Walter V. Maresch

Plate-kinematics and crustal dynamics of circum-Caribbean arc-continent interactions: Tectonic controls on basin development in Proto-Caribbean margins

The American margins of the Caribbean comprise basins and accreted terranes recording a polyphase tectonic history. Plate kinematic models and reconstructions back to the Jurassic show that Mesozoic separation of the Americas produced passive margins that were overridden diachronously from west to east by allochthonous Caribbean plate–related arc and oceanic complexes. P-T-t and structural data, sedimentary provenance, and basin-subsidence studies constrain this history. Caribbean lithosphere is Pacifi c-derived and was engulfed between the Americas during their westward drift as the Atlantic Ocean opened. This began ca. 120 Ma with development of a west-dipping Benioff zone between Central America and the northern Andes, now marked by the Guatemalan and Cuban sutures in North America and by the northern Colombian and Venezuelan “sutures” of South America, persisting today as the Lesser Antilles subduction zone. Most Caribbean high-pressure metamorphic complexes originated at this subduction zone, which probably formed by arc-polarity reversal at an earlier west-facing Inter-American Arc and was probably caused by westward acceleration of the Americas. The mainly 90 Ma Caribbean basalts were extruded onto preexisting Caribbean crust ~30 m.y. later and are not causally linked to the reversal. The Great Caribbean Arc originated at this trench and evolved up to the present, acquiring the shape of the preexisting ProtoCaribbean Seaway. The uplift and cooling history of arc and forearc terranes, and history of basin opening and subsidence, can be tied to stages of Caribbean plate motion in a coherent, internally-consistent regional model that provides the basis for further studies.

Foundations of Gulf of Mexico and Caribbean evolution: eight controversies resolved

Eight points of recurring controversy regarding the primary foundations of models of Gulf of Mexico and Caribbean tectonic evolution are identified and examined. The eight points are controversial mainly because of the disconnect between different scales of thinking by different workers, a common but unfortunate problem in the geological profession. Large-scale thinkers often are unaware of local geological detail, and local-scale workers fail to appreciate the level of evolutionary precision and constraint provided by regional tectonics and plate kinematics. The eight controversies are: (1) the degree of freedom in the Gulf-Caribbean kinematic framework that is allowed by Atlantic opening parameters; (2) the existence of a South Bahamas-Guyana Transform, and the role of this structure in Cuban, Bahamian, Trinidadian, and Guyanese evolution; (3) the anticlockwise rotation of the Yucatan Block during the opening of the Gulf of Mexico; (4) the Pacific origin of the Caribbean oceanic crust; (5) the Aptian age and plate boundary geometry of the onset of west-dipping subduction of Proto-Caribbean beneath Caribbean lithospheres; (6) the origin and causal mechanism of the Caribbean Large Igneous Province…not Galapagos!; (7) the number and origin of magmatic arcs in the northern Caribbean; and (8) the origin of Paleogene “flysch” deposits along northern South America: the Proto-Caribbean subduction zone. Here we show that there are viable marriages between the larger and finer scale data sets that define working and testable elements of the region’s evolution. In our opinion, these marriages are geologically accurate and suggest that they should form discrete elements that can and be integrated into regional models of Gulf and Caribbean evolution. We also call upon different facets of the geological community to collaborate and integrate diverse data sets more openly, in the hopes of improving general understanding and limiting the publication of unnecessary papers which only serve to spread geological uncertainty.

Experimental studies on glaucophane: An analysis of present knowledge

Abstract A review of the evolution of experimental work on synthetic, end-member, iron-free glaucophane, Na2Mg3Al2SigO22(OH)2, leads to the conclusion that it is unlikely that an amphibole of this composition has yet been synthesized. Although some amphiboles obtained in these studies at synthesis pressures of 20–30 kbar PH2O may approach end-member glaucophane reasonably closely, the exact compositions as well as the geologically relevant stability limits of these synthetic products are still only poorly defined. Moreover, the concept of polymorphism cannot be upheld, because “glaucophane I” and “glaucophane II” differ fundamentally in composition. It is therefore not possible to extract data meaningful to natural glaucophane stability from these experimental studies. To shed new light on this problem, stability data for a relatively Fe-poor glaucophane from Ile de Groix, France, are presented with which a maximum PH2O—T stability field for this amphibole may be mapped out. This particular glaucophane requires a minimum of 4 kbar PH2O and is probably not stable above 550°C. Between 350 and 550°C, the minimum pressure required rises from 4 to approximately 10 kbar. These maximum possible stability limits are essentially compatible with the high-pressure, low-temperature conditions deduced for glaucophane from metamorphic terranes.

