TECTONICS AND SEDIMENTATION ALONG ACTIVE CONTINENTAL MARGINS

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Tectonics and sedimentation along active continental margins

Active continental margins are the most dynamic regions tectonically speaking, where the primary process of rock recycling and continental crust generation occurs. These margins are characterized by the subduction of oceanic lithosphere beneath the continental lithosphere, with or without a volcanic arc built directly on the adjacent continent and connected directly to the hinterland (Frisch et al., 2011). The plate boundary, represented by the trench, conforms to a broad zone of hundreds to thousands of kilometres in length (Figure 2.1). Landwards from the trench, the magmatic arc (when present) is located at a distance that is dependent on the angle of subduction, and the area in between the arc and the trench is denominated the forearc region. The sedimentary basins that develop across this region (s.s. forearc basins) are highly dependent on the volcanism developed landwards and the tectonic processes at the trench. Thus, it sounds imperative to consider all the different processes and their interplay across this region, both in space and through time searching for evidence or remnants in the geological record. This chapter provides an abridged overview of the main processes, highlighting their importance on margin configuration, their reconnaissance through the use of geological and geophysical tools, and their possible role through time.

Subduction initiation

Before we focus our view on the different tectonic processes involved in the margin configuration, I would like to highlight the different subduction initiation processes. It is recognized that the reconstruction of a subduction initiation process remains enigmatic and controversial for advancing in our understanding of plate tectonics. The ephemeral nature of a subduction initiation process and the constraints imposed by buried evidence in forearc regions impair the access to tectonic, magmatic, and sedimentary responses to subduction initiation processes (Stern et al., 2012). Furthermore, significant parts of the forearc may be lost by tectonic erosion (Scholl and von Huene, 2009; Draut and Clift, 2013); nevertheless, the preserved parts may contain a record of the processes that accompanied subduction initiation of a particular convergent margin. On land, ophiolites may provide access to the forearc composition and magmatic stratigraphy and subsequently understand the subduction initiation process. There are two recognized mechanisms in the literature: spontaneous and induced nucleation of subduction zones (Stern, 2004). However, I will also refer to a particular case of induced nucleation (plume-induced), which may be highly relevant in the southernmost Northern Andes domain.

Induced nucleation of subduction

The existing plate motion causes compression and lithospheric rupture at the boundary between two converging plates forming a subduction zone. Figure 2.2 shows two ways where a continuous plate convergence may yield a new subduction zone. The “transference” way is of particular interest in this work, as it is related to the presence or arrival of a buoyant crustal block at the subduction zone, a topic discussed in chapter 3 for the study area.

Spontaneous nucleation of subduction

Spontaneous subduction occurs when dense and old oceanic lithosphere sinks into the underlying asthenosphere, developing a down-dip sense of motion. A lithospheric weakness is required to overcome the lithospheric strength allowing the collapse (Figure 2.2) (Stern, 2004).

Plume-induced subduction initiation

The Interaction of a large plume head with dense oceanic lithosphere could weaken the lithosphere by developing transform faults or subduction zones (Whattam and Stern, 2015). Indeed, thermo-mechanical numerical models investigate the lithosphere’s interaction with a buoyant mantle plume, demonstrating that a mantle plume can break the lithosphere and initiate self-sustaining subduction (Ueda et al., 2008; Burov and Cloetingh, 2010). Based on trace element chemistry from most of the 100 Ma and younger units exposed along the southern margin of the Caribbean Plate and NW South America, Whattam and Stern (2015) suggest that there is a continuous evolution from plume magmatism into subduction-related magmatism with time, supporting the plume-induced subduction initiation process (Figure 2.3). The crustal rocks along the Western Cordillera and Coastal region in Ecuador relate to a sliver of the Caribbean Large Igneous Province (CLIP) accreted to the Northern Andes relevance of studying the remnant of such processes to constrain the pre-accretion geometry of the margin better. A combined geological and geophysical view of a remnant of a sliver from the CLIP is presented in chapter 3.
Figure 2.3: (a-e) thermo-mechanical models of Plume induced subduction initiation based on Ueda et al. (2008). (f-j) 100-60 Plume induced subduction initiation to volcanic arc generation tectonic reconstruction (further details in Whattam and Stern (2015)).

