Development of Structures and Evolution of Morphology in Large Debris Slides

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The Macolod Corridor

The Macolod Corridor is a 40-km wide, NE-SW area of approximately 200 monogenetic volcanoes with two distinct clusters of scorias and maars (Calibo et al., 2009). These clusters of volcanoes were generated by a counter-clockwise block rotation in southwestern Luzon that is linked to the opposite motions of the subducting South China Sea under the Manila Trench on the west and the strike-slip Philippine Fault on the east coupled with shearing from the Verde Fault (Fig. 1.3). This block rotation resulted in localised extension along the sides of the blocks that led to the partial melting of the crust (Calibo et al., 2009; Galgana et al., 2007).

The Palawan Volcanic Field

The geomorphologically young Palawan Volcanic Field is probably the largest exposure of basaltic lava flows in the Philippine archipelago (Arcilla et al., 2003). This volcanic field is not influenced by recent subduction processes due to the absence of trenches in its surroundings. It also postdates collision of Northern Palawan with the Proto-Philippine arc (Arcilla et al., 2003). The high field strength element concentrations of the lavas show gradation with those of the South China Basin Seamounts and primitive Jolo arc rocks sug-gesting cogenetic origins from geochemically enriched sources (Arcilla et al., 2003).
The Central Mindanao Volcanic Field is the most extensive field of active volcanoes in the Philippines (Sajona et al., 1993). Several seismic studies (Besana et al., 1997; HK and YP, 1980; Pubellier et al., 1991) and reconnais-sance petrologic investigations (e.g. Maury et al., 1996; Sajona et al., 1994, 1993; Sajona F.G. et al., 1997) have suggested that this volcanic field is as-sociated with remnants of the subducted Molluca Sea Plate under parts of Mindanao island. Parental magmas of the central Mindanao volcanic field most likely came from the mantle wedge metasomatized by fluids dehydrated from subducted sediments and oceanic crust (Castillo et al., 1999). Based on geographic and structural setting, tectonic, and geochemistry, volcanoes in central mindanao has a northwest migration of volcanism and Camiguin is the northernmost extension of this volcanic field (e.g. Castillo et al., 1999; Corpuz, 1992; Sajona et al., 1994; Sajona F.G. et al., 1997).
Grosse et al. (2009) presented a quantitative morphometric classification and interpretation of evolutionary trends for arc volcanoes (see Fig. 1.4). In their work, morphometric analysis of two contrasting arcs, Central America and the southern Central Andes, was used to interpret processes operating during volcano construction.
A similar morphometric analysis is presented here for the Philippine vol-canoes. The analysis aims to obtain quantitative morphological data of the active and potentially active volcanoes in the Philippines and interpret pos-sible shape evolution trends. Also, it will hopefully add to the global docu-mentation of volcanoes and give useful information on the factors controlling volcano shape and evolution, especially in areas with high erosion rates like the Philippines.
The morphometric analysis uses the Morvolc code (Grosse et al., 2012, 2009). It focuses only on the 39 morphologically young-looking active and potentially active volcanoes and uses the 90 m resolution Shuttle Radar To-pography Mission (SRTM) digital elevation model (DEM) to compute and define parameters for size: height, width and volume; plan shape: ellipticity and irregularity; and profile shape: height-width ratio, summit width-basal width ratio and slope.

The MORVOLC code

Morvolc is an interactive data language (IDL) code developed by (Grosse et al., 2012, 2009) for characterising volcano morphometry. The extent of each volcano is manually delineated mostly by following the breaks in slope around the edifice base also considering the extent of deposits, using DEM-derived slope maps, 3D surfaces and shaded relief images. Thus, far-reaching fall and flow products are not considered in the delineation, just the visible edifice. A 3D basal surface is then calculated from the edifice outline using a least-square criterion and this is used to estimate volume and height parameters. Elevation contour lines are then generated from the DEM, and a summit region is defined at the elevation where the edifice starts flattening out.

