angle, plate convergence rate and distance of the Cordillera de Talamanca to the trench implies that the Cocos ridge entered the Middle America Trench between 5.5 and 3.5 Ma.

The North American-Caribbean Plate boundary in Mexico-Guatemala-Honduras

Abstract New structural, geochronological, and petrological data highlight which crustal sections of the North American–Caribbean Plate boundary in Guatemala and Honduras accommodated the large-scale sinistral offset. We develop the chronological and kinematic framework for these interactions and test for Palaeozoic to Recent geological correlations among the Maya Block, the Chortís Block, and the terranes of southern Mexico and the northern Caribbean. Our principal findings relate to how the North American–Caribbean Plate boundary partitioned deformation; whereas the southern Maya Block and the southern Chortís Block record the Late Cretaceous–Early Cenozoic collision and eastward sinistral translation of the Greater Antilles arc, the northern Chortís Block preserves evidence for northward stepping of the plate boundary with the translation of this block to its present position since the Late Eocene. Collision and translation are recorded in the ophiolite and subduction–accretion complex (North El Tambor complex), the continental margin (Rabinal and Chuacús complexes), and the Laramide foreland fold–thrust belt of the Maya Block as well as the overriding Greater Antilles arc complex. The Las Ovejas complex of the northern Chortís Block contains a significant part of the history of the eastward migration of the Chortís Block; it constitutes the southern part of the arc that facilitated the breakaway of the Chortís Block from the Xolapa complex of southern Mexico. While the Late Cretaceous collision is spectacularly sinistral transpressional, the Eocene–Recent translation of the Chortís Block is by sinistral wrenching with transtensional and transpressional episodes. Our reconstruction of the Late Mesozoic–Cenozoic evolution of the North American–Caribbean Plate boundary identified Proterozoic to Mesozoic connections among the southern Maya Block, the Chortís Block, and the terranes of southern Mexico: (i) in the Early–Middle Palaeozoic, the Acatlán complex of the southern Mexican Mixteca terrane, the Rabinal complex of the southern Maya Block, the Chuacús complex, and the Chortís Block were part of the Taconic–Acadian orogen along the northern margin of South America; (ii) after final amalgamation of Pangaea, an arc developed along its western margin, causing magmatism and regional amphibolite–facies metamorphism in southern Mexico, the Maya Block (including Rabinal complex), the Chuacús complex and the Chortís Block. The separation of North and South America also rifted the Chortís Block from southern Mexico. Rifting ultimately resulted in the formation of the Late Jurassic–Early Cretaceous oceanic crust of the South El Tambor complex; rifting and spreading terminated before the Hauterivian (c. 135 Ma). Remnants of the southwestern Mexican Guerrero complex, which also rifted from southern Mexico, remain in the Chortís Block (Sanarate complex); these complexes share Jurassic metamorphism. The South El Tambor subduction–accretion complex was emplaced onto the Chortís Block probably in the late Early Cretaceous and the Chortís Block collided with southern Mexico. Related arc magmatism and high-T/low-P metamorphism (Taxco–Viejo–Xolapa arc) of the Mixteca terrane spans all of southern Mexico. The Chortís Block shows continuous Early Cretaceous–Recent arc magmatism.

Second look at suspect terranes in southern Mexico

The boundary between the Xolapa and the Guerrero, Mixteca, and Juarez (or Oaxaca) terranes is a zone of normal faulting indicating north-south subhorizontal extension. Stratigraphic and geochronometric evidence dates tectonic uplift of the Xolapa terrane as Late Cretaceous and Tertiary. We propose that the Xolapa terrane represents a late Mesozoic-early Tertiary magmatic arc built near or on North American continental crust, and we discuss, as possible tectonic uplift mechanisms, (1) extension associated with back-arc rifting, (2) extension during gravitational spreading of the upper and middle crust, and (3) transtension within a strike-slip regime established during formation of the Caribbean. Both far- and near-field deformations indicate distributed transtension. Therefore, a single regional tectonic framework can account for the Mesozoic and Cenozoic geologic history of these terranes.

STRESS TRANSMISSION ACROSS AN ACTIVE PLATE BOUNDARY : AN EXAMPLE FROM SOUTHERN MEXICO

Abstract Paleostress tensors calculated from inversion of fault-slip data at 60 stations between Zihuatanejo and Tehuantepec in southern Mexico were separated into four distinct groups and assigned to time intervals between 150 Ma and Recent. The change in orientation of the maximum horizontal stress closely traces the change in direction of convergence between the North American and the Farallon/Cocos plates for the same time interval. Depending on the angle between the convergence direction and the plate margin, both thrust fault and strike-slip stress regimes were active. Based on this comparison, we infer that the stress distribution in the continental crust of southern Mexico has been controlled by interplate stress transmission. Latest Cretaceous-Early Tertiary mylonites at the northern edge of the Xolapa complex portray left-lateral oblique crustal extension and oceanward migration of the upper plate. The orientation of extension at mid-crustal level grossly correspond to the orientation of the minimum compressive stress at upper crustal level. During the last 20 Ma a tensional stress regime as a response to surface uplift has been dominant in southern Mexico.

The Pieniny Klippen Belt in the Western Carpathians of northeastern Slovakia: Structural evidence for transpression

Abstract The Pieniny Klippen Belt represents a 600-km-long but only a few kilometers wide suture zone in the Carpathian orogenic belt. Based on a quantitative analysis along a part of its NW-trending segment in northeastern Slovakia, we present structural data supporting transpression, the continuous interaction of strike-slip shearing, horizontal shortening, and vertical lengthening, as a major deformation style in its polyphase deformation history. Dextral transpression is expressed in the map scale and outcrop fault pattern, the oblique orientation of fold axes to the faults bounding the Klippen Belt, and extension parallel to the fold axes. The transpression-related strain field is described and quantified by the analysis of: (1) orientation of rotated fold axes (displaying an acute angle to the margins of the Klippen Belt); (2) orientation and geometry of paleostress derived from mesoscale fault-striae analysis (E-trend of σ3-trajectories and flattening geometry); and (3) the deformation history indicated by extension veins (non-coaxial regime). Different techniques using fault-striae data quantify paleostress and subdivide heterogeneous data sets mathematically into homogeneous subsets. The observed deformation history is modelled as a homogeneous transpression deformation. The best-fitting model requires a NW-trending (present-day orientation) external contraction direction (e.g., plate-slip vector), and predicts 16% fold axes parallel extension and 23% axial plane normal shortening.

Structure, inferred mechanical properties, and implications for fluid transport in the décollement zone, Costa Rica convergent margin

Faults in a variety of tectonic settings can act as both conduits for and barriers to fluid flow, sometimes simultaneously. Documenting the interaction between hydrologic and tectonic processes in active faults in situ is the key to understanding their mechanical behavior and large-scale fluid transport properties. We present observations of the plate boundary decollement zone at the Middle America Trench off Costa Rica, showing that it is structurally divisible into an upper brittle-fracture–dominated domain overlying a lower, ductile domain. Pore-water geochemical evidence shows that along-fault flow is occurring specifically in the upper brittle domain, but is hydrologically isolated from fluids in the underlying footwall sediments. We propose a model for the mechanics of these contrasting domains in which differing stress paths coexist in the upper and lower parts of the decollement zone. The data suggest a mechanically controlled permeability anisotropy at a scale of several meters to ∼10 m across the decollement zone. This documentation of separate yet simultaneously active mechanical and hydrologic subregimes within a decollement provides a relatively simple explanation for enhanced along-fault permeability coexisting with reduced cross-fault permeability, without requiring matrix-scale permeability anisotropy.