Numerical modelling of PT-paths related to rapid exhumation of high-pressure rocks from the crustal root in the Variscan Erzgebirge Dome (Saxony/Germany)

Abstract The Bohemian Massif in the Central European Variscides contains many crustal slices with (ultra-)high-pressure rocks related to continent-continent collision. After closure of pre-existing oceans during the Devonian, excess crustal thickness was maintained for about 50 Ma until at around 340 Ma large volumes of high-pressure rocks from the crustal root were exhumed within a few million years. We relate this event to delamination and complete detachment of the lithospheric mantle, causing a crustal-scale isostatic instability. In the Erzgebirge dome, a model region in the northern part of the massif, an array of interrelated PTtd-paths with “decompression/cooling” and “decompression/heating” trajectories in juxtaposed tectonometamorphic units has been established. Numerical 2D-experiments using a rheologically, thermally and dynamically consistent convection technique show three stages of the crustal evolution related to delamination and detachment of mantle lithosphere under the crustal root: (1) During delamination a rapid overthickening of the crust can occur with the crust penetrating down to >160 km depth. (2) After detachment extensional crustal thinning controlling exhumation occurs with escape of rocks from the crustal root towards the margins of the orogen through tectonically weak zones. Horizontal displacement exceeds vertical by a factor of ∼3. (3) Forced circulation in the weak zones follows and upward flow of lower crustal rocks is compensated by subduction of upper crustal rocks in the footwall of these zones. One-dimensional modelling was used in order to further understand basic processes and to simulate the rock record in detail. According to 2D and 1D modelling, strongly decelerating exhumation rates with decreasing overburden and a late increase in the geothermal gradient due to upward heat transfer are necessary corollaries of this scenario, in keeping with observations from the Erzgebirge dome. Exhumation PT-paths do not conform to one single uniform exhumation trajectory; rather, assemblages of interrelated PTtd-paths are characteristic.

Plate Tectonics Origin of the Caribbean Mountain System of Northern South America: Discussion and Proposal

A brief summary of the petrotectonic assemblages involved in the Cretaceous to early Tertiary orogeny of the Caribbean Mountain System is presented in order to delineate those geological constraints pertinent to a plate tectonic interpretation of the area. In particular, it is concluded that the Mesozoic sediments caught up in the orogeny were originally deposited along a quiescent Atlantic-type continental margin or “tensile open series geosyncline” (Wang, 1972) and not in association with an active subduction zone. Moreover, paired metamorphic belts are not found, and all metamorphic rocks, whose grade increases from south to north across the orogenic belt, are of medium- to high-pressure type. Thus a collision-type model appears to be more appropriate as the major mountain-building mechanism than the cordilleran-type mechanism proposed for the Caribbean Mountain System by various authors. An alternate tectonic model which incorporates the following main stages in tectonic development is suggested: (1) formation of an Atlantic-type continental margin along northern South America by continental rifting in middle Mesozoic time or earlier; (2) movement of an oceanic plate with a leading edge characterized by a subduction-zone–island-arc complex toward this continental margin; (3) collision in Late Cretaceous time; (4) overriding of the island arc over the continental margin, resulting in widespread gravity sliding and thrusting, and high p/T metamorphism of the shelf and rise sediments; and (5) isostatic readjustment due to reorganized plate movement in the early Tertiary. Ephemeral subduction zones active during this sequence of events explain the singular pattern of sporadic volcanism observed during this time span. The direct applicability of this model to areas outside of the region of interest is limited. Necessary restrictions on timing and plate movement in the Caribbean area, however, suggest that a probable feature of the most likely model for the tectonic evolution of this area will be the insertion of an oceanic plate from the Pacific between North and South America in late Mesozoic time.