Morphometric Parameters

Morvolc generates an array of morphometric parameters for detailed quantifi-cation and characterisation of volcano morphology including the size, shape, and morphometric ratios considered here (Table 1.2). The edifice and sum-mit outlines give the basal and summit areas and average basal and summit widths. Height and volume are computed using the edifice outline and the 3D basal surface. Plan shape is characterised by the shape of the elevation contours, quantified into two dimensionless shape descriptor indices: elliptic-ity index (ei) and irregularity index (ii), which estimate and quantify contour elongation and complexity, respectively, starting at a value of 1 for circular and regular contours. Profile shape of the volcanic edifice is summarised with two ratios of size parameters: the height/basal width ratio (H/WB ) estimates the edifice steepness, and the summit width/basal width ratio (WS /WB ) es-timates the truncation of the edifice and shows the relative importance of the summit region.

Quantitative Morphometry of Philippine Volcanoes

The active and potentially active volcanoes of the Philippines have a wide variety of sizes and shapes (Table 1.2, Fig. 1.5-1.6). Their edifice heights vary from 150 to 2090 m and their volumes range from 0.4 to 550 km3. Edifice shapes vary from smooth, steep and conical to very irregular and flat.
The edifices can be grouped into three main classes according to their size (basal width and volume), profile steepness (H/WB ) and plan shape (average ii and ei) (Fig. 1.5E): cones, subcones and massifs; a subclass of breached edifices is also identified. Considering basal width and volume, two size groups can be defined, small and large (Fig. 1.5A, B, C). Small edifices have basal widths ≤ 10 km and volumes ≤ 20 km3. Large edifices have basal widths ≥ 15 km and volume ≥ 30 km3. Two volcanoes, Banahaw and Kalatungan have intermediate basal widths but are large considering their volumes. Their heights are in the higher end of the spectrum for the cone and subcone classes, respectively.
Edifices classified as cones (n=7) are steep, with H/WB ≥ 0.13. They have circular and regular plan shapes, having low ei (average ei ł) 1.7 and ii (average ii ≤ 1.1) values, although two edifices are more irregular, Maripipi, because of strong erosion, and Bulusan as it is a vent of a caldera system. Most cones are small, except for large and very active Mayon.
Massifs (n=9) are large volcanoes with low H/WB (≤ 0.07). Their plan shapes are very irregular (average ii < 2.6), with intermediate to high ellip-ticity (average ei < 1.9).
Subcones (n=11) have shape parameters with intermediate values. They have H/WB ratios mostly between 0.08 and 0.12 and average ii values between 1.2 and 1.8; their average ei values are very variable. Most subcones are small. Ten volcanoes have breached edifices, qualitatively defined by collapse-scars and horseshoe-shaped summits. These edifices can be grouped into a morphometric subclass; they are edifices belonging to any of the three main classes that show breaching. They have variable H/WB ratios mostly below 0.12, within the range of subcones (3 edifices) and massifs (6 edifices); only Banahaw has a high H/WB ratio within the range of cones. They have a variable ei values and intermediate to high ii values (average ii < 1.2). Most breached edifices are large, only Cagua and Iriga are small. It is diﬃcult to distinguish breached edifices quantitatively because their morphometric parameters are variable as a result of their diﬀerent ‘pre-breaching’ edifice morphologies. Their common feature is the horseshoe shape of their summits. Thus, their uppermost elevation contours generally have very high ii and ei values. Figure 1.6 shows height vs ii graphs of diﬀerent edifice types. Breached edifices have increasing irregularity towards the summit.
From the morphometric data and classification (Fig. 1.5, 1.7) of the Philippine volcanoes into cones, subcones, massifs, and breached edifices, an evolutionary trend is suggested. The wide variety of volcano sizes and shapes may represent diﬀerent growth stages. At the earliest stage of volcano formation, an edifice can either be a cone if it has a simple morphology and a single summit vent or a subcone if it has more than one eruption centre. The cone can continue to grow by eruption from its single vent and deposition of products on its flanks resulting in an increase in height and volume (Fig. 1.3B) without a change or addition of vents, thus maintaining its simple conical shape. Conversely, it can grow new domes on its flank or its eruption centre can migrate evolving into a subcone. Small cones and subcones can grow into larger cones and subcones if no further complexities arise, or they can grow wider into massifs if their complexities increase.