Ophiolites and mechanisms for their emplacement

Our understanding of ophiolite emplacement or obduction has been primarily based on the North American Cordillera (Kearey et al., 2009). Their incorporation into land constitutes a fundamental aspect of continental growth and a problem on plate tectonics, as dense oceanic crust becomes emplaced over less dense material of continental margins (Wakabayashi and Dilek, 2003).
Ophiolites are recognized as fossil fragments of upper mantle and oceanic crust exposed on land usually incorporated into the continental margin during continent-continent and arc-continent collisions (Kearey et al., 2009; Dilek and Furnes, 2011). They are mostly found along sutures zones in both collisional-type and accretionary-type orogenic belts. The association of deep-sea sediments, basalts, gabbro, and ultramafic rocks suggests that they originated as oceanic lithosphere and subsequently thrust up into their continental setting by obduction. However, it has been noticed in the past that ophiolites may represent fragments of crust and mantle formed by spreading at a ridge beneath an ancient ocean (Mason, 1985). Several examples suggest that ophiolites may have originated near their emplacement site through the development of Large Igneous Provinces (LIP) (Whattam and Stern, 2015), based on geochemical signatures and the short time between crystallization, collision, and accretion. Ophiolites constitute rock fragments that provide insight into ocean floor construction and continental growth, constituting key petro-tectonic elements for geodynamic reconstructions. Furthermore, their emplacement and nature are expected to vary depending on the age, thickness, and thermal state of oceanic crust, the nature and geometry of the plate boundaries involved (Wakabayashi and Dilek, 2003).
A thorough review of the genesis and tectonics of ophiolites is given by Dilek and Furnes (2011). These authors present a new classification that incorporates the diversity in their structural architecture and geochemical signatures. A particular type of ophiolites that have a mid-ocean-ridge basalt (MORB) composition is subduction-unrelated ophiolites such as the case of the Northern Andean Sliver, where geochemical analysis of surface exposures indicates a MORB signature.
Wakabayashi and Dilek (2003) distinguish four types of ophiolites based on their emplacement mechanism and the nature of the underlying tectonic basement. However, only the first two are discussed in this chapter, given their abundance and importance for the present study: 1) Tethyan; and 2) Cordilleran (Figure 2.4).
Tethyan-type ophiolites structurally overlie passive continental margins, microcontinental fragments, or island arcs. The extrusive section of most of these ophiolites does not have volcaniclastic rocks typical of a volcanic arc. However, many of these ophiolites display geochemical characteristics of subduction zone environments in their upper-crustal part.
Cordilleran-type overlies subduction-accretion complexes instead. The volcaniclastic and intermediate silicic volcanic rocks that are generally associated with island arc development are widespread in the extrusive sections of Cordilleran ophiolites (Wakabayashi and Dilek, 2003). Upper-crustal rock units in Cordilleran ophiolites display island arc tholeiite to calc-alkaline chemical affinities indicating a subduction zone origin. The existence of volcaniclastic rocks is indicative of volcanic arc edifices construction during the evolution of these ophiolites. This last aspect, considered in this work, is further discussed in Chapter 3.
Many ophiolite complexes are structurally underlain by fault-bounded sheets of highly strained high-grade metamorphic rocks, called metamorphic soles (Figure 2.4). Metamorphic soles constitute witnesses of the earliest stages of obduction in intra-oceanic settings preceding final emplacement onto continental margin, which is a constraint when building any initial geodynamic stage at convergent plate margins (Wakabayashi and Dilek, 2003; Agard et al., 2012).
As shown in Figure 2.5, major and voluminous magmatic pulses occurred in the Mesozoic at a hemispheric scale (Stern et al., 2012), located along the Caribbean region and Western Pacific (Figure 2.6), and therefore their importance of study to better understand continental growth processes.

