TECTONIC VERSUS CLIMATIC CONTROLS ON EXHUMATION IN THE VENEZUELAN ANDES

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Current tectonic models for the origin of the Venezuelan Andes

From the work of Gansser (1973) it is clear that the Venezuelan Andes are a distinct mountain chain, separated from the Eastern Cordillera of Colombia. The separation is based on changes in orientation of structural features and the presence of Precambrian crystalline rocks in the Venezuelan Andes. Since the 1950s, two different types of tectonic models were proposed to explain the origin of the Venezuelan Andes, symmetric and asymmetric models.

Symmetric models

In this type of model the Venezuelan Andes are regarded as a symmetric chain with a major strike-slip fault in the centre, and with bounding reverse faults on both sides of the mountain belt (Audemard and Audemard, 2002). Dewey (1972) and Schubert (1981) postulated that the Boconó fault represents the plate boundary between the South America and the Caribbeanplate or Maracaibo block. The Venezuelan Andes would then be the result of compression between two plates and would comprise two separate chains, split by the Boconó fault (Soulas, 1985).
Bucher (1952), González de Juana (1952), Hargraves and Shagam (1969), and Shagam (1972) proposed for the origin of the Venezuelan Andes a mega-anticline, containing a horst and graben block complex in the centre and with both flanks bounded by high-angle reverse faults (Fig. 2.7). In contrast, Rod (1956), Deratmiroff (1971), Schubert (1985), White (1985), Stephan (1982), Boesi et al. (1988), and Monsalve (1988) proposed a mushroom-like transpressional uplift with shear deformation along the Boconó fault and imbricate thrusting toward both flanks.

Present-day tectonic

The Cenozoic evolution of the Northern Andes, and in particular of the Venezuelan Andes has been dominated by the relative north-eastward, and subsequent eastward advance of the buoyant Caribbean plate with respect to stable South America (Fig. 2.10; Molnar and Sykes, 1969; Case et al. 1971; Bell 1972; Malfait and Dinkelman 1972; Jordan 1975; Pindell and Dewey 1982; Sykes et al. 1982; Wadge and Burke 1983; Burke et al. 1984; McCourt et al. 1984; Laubscher, 1987; Avé Lallemant, 1997; Villamil and Pindell, 1998; James, 2000a; Colmenares and Zoback, 2003; Audemard et al. 2006). This eastward motion of the Caribbean plate is supported by results of recent GPS surveys (Fig. 2.10A; Freymueller et al. 1993, Kaniuth et al. 1999, Weber et al. 2001, Pérez et al. 2001, Trenkamp et al. 2002, Colmenares and Zoback, 2003) and the seismicity record (Fig. 2.10B; Colmenares and Zoback, 2003; Cortés and Angelier, 2005). The eastward movement of the Caribbean plate is accommodated by well-developed transcurrent plate boundaries to the north (Rosencrantz et al. 1988) and south (Kafka and Weidner, 1981; Pennington, 1981). Unlike the sharp northern boundary, however, the southern Caribbean plate boundary is a collection of continental fragments that resisted the advance of the Caribbean plate (Montes et al. 2005). Continued movement of the Caribbean plate led to the progressive dextral transpressional distortion, dismembering, rigid-body translation, and clockwise rotation of the continental fragments that make up the northwestern Andes. This process is responsible for the difficulties ininterpreting and locating the Caribbean-South America plate boundary (Soulas 1986, Beltrán 1994). In western Venezuela, the plate boundary covers a 600 km wide zone and comprises a set of discrete tectonic blocks or microplates (Fig. 2.2, Fig. 2.10A), which move independently among the surrounding larger plates (Caribbean, South America and Nazca).
One of these tectonic blocks is defined between the Boconó, Oca, and Bucaramanga-Santa Marta faults, which define a roughly triangular block (Fig. 2.10; Audemard 1993, 1998; Dhont et al. 2005; Backé et al. 2006; Audemard et al. 2006) with an intervening northeasttrending foldbelt (Perijá mountains, Kellogg, 1984), a northwest corner out of isostatic equilibrium (Sierra Nevada de Santa Marta, Tschanz et al. 1974), a northeast region limited to the east by allochthonous oceanic sequences (Villa del Cura, Bell, 1971), and a central depression where a great thickness of sediment has accumulated (Maracaibo basin, James, 2000a). The relatively undeformed stratal geometry reported in the central part of this block (Maracaibo basin, Fig. 2.10A) is evidence of its relative rigidity.