During growth from small to larger cones (e.g. Mayon) or to a subcone or massif, the chance of structural failure of the edifice increases. This increase in probability can be due to growth of the volcano by magma intrusion or weakening of its edifice by hydrothermal alteration or if the volcano is spread-ing due to its volume and size over a substratum that is too weak to support it. These causes happen during the development of a volcano. Large land-slides leave large collapse scars on the volcano summit and abruptly change the shape of the volcano, having a very irregular summit shape, for example.
From this suggested volcano evolution trend, we have an idea on whether a certain volcano is likely to be aﬀected by breaching and at what particular stage of evolution it is based on its morphometry. These morphological trends should be integrated with geological, geophysical and geochemical data in the future works to improve the volcano evolution models, all the more in trying to find out whether a volcano is prone to structural failure and breaching.
Mt Iriga in southeastern Luzon is known for its spectacular collapse scar pos-sibly created in 1628 AD by a 1.5 km3 debris avalanche, spread over 70 km2 and dammed the Barit River to form Lake Buhi. The collapse has been as-cribed to a non-volcanic trigger related to a major strike-slip fault under the volcano. Using a combination of fieldwork and remote sensing, we have iden-tified a similar size, older debris avalanche deposit (DAD) to the southwest of the edifice that originated from a sector oblique to the underlying strike-slip fault. Both deposits cover wide areas of low, waterlogged plains, to a distance of about 16 km for the oldest and 12 km for the youngest. Hundreds of m wide, and up to 50 m high hummocks of intact conglomerate, sand, and clay units derived from the base of the volcano show that the initial failure planes cut deep into the substrata. In addition, large proportions of both DAD con-sist of ring-plain sediments that were incorporated by soft-sediment bulking and extensive bulldozing. An ignimbrite unit incorporated into the younger Buhi DAD forms small (less than 5 m high) discrete hummocks between the larger ones. Both debris avalanches slid over water-saturated soft sediment or ignimbrite, and spread out on a basal shear-zone, accommodated by horst and graben formation and strike-slip faults in the main mass. The observed faults are listric and flatten into a well-developed basal shear zone. This shear zone contains substrate material and has a diﬀuse contact with the intact substrata. Long, transport-normal ridges in the distal parts are evidence of compression related to deceleration and bulldozing. The collapse orientation and structure on both sectors and DAD constituents are consistent with predictions from analogue models of combined transtensional faulting and gravity spreading. Iriga can serve as a model for other volcanoes, such as Mayon, that stand in sedimentary basins undergoing transtensional strike-slip faulting.
Keywords: Mt Iriga, debris avalanche deposit, volcano-tectonics, transten-sional faulting, emplacement kinematics Flank destabilization may occur during the development of a stratovolcano. Such destabilisation may be caused by tectonic activity (Lagmay et al., 2000; Vidal N. and Merle O., 2000), internal growth by magmatic intrusion (Don-nadieu and Merle, 1998; Tibaldi, 2001) or weakening by hydrothermal alter-ation (Reid et al., 2001; van Wyk de Vries and Francis, 1997) and gravitational spreading (Borgia et al., 1992; van Wyk de Vries and Francis, 1997). Failure of destabilized volcano flanks can generate large debris avalanches, triggered by one or a combination, of earthquakes (Montaldo et al., 1996), magmatic in-trusions (Elsworth and Voight, 1996; Voight et al., 1983), and meteoric events (van Wyk de Vries et al., 2000). Evidence and triggering mechanisms of vol-canic debris avalanches may be preserved within the debris avalanche deposits (DAD).
The hazard of landslides, rockslide avalanches, and debris flows on active and even extinct volcanoes can be significant. Examples are the 1998 avalan-che and debris flow at Casita, Nicaragua that killed ∼2,500 people (Kerle and van Wyk de Vries, 2001, e.g.), mass movements on the Monts Dore volcano in the French Massif Central over the last century (Bernard et al., 2009), and the collapse of Mayu-yama lava dome at the Unzen Volcanic Complex, Japan in 1792 (Siebert, 2002).
The Macolod Corridor is a 40-km wide, NE-SW area of approximately 200 monogenetic volcanoes with two distinct clusters of scorias and maars (Calibo et al., 2009). These clusters of volcanoes were generated by a counter-clockwise block rotation in southwestern Luzon that is linked to the opposite motions of the subducting South China Sea under the Manila Trench on the west and the strike-slip Philippine Fault on the east coupled with shearing from the Verde Fault (Fig. 1.3). This block rotation resulted in localised extension along the sides of the blocks that led to the partial melting of the crust (Calibo et al., 2009; Galgana et al., 2007).