Accretionary vs. erosional regimes

Subduction zones as shown in Figure 2.7 can be erosive or accretionary. It has been recognized that the sediments attached to the subducting oceanic plate may underthrust the overriding plate, a process called sediment subduction, and that material from the upper plate can be removed or scraped-off and subducted by a process of subduction erosion (Von Huene and Scholl, 1991).
The notions of sediment subduction arise from unbalanced volumes in regional sediment budgets and the absence of accretionary wedges along some convergent margins. The input material attached to the oceanic crust is primarily composed of clay, far-travelled terrigenous detritus, and the carbonate and siliceous material supplied by planktonic and benthic organisms. These deposits typically reach only 200-400 m thick, and in the presence of large drainage systems derived from the continent to the ocean, the thickness can be greater than 500m. Such deposits, often transported by some turbidity currents may be concentrated along the trench axis forming a wedge-shaped body that may be added to the upper plate by processes of frontal accretion and basal underplating (Figure 2.8) (Scholl et al., 1980).
A region of well-lithified rocks seaward of the volcanic arc, known as the margin mechanical backstop, is characterized by a greater shear strength than the sediments lying trenchward (Byrne et al., 1993; Kopp and Kukowski, 2003). It plays an essential role in the overall growth of forearcs (Byrne et al., 1993). Indeed, numerical and analogue modelling shows that a reasonable contrast in mechanical properties with the sediments trenchward, may result in the development of an outer forearc high, which may bound a relatively undeformed forearc basin landward.
Analogue or numerical modelling has led to a better understanding of the evolutionary processes taking place during an arc-continent collision (see next sections).
Figure 2.8: Diagram of an accretionary prism/outer wedge and the processes of frontal accretion and underplating that contribute to its volumetric growth (modified after Von Huene and Scholl, 1991).

Arc-continent collision: insights from analogue and numerical modelling

The natural examples of arc-continent collisions available in the literature show very different geometries and complexities that appear to depend on several first-order parameters, such as the age of the oceanic crust, and the pre-existing structure of the margin and the arc (Brown et al., 2011). Through the aid of geophysical and geological tools (discussed in the last section of this chapter), we may get insights into some of these parameters. Especially for ancient systems, which have undergone post-collision deformation or erosion, leaving us with partial evidence of the original arc-continent collision (Brown et al., 2011). To better understand the evolutionary processes taking place during an arc-continent collision, the use of analogue modelling (Figure 2.9) or numerical techniques provide insights into the geodynamic evolution of such process. Bellow, I present a non-extensive summary of these techniques and theirmain conclusions.

Physical modelling of arc-continent collisions

The partial insight given by the geological data on the lithospheric processes involved in arc-continent collisions is limited, requiring to invoke physical or numerical techniques to understand better the evolution of the plausible processes in nature (Boutelier and Chemenda, 2011).
Figure 2.9: Analogue experimental setup (details are provided in Boutelier and Chemenda (2011)).

Oceanic subduction

To better illustrate the effects of the bending strength of the subducting lithosphere and the force due to its negative buoyancy on the distribution of stresses along the interplate zone, two regimes of oceanic subduction are identified, which are associated with the compressive or tensile regimes in the arc/back-arc areas.
Under a compressive regime, the subducting lithosphere resists to bending during subduction causing a compressive non-hydrostatic normal stress whose magnitude increases with depth along the interplate zone (Figure 2.10). In the tensile regime, the density of the subducted lithosphere is significantly larger than that of the surrounding mantle, creating a negative buoyancy that generates a downward pull force, which results in a tensile non-hydrostatic normal stress on the interplate zone. However, when the length of the subducted slab reaches a certain critical value, the slab pull force becomes sufficient to cause slab break-off, after which oceanic subduction switches to the compression regime (Boutelier and Chemenda, 2011). Several numerical models of slab breakoff have shown that the age influences the depth and the implemented rheology in the subducting plate, as a result there is a broad range of depths between 80-510km, requiring 1 to 50 Ma (Fernández-García et al., 2019).