External detector method and z calibration method or Z factor

In practical terms, during fission track counting we determine the density of etched tracks on internal crystal surface (ρs and ρi) and not the number of tracks per volume (Ns and Ni). Therefore Ns,i is substituted by ρs,i in the following equations, for details see Wagner and Van den Haute (1992).
A major problem is the accurate measurement of the decay constant for spontaneous fission, where individual determinations range from 7 to 8.5´10-17 y-1 (cf. compilation in Wagner and Van den Haute (1992). In addition, uncertainties are related to the determination of the thermal neutron flux f (Hurford and Green, 1982), which is determined by using glass monitors with known uranium concentration. To overcome the uncertainty of the λf, value, Fleischer and Hart (1972) introduced the ζ-dating method, which affords to determine a personal calibration factor ζ for every individual dating method (e. g. apatite and zircon). To calculate this value, is usually applied the External Detector Method (Wagner and van den Haute, 1992; Gallagher et al. 1998). In this process, samples are irradiated and bombarded with neutrons which induce fission tracks that can be measured. In order to produce a surface to observe the induced fission tracks, each grain mount (Fig. 3.3) is covered by a thin, lowuranium muscovite mica sheet placed in intimate contact with the polished and etched apatite crystals. Following irradiation and subsequent cooling down, the mica sheets are etched to reveal the induced tracks resulting from the induced fission of the 235U in the sample. For standardisation a similar mica sheet is also placed in contact with a small chip of 235U doped glass. This is dealt with similarly and the results from the induced track densities for the apatite grains and the doped glass allow the 238U concentration in the apatite grain to be calculated (Donelick et al. 2005).

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Fission-track annealing and modeling

The fission event causes damage in the solid crystal structure of apatite or zircon crystals. The latent tracks in the crystal may be repaired, mainly depending on ambient temperature and residence time at a given temperature (Fleischer et al. 1975). This process, also known as annealing, leads to a reduction in track length and density, making it a unique tool for determining time-temperature (t-T) histories of rocks (Wagner and Van den Haute, 1992). Annealing of fission tracks depends mainly on the temperature and the kinetic and chemical properties in case of apatite of single crystals. The properties must be approximated to derive reliable t-T history predictions. Apatite can have a wide variety of chemical composition with coupled substitution within the cation and anion positions, resulting in F, Cl or OH apatites.
The relationship between composition, crystal structure and annealing kinetics leads to a complex behaviour of FT annealing in apatite (Carlson et al. 1999; Barbarand et al. 2003a, b). Fission-track annealing experiments on apatites clearly show that the track lengths in compositionally different apatites are shortened by different amounts given the same  temperature and residence time (Barbarand et al. 2003a, b). This behaviour represents the summed effect of the type and amount of elemental substitution, and site of substitution in the crystal structure. The substitution directly affects the apatite unit cell, resulting in different FT annealing rates. Barbarand et al. (2003a, b) found a strong correlation (greater than 0.8) between the level of annealing and the unit-cell parameters (positive for unit cell axes dimensions a and c, respectively). Thus, the unit cell parameter can be used as a key indicator of the response to annealing in apatite (Barbarand et al. 2003a, b). Unit cell parameter variations can be partly explained by levels of Chlorine, Fluor or Hydroxyl ion substitutions.
This indicates that no single substitution can explain the bulk change in lattice parameters although Green et al. (2005) proposed that even for fluorine apatite a good correlation between annealing and wt.% Cl exists. An alternative approach for describing the annealing properties of apatite is to use its solubility as a proxy for the bulk composition. For this reason Dpar can be measured; the mean length of well-defined etch pits of fission tracks parallel to the crystallographic c-axis (Burtner et al. 1994). Dpar values vary according to apatite composition (Barbarand et al. 2003a, b), and show a positive correlation with cell parameter a and MTL.

Table of contents :