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The Palawan Volcanic Field

Table of contents :

1 Introduction
1.1 Introduction
1.2 Motivation of the Present Work
1.3 This Thesis
1.4 The Philippines
1.5 The Philippine Volcanoes
1.5.1 Spatial Distribution of Philippine Volcanoes
1.5.2 Morphometry and Evolution of Philippine Volcanoes
1.5.3 Evolution
2 Controls and emplacement of the Iriga DAD
2.1 Abstract
2.2 Introduction
2.3 Regional Setting
2.4 Mt Iriga
2.5 Methods and Terms
2.6 Mt Iriga Morphology
2.6.1 Iriga DAD1 Geometry, Structure, and Morphology
2.6.2 Iriga DAD1 Lithology and Stratigraphy
2.6.3 Buhi DAD2 Geometry, Structure, and Morphology
2.6.4 Buhi DAD2 Lithology and Stratigraphy
2.7 Discussion
2.7.1 Comparison of the Iriga DAD1 and Buhi DAD2
2.7.2 Collapse Mechanism
2.7.3 Mode of Transport
2.7.4 Substrate Incorporation
2.7.5 Mobility
2.8 Conclusions
3 Hummocks: how they form and how they evolve
3.1 Abstract
3.2 Introduction
3.3 Methodology
3.3.1 Analogue Models
3.3.2 Model Set-Up and Parameters
3.3.3 Scaling
3.4 Results
x Contents
3.4.1 Standard Experiment
3.4.2 Model Avalanche Class
3.4.3 Surface Morphology and Structures
3.4.4 Plan View Shape
3.4.5 Subsurface Deformation
3.4.6 Hummocks
3.4.7 Sequence of Events
3.5 Discussion
3.5.1 Avalanche Characteristics
3.5.2 Hummock Description
3.5.3 Avalanche Stages and Hummock Formation
3.5.4 Structural Models: Layers and Structural Interface
3.6 Conclusions
4 The anatomy of avalanche hummocks
4.1 Abstract
4.2 Introduction
4.2.1 Hummock Interpretations
4.2.2 Hummock Types
4.2.3 Iriga volcano and her DADs
4.2.4 Objectives
4.3 Methodology
4.4 Results
4.4.1 Hummock Exploration and Accounting
4.4.2 Field Description
4.5 Discussion
4.6 Conclusions
5 The development of structures in avalanches
5.1 Abstract
5.2 Introduction
5.3 Model Set-up
5.4 Scaling
5.5 Reproducibility, Model Limitation, Initiation
5.6 Results
5.6.1 The Curved Ramp Experiments: Set 1
5.6.2 The Inclined Straight Ramp Experiments: Set 2
5.6.3 Natural Debris Avalanche Deposits
5.7 Discussion
5.7.1 Morphology and Structures in Curved- and Straight- Based Analogue Slides
5.7.2 Development of Structures and Evolution of Morphology in Large Debris Slides
5.7.3 Implications of Analogue Morphology and Structure on Natural Prototypes
5.8 Conclusions
6 Structural and morphological mapping of DAD
6.1 Abstract
6.2 Introduction
6.2.1 Structures and Morphological Features in Analogue and Natural DAD
6.2.2 Remote Sensing and GIS in Mapping DAD
6.2.3 Objectives and Limitations
6.3 Study Sites
6.3.1 Cerro Pular-Pajonales (Chile-Argentina)
6.3.2 Süphan Dağı (Turkey)
6.3.3 Tacna (Peru)
6.4 Methodology
6.4.1 Data Gathering and building the GIS
6.4.2 Topographic Modelling: SRTM DEM and ASTER GDEM139
6.4.3 Colour Composites: Landsat ETM+
6.4.4 RS and GIS Interpretation and Surface Mapping
6.5 Results
6.5.1 Cerro Pular-Pajonales
6.5.2 Süphan Dağı
6.5.3 Tacna
6.6 Dynamics and Kinematics
6.6.1 Cerro Pular-Pajonales
6.6.2 Süphan Dağı
6.6.3 Tacna
6.7 Conclusion
7 Summary and future work
7.1 Introduction
7.2 The Philippine Volcanoes
7.3 Iriga volcano (Philippines) and her 2 DAD
7.4 Hummocks: How They Form, What They Mean, and Anatomy
7.5 Structures in Long-Runout Avalanches and Large Landslides
7.6 Mapping Remote DAD
7.7 Future Work