Continental subduction

Due to the high buoyancy of the subducting continental crust, continental subduction is primarily associated with two principal regimes (Figure 2.11), a high and low compressional regime, characterized by high and low pressure between the overriding and subducting plates respectively (Chemenda et al., 1996). As for oceanic subduction, the regime of continental subduction is mainly controlled by the slab-pull force due to the negative buoyancy of the subducted lithospheric mantle and the bending strength of the lower plate (Boutelier and Chemenda, 2011).
Figure 2.11: Two principal regimes of continental subduction obtained in purely mechanical experiments (Boutelier and Chemenda, 2011).

Numerical models

I have placed an interest in the collision and accretion of oceanic plateaus and their role in continental growth. The numerical models presented by Vogt and Gerya (2014) resulted in oceanic plateau either being lost by subduction (>40 Ma oceanic lithosphere) or accreted onto continental margins (younger oceanic lithosphere). This study identifies one mode of complete plateau subduction and three modes of terrane accretion, they include: 1) frontal plateau accretion, 2) basal plateau accretion, and 3) underplating plateaus. Although, complete plateau subduction is the dominant process, the other three accretionary modes show that following a collision process the time to re-establish a stable subduction varies depending on the accretion mode. The basal plateau accretion shows an outward migration of the subduction zone, with the incoming oceanic crust underthrusting the fractured terrane, forming a new subduction zone behind the accreted terrane. This result is of crucial importance in this work as it may partly explain the local development of some structural highs following the re-establishment of the new subduction zone (Figure 2.12), an aspect discussed further in Chapter 4.

Table of contents :