I. GENERAL INTRODUCTION
1.0 Introduction
1.1 Overview of chapters and results
1.2 Publications and abstracts from this dissertation
1.2.1 Publications
1.2.2 Abstracts
II. PLATE TECTONIC EVOLUTION OF THE NORTHERN ANDEAN REGION 
2.0 Introduction
2.1 Tectonic assembly of the Northern Andes
2.2 Pangaea break-up and origin of the Caribbean plate
2.2.1 Continental rifting and formation of structural discontinuities
2.2.2 Origin of the Caribbean plate, the western and central Cordillera (Ecuador and Colombia)
2.2.3 Maracaibo block rotation, reactivation of paleo-structures
2.3 Current tectonic models for the origin of the Venezuelan Andes
2.3.1 Symmetric models
2.3.2 Asymmetric models
.3.3 Present-day tectonic
III. APATITE FISSION-TRACK THERMOCHRONOLOGY
3.0 Introduction
3.1 Fission-track formation theory
3.2 Fission-track age equation
3.2.1 External detector method and z calibration method or Z factor
3.3 Conventional statistics for in-situ FT thermochronology
3.3.1 Estimators for R
3.3.1.1 Isochron fitting
3.3.1.2. Mean ratio
3.3.1.3 Pooled Mean
3.3.1.4. Central Age
3.3.2 Standard error on fission-track age
3.3.3 Galbraith test
3.4 Fission-track annealing and modeling
3.5 Length measurements
3.6 Analytical procedure
3.7 Detrital apatite fission-track thermochronology
3.8 Conventional statistics for detrital FT thermochronology
3.9 Data interpretation and quantitative thermochronology methods
3.9.1 Quantitative thermochronology
IV. SPATIAL AND TEMPORAL PATTERNS OF EXHUMATION ACROSS THE VENEZUELAN ANDES: IMPLICATIONS FOR CENOZOIC CARIBBEAN GEODYNAMICS
4.0 Abstract
4.1 Introduction
4.2 Geodynamic setting and structure of the Venezuelan Andes
4.3 Methods and analytical procedures
4.3.1 Topographic characteristics
4.3.2 Apatite fission-track thermochronology
4.3.3. Thermal history modeling
4.4 Results
4.4.1 Topographic characteristics
4.4.2. Apatite fission-track data
4.5 Discussion
4.6 Conclusions
4.7 Acknowledgements
V. THERMOCHRONOLOGIC EVIDENCE FOR KM-SCALE VERTICAL OFFSET ACROSS THE BOCONÓ STRIKE-SLIP FAULT, CENTRAL VENEZUELAN ANDES. 
5.0 Abstract
5.1 Introduction
5.2 Tectonic setting
5.3 Thermochronology data
5.3.1 Methods
5.3.2 Results
5.4 Numerical modeling
5.5 Discussion and conclusions
5.6 Acknowledgements
VI. TECTONIC VERSUS CLIMATIC CONTROLS ON EXHUMATION IN THE VENEZUELAN ANDES
6.0 Abstract
6.1 Introduction
6.2 Tectonic, geomorphic and climatic setting
6.2.1 Geological evolution and exhumation history
6.2.2 Relief
6.2.3 Seismicity
6.2.4 Precipitation pattern
6.3 Detrital apatite fission-track thermochronology
6.3.1. Data collection and discrimination of age components
6.3.2 Comparison of detrital and bedrock apatite FT ages
6.3.3 Implications for sediment provenance
6.4 Predicted exhumation and erosion patterns
6.4.1 Long-term exhumation rates
6.4.2 Short-term erosion patterns
6.5 Discussion
6.5.1 Relations between tectonics, climate and erosion in the VA
6.6 Conclusions
6.7 Acknowledgements
VII. STRATIGRAPHY AND PROVENANCE OF THE MÉRIDA ANDES PRO- AND RETRO-SIDE FORELAND BASIN DEPOSITS: INSIGHTS FROM DETRITAL APATITE FISSION-TRACK THERMOCHRONOLOGY, PALYNOLOGY, AND SEDIMENT PETROLOGY.
7.0 Abstract
7.1 Introduction
7.2 Geologic setting
7.3 Stratigraphic analysis and sample collection
7.4 Methods
7.4.1 Detrital apatite fission-track thermochronology
7.4.2 Palynological analysis
7.4.3 Characterization of organic matter
7.4.3.1 Characterization by Rock-Eval pyrolysis
7.4.3.2 Determination of total carbon content by elementary analysis (LECO)
7.4.3.3 Calcimetric measure
7.4.4 Sediment petrology
7.5 Results
7.5.1 Detrital apatite fission-track thermochronology
7.5.2 Palynological analysis
7.5.4 Sediment petrology
7.5.5 The Río Hoyos-Río Vichú stratigraphic section
7.5.6 Parángula River stratigraphic section
7.6 Discussion
7.6.1 Stratigraphy and provenance of the two foreland basins
7.6.1.1 Depositional environments
7.6.1.2 Depositional ages
7.6.1.3 Sediment provenance
7.6.2 Neogene exhumation history of the Mérida Andes
7.7 Conclusions
7.8 Acknowledgements
VIII. GENERAL CONCLUSIONS
8.1 Conclusions
8.2 Synthesis
8.3 Future perspectives
VIII. CONCLUSIONS GÉNÉRALES
8.1 Conclusiones
8.2 Síntesis
8.3 Perspectivas a futuro
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