Interrelated P-T-t-d Paths in the Variscan Erzgebirge Dome (Saxony, Germany): Constraints on the Rapid Exhumation of High-Pressure Rocks from the Root Zone of a Collisional Orogen

The Erzgebirge dome consists of several superimposed composite tectonometamorphic units of medium- to high-grade metamorphic rocks from different crustal depths. These exhibit high pressure-high temperature and even ultrahigh-pressure imprints inherited from the root zone of a Variscan orogen and were exhumed almost immediately after attainment of maximum pressures at ∼341 Ma. At present, the entire stack of tectonometamorphic units lies underneath an upper-crustal sequence of Paleozoic metasediments and tectonic slivers of pre-Carboniferous metamorphic rocks. Shear zones active at different times and at different depths are preserved, mainly recording two successive stages of the exhumation history between 340 and 330 Ma. Tectonic transport during exhumation was remarkably constant in an E-W direction, swinging to NW-SE in the eastern part of the Erzgebirge parallel to a ductile transtensional zone (Elbe zone) that was concomitantly active. The various tectonometamorphic units have characteristically correlated, convergent P-T-t-d paths (both “cooling during decompression” and “heating during decompression”) that can be deduced from the dominant quartzofeldspathic rocks. These paths indicate successive exhumation of hotter rocks from increasingly deeper structural positions and juxtaposition against cooler rocks in higher positions, concomitant with the excision of intermediate crustal levels. We interpret this type of successive vertical telescoping of the metamorphic profile to be the result of extension of the thickened tectonometamorphic stack. Extensional unroofing in the middle and upper crust was contemporaneous with and outlasted underthrusting and hence prograde metamorphism and deformation at deeper levels of the tectonometamorphic pile. Underthrusting is documented by a major inversion of the maximum pressure conditions in the lowermost units. However, structures related to compressional stacking now generally occur only as relics transposed by extensional deformation at lower pressure, or are restricted to rare small slivers with preserved prograde structures. Sedimentation of Lower Dinantian turbidites occurred along the flanks of the Erzgebirge dome during the exhumation process. The extrusion of high-pressure rocks is interpreted to have been driven mainly by a major regional buoyancy instability caused by the delamination of the lithospheric mantle underneath the neighboring Bohemian Massif, which represented overthickened crust at least from the Devonian to the early Visean. Major controlling factors were boundary forces exerted by the thickened crustal bulge on the neighboring thin crustal segments in the north and east, effecting lateral extension of this orogenic wedge and extrusion-i.e., convective upward flow of gravitationally unstable crustal material.

New occurrences of jadeitite, jadeite quartzite and jadeite-lawsonite quartzite in the Dominican Republic, Hispaniola: petrological and geochronological overview

New occurrences of jadeitite and jadeite-rich rocks have been discovered in the Rio San Juan Complex (RSJC) of the northern Dominican Republic in serpentinite melanges associated with a former intra-oceanic subduction zone. Allochthonous blocks in lag deposits developed on the melange outcrops or boulders in river beds are common. A very unusual feature for the RSJC is the occurrence of concordant layers and discordant veins of cm to dm thickness in blocks of jadeite±lawsonite- or omphacite-garnet-bearing blueschist of the melange. Two suites of jadeite-rich rocks can be recognized. The first is represented by quartz-free jadeitite s.str . (>90 vol% jadeite) found so far only as blocks and boulders. The second suite comprises quartz-bearing jadeitite s.str . grading into jadeitite quartzite (JQ), jadeite-lawsonite quartzite (JLQ) and jadeite-free lawsonite quartzite (LQ). The second suite is found both as blocks and boulders as well as layers and veins in blueschist blocks. One single occurrence of a cross-cutting omphacitite vein in blueschist has also been observed. Additional important phases so far found in both suites are omphacite, phengite, glaucophane, epidote, albite, calcite, titanite and zircon. Apatite and pumpellyite have only been identified in quartz-free jadeitite s.str .; almandine-rich garnet has so far been observed only in JLQ. The two suites of jadeite-bearing rocks occur in various shades of green, are fine- to coarse-grained, and usually equigranular. Mineral distribution is commonly homogeneous, but may be patchy in JLQ, giving this rock type a distinctly mottled appearance. Cathodoluminescence (CL) images show oscillatory zoning patterns in jadeite, zircon, apatite and calcite; this is evidence for crystallization from an aqueous fluid under open-system conditions. Zircons separated from a sample of quartz-free jadeitite s.str . contain primary inclusions of high-pressure matrix minerals such as jadeite and omphacite, indicating coeval zircon growth. The cores of the zircons yield ages of 114.9 ± 2.9 Ma, thus defining a crystallization age close to the initiation of subduction in the Rio San Juan Complex, when “warm” geotherms of ≈15°/km prevailed. These ages are in contrast with the crystallization ages of the blueschists hosting the second, quartz-bearing suite of jadeite-rich rocks. These range from 80 to 62 Ma, towards the end of subduction-zone activity at ≈55 Ma and “cool” geotherms of 8–9 °C/km. For the younger quartz-bearing suite, the combination of phengite compositions with the available P-T-t paths of the host blueschists suggests crystallization temperatures of ≈ 350 to ≈ 500 °C at minimum pressures of 15–16 kbar. The P-T conditions for the older quartz-free suite are more difficult to constrain, but the combination of phengite compositions with the prevailing geotherms in the young and warm subduction zone suggest minimum conditions of at least 500 °C and 11 kbar. However, temperatures and pressures as high as 600 °C and 15 kbar, as documented for jadeitites of similar age in the same subduction zone exposed in neighbouring eastern Cuba, are possible. Jadeitites and jadeite-rich rocks of the RSJC are thus interpreted to have crystallized over a time-span of ≥ 60 Myr at initial temperatures of at least 500 °C, later evolving down to 350 °C in a single, thermally self-organizing, cooling subduction zone. The P-T conditions suggested for the younger quartz-bearing suite correlate well with those of jadeitite formation in Guatemala south of the Motagua Fault Zone, the only other occurrence world-wide where jadeitite with both lawsonite and quartz appears to be common. Further evidence is needed to corroborate that the older quartz-free suite represents another example of rare high-temperature jadeitite as documented in Cuba.