INTRODUCTION
1.1 THESIS OBJECTIVE
1.2 MOTIVATION
1.3 ORGANIZATION OF THE THESIS
TECTONICS AND SEDIMENTATION ALONG ACTIVE CONTINENTAL MARGINS
2.1 SUBDUCTION INITIATION
2.1.1 Induced nucleation of subduction
2.1.2 Spontaneous nucleation of subduction
2.1.3 Plume-induced subduction initiation
2.2 OPHIOLITES AND MECHANISMS FOR THEIR EMPLACEMENT
2.3 ACCRETIONARY VS. EROSIONAL REGIMES
2.4 ARC-CONTINENT COLLISION: INSIGHTS FROM ANALOGUE AND NUMERICAL MODELLING
2.4.1 Physical modelling of arc-continent collisions
2.4.2 Numerical models
2.5 GRAVITY AND MAGNETIC RESPONSES OF TRAPPED OCEANIC TERRANES
2.6 FOREARC BASINS
CRUSTAL STRUCTURE OF WESTERN ECUADOR
3.1 INTRODUCTION
3.2 REGIONAL GEOLOGY
3.2.1 Western Cordillera crustal blocks
3.2.2 Volcanic and oceanic plateau remnants in the forearc region
3.2.3 Amotape-Tahuin Massif along NW Peru
3.3 PREVIOUS STUDIES
3.4 GEOPHYSICAL DATA AND METHODS
3.4.1 Analysis of gravity and magnetic anomalies
3.4.2 Data Constraints for 2-D forward models
3.5 ANALYSIS OF REGIONAL GEOPHYSICAL DATA
3.5.1 Seismic, gravity and magnetic anomalies
3.5.2 Forward models
3.6 DISCUSSION
3.6.1 Split of Rio Cala-San Lorenzo arc and development of a marginal basin?
3.6.2 Esmeraldas block – trailing edge of a different accreted sliver?
3.6.3 The southern suture zone (Gulf of Guayaquil) – a transform fault boundary
3.7 CONCLUSIONS
CENOZOIC TECTONIC EVOLUTION OF SW ECUADOR
4.1 INTRODUCTION
4.2 GEODYNAMIC AND GEOLOGICAL SETTINGS
4.2.1 North Andean Sliver
4.2.2 Basin Stratigraphy of SW Ecuador
4.2.3 Northern Peru
4.3 DATASET AND METHODOLOGY
4.4 RESULTS
4.4.1 Santa Elena High
4.4.2 Progreso Basin
4.4.3 Gulf of Guayaquil-Tumbes Basin
4.5 DISCUSSION
4.5.1 Possible effects on the margin following the arrival of the Caribbean Large Igneous Province (CLIP)
4.5.2 From an unstable to a stable margin (Paleocene-Eocene stage)
4.5.3 Preservation of the Santa Elena accretionary wedge in SW Ecuador
4.5.4 Development of the Progreso and Gulf of Guayaquil – Tumbes basins
4.6 CONCLUSIONS
STRATIGRAPHY CONTROLLED BY THE LOCAL DEVELOPMENT OF AN OUTER-FOREARC HIGH: PROGRESO BASIN
5.1 INTRODUCTION
5.2 FOREARC DEVELOPMENT DURING THE CENOZOIC
5.3 SAMPLING AND METHODS
5.4 STRATIGRAPHIC EVOLUTION AND U-PB CONSTRAINTS
5.4.1 Youngest U-Pb zircon dates point to depositional age
5.4.2 The accretion series: the Paleocene Azúcar Formation
5.4.3 The Upper Paleocene – Lower Eocene accretionary series close to the Chongón-Colonche hills
5.4.4 The post-accretionary Eocene Ancón Group
5.4.5 The Progreso forearc basin and coeval sediments in the North Peninsula
5.5 SEISMIC INTERPRETATION
5.6 DISCUSSION
5.6.1 The accretionary prism and the post-accretion series
5.6.2 The outer forearc high
5.6.3 The shallow-water forearc basin infilling
5.7 CONCLUSIONS
TECTONOSTRATIGRAPHIC EVOLUTION AT THE TERMINATION OF A TRENCH-LINKED CONTINENTAL TRANSFORM BOUNDARY: GULF OF GUAYAQUIL-TUMBES BASIN, SOUTHERNMOST NORTHERN ANDES
6.1 INTRODUCTION
6.2 REGIONAL GEOLOGICAL FRAMEWORK
6.2.1 Structure
6.2.2 Stratigraphic framework
6.3 DATA & METHODS
6.3.1 Structural & stratigraphic seismic interpretation
6.3.2 Outcrop exposures
6.4 TECTONO-STRATIGRAPHIC UNITS OF THE GGTB
6.4.1 Early to Middle Miocene (Unit 1)
6.4.2 Middle to Late Miocene (Unit 2)
6.4.3 Early Pliocene (Unit 3)
6.4.4 Plio-Pleistocene (Unit 4)
6.5 DISCUSSION
6.5.1 Basin type development at the termination of a trench-linked continental transform boundary
6.5.2 Oblique ridge development and implication to the depositional environment 188
6.5.3 Basin evolution of the GGTB
6.6 CONCLUSIONS
CONCLUSIONS AND FURTHER PERSPECTIVES
7.1 REVEALING THE UNDERLYING FOREARC CRUSTAL STRUCTURE: FROM AN EARLY SPLIT-ARC TO A BUILT-IN MAGMATIC EVENT
7.2 AN INHERITED ACCRETIONARY WEDGE AND ITS INTERACTION TO THE PIÑON BACKSTOP
7.3 DEVELOPMENT OF A LOCALIZED OUTER FOREARC HIGH (OFH) AND FURTHER CONTROL ON FOREARC BASIN DEVELOPMENT
7.4 FURTHER PERSPECTIVES

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