Crustal history of Margarita Island (Venezuela) in detail: Constraint on the Caribbean plate-tectonic scenario

The pressure-temperature-time-deformation evolution for the crust of Margarita Island (Venezuela) has been established to allow comparison with current plate-tectonic models for the Caribbean region. On Margarita, the 12 recognizable stages of development can be summarized in terms of the following evolving tectonic settings: Protolith evolution as Aptian-Albian or older oceanic crust, as well as continental crust with Paleozoic basement (stages 1 and 2); accretion and high-pressure metamorphism (500–600 °C, 10–14 kbar) as the Margarita Complex in the deep level of a fore arc at 100–90 Ma (stage 3); ascent, cooling, and emplacement into the intermediate crustal level of a volcanic arc at 90–80 Ma (stage 4); transform plate-margin setting at a comparable level at 80–50 Ma (stage 5); second episode of rapid uplift and cooling (stages 6 and 7); and shallow crustal level close to transform plate margin from 50 Ma to present (stages 8 to 12). This complex sequence is in excellent agreement with plate-tectonic scenarios that require a Pacific origin for the Caribbean plate and eastward migration of the Margarita Complex and its correlatives along northern South America since the Cretaceous.

Inherent gravitational instability of thickened continental crust with regionally developed low- to medium-pressure granulite facies metamorphism

Abstract Petrological arguments show that regionally developed low- to medium-pressure, high-temperature granulite facies metamorphism may critically enhance the lowering of crustal density with depth. This leads to gravitational instability of homogeneously thickened continental crust, mainly due to changes in mineral assemblages and the thermal expansion of minerals in conjunction with the exponential lowering of the effective viscosity of rocks with increasing temperature. It is argued that crustal processes of gravitational redistribution (crustal diapirism) contributing to the exhumation of granulite facies rocks may be activated in this way.

Structure, tectonics and metamorphic development of the Sancti Spiritus Dome (eastern Escambray massif, Central Cuba)

The Sancti Spiritus Dome of the eastern Escambray (Cuba) represents a metamorphic fold and thrust structure which was part of the Cretaceous subduction-accretion complex of the Greater Antillean Arc. On the basis of structural data and pressure-temperature-time evolution the metamorphic complex can be subdivided into four units interpretable as nappes: a high-grade greenschist-facies unit (Pitajones unit), a high-pressure tectonic melange (Gavilanes unit), high-pressure amphibolites (Yayabo unit) and – tectonically overlying - low-pressure metagabbros of the Greater Antillean Arc (Mabujina unit). The oldest rock fabrics are preserved in eclogite- and blueschist-facies rocks of the Gavilanes unit, indicating arc-parallel extension. Maximum metamorphic conditions are recorded in eclogites (16-20 kbar, 580-630 °C) and garnet-mica schists (16-23 kbar, 530-610 °C) of the Gavilanes unit. Field observations and fabric studies show that greenschist-facies dynamic indicators are dominated by top-to-NE tectonic transport in the lowermost nappes. The greenschist-facies shear zone between the Yayabo unit and the Mabujina unit is viewed as the main detachment zone between the subduction complex and the overlying arc complex. Active subduction ceased at about 70 Ma, followed by rapid uplift, exhumation and thrusting to the north.