Geomorphology of the Clarion Clipperton Zone, tropical North Pacific Ocean

ABSTRACT The Clarion Clipperton Zone (CCZ) hosts a valuable deposit of polymetallic nodules. Understanding the geology of this deposit is key to its effective exploration and future mining. Interpretation of satellite derived bathymetry elucidates large scale geomorphological units and structural units that characterize the area. The Main Map is includes distinct plains, rises, and lows of abyssal hills, steps and a trough along parts of the bounding fracture zone, seamount chains, fault zones and rifts.


Introduction
The release of an updated global bathymetric product by GEBCO (British Oceanographic Data Centre, 2020) provides an opportunity to interpret the regional and local seabed bathymetry of the Clarion-Clipperton Zone (CCZ) oceanic plate segment at a large scale (1:6,000,000). The CCZ segment is a 6000 × 1000 km region in the tropical North Pacific (Figure 1). It contains polymetallic nodules, which accumulate manganese, nickel, copper and cobalt to levels that have reasonable prospects for eventual economic extraction (Menard & Shipek, 1958). In combination with other data, interpretation of the global bathymetric product (GEBCO grid) elucidates the geomorphological structure within the CCZ and, the systematics of this structure helps explain the formational history of the plate segment.
The interpreted age of the seafloor over the extent of the CCZ increases from east to west (Figure 1), consistent with increasing distance from the East Pacific Rise spreading centre (EPR). The seafloor deepens progressively from about ∼4200 m at 115°W to ∼4800 m at 128°W, then it varies between ∼4800 m and ∼5500 m until 161°W (Figure 1). The CCZ has other plate segments to its south (Clipperton -Galapagos segment) and north (Molokai -Clipperton segment), and there is a general increase in depth from south to north between the three segments ( Figure  1). To the west the CCZ is bound by the Line Islands rise; further west is the Central Pacific Basin and the SE corner of the Pacific Triangle (Boschman & van Hinsbergen, 2016). To the east the CCZ is bound by the Mathematician Seamounts, further east is the EPR and the Rivera and northernmost Cocos microplates (Manea et al., 2013).
Polymetallic (also called ferro-manganese) nodules are found in all of the world's oceanic basins (McKelvey et al., 1983). The high Ni, Co, Cu, Mn grades in CCZ nodules are thought to result from: significant metal supply via net biological carbonate-silicate export from the surface; metal release near the nodules at the seabed via a combination of water depth relative to the lysocline; and distance from continental land masses resulting in an absence of competitor detrital sediments (International Seabed Authority, 2010a; Lipton et al., 2016;Morgan, 2003). High grade nodules are found at the seabed surface in a very soft clayoozeseawater mix termed as the semi-liquid layer or geochemically active layer (Bender et al., 1966;Callender & Bowser, 1980). These metals are adsorbed into the open structures of the slowly precipitating phyllomanganate minerals within the nodules. The 4500 km by 500 km extent of the CCZ nodule deposit can be approximated from exploration contracts issued by, as well as reserved areas held by, the International Seabed Authority (ISA) (International Seabed Authority, 2020), Figures 1 and 2. No prior regional scale geomorphological maps of the CCZ are known. (Neizvestnov et al., 2004) and (Kotlinski et al., 2009) published a lineament and feature map and (Kazmin, 2003) and (International Seabed Authority, 2010b) reference some of the bathymetric rises and other features in the map (e.g. Eastern Central and Cooper Rises). CCZ scale maps of nodule chemistry, abundance, seabed sediment type are contained in (International Seabed Authority, 2010a) and (International Seabed Authority, 2010b). The interpretation described here is compatible with the higher-level global seabed classification of (Harris et al., 2014), and the habitat classification of (McQuaid et al., 2019).
The seabed in the CCZ is mostly composed of basaltic abyssal hills. These are typically covered in a variety of siliceous and calcareous sediment, e.g. (van Andel et al., 1975;Pälike et al., 2009). In much of the CCZ, thin and often poorly consolidated siliceous clays and oozes (Neogene to modern Clipperton Oceanic Formation) overly thicker calcareous nannofossil chalk (Eocene to Neogene Marquesas Oceanic Formation), that formed when the crust was located further south in the equatorial region (International Seabed Authority, 2010b). A third, mostly siliceous sedimentary unit is also found in the western part of the CCZ (Line Island Oceanic Formation).
The majority of abyssal hills are interpreted to have formed via normal faulting (i.e. horst and graben structures) in turn resulting from shrinking and subsiding driven by progressive plate cooling after formation at a spreading centre (Macdonald et al., 1996). Abyssal hills are not apparent in most of the GEBCO grid and can only be seen in 12 kHz multibeam coverage and along some ship-tracks. The hills are typically: around 100-300 m high (trough to crest); have a frequency of 1-5 km; and strike for ten to several hundred kilometres. In the central and eastern CCZ the abyssal hills mostly strike 350°( perpendicular to the direction of current plate spreading), while in the western CCZ the hills strike mostly N-S ( Figure 3). Interspersed hills with an overall secondary strike are found in some areas, most commonly to ±010°(i.e. roughly perpendicular to the direction of plate motion).
A range of other morphological features are present and mostly only resolvable in the 12 kHz bathymetry and/or backscatter. These include: . volcanic knolls from ∼50 m in height, up to seamounts that rise ∼4 kilometres above the surrounding seabed; . outcropping dyke/fissure lavas and sheet flows that are interpreted to have formed in relatively recent times (i.e. post much of the sediment record); and . potholes in the Marquesas Formation carbonate strata, see also (Fouquet & Depauw, 2014;Mayer, 1981;Moore et al., 2007). Note that seabed carbonates over much of the CCZ are currently unstable at this water depth relative to the interpreted position of the lysocline (Broecker, 2003;Chen et al., 1988).

Methods
The new interpretation in the map (also Figure 4) is based on the following data sources (e.g. Figures 3 and 4): . 2014 and then 2020 GEBCO grids from (British Oceanographic Data Centre, 2014) and (British Oceanographic Data Centre, 2020) respectively. Within the CCZ the GEBCO, 2020 grid is 87% an interpolated value guided by satellite-derived gravity data, 11% from provided multibeam data and 2% from soundings (that serve to constrain the satellite data); . 12 kHz bathymetric images from public sources i.e. (Charlet et al., 2015 (Kim & Wessel, 2011;Yesson et al., 2011).
Note that ship-track data is incorporated into the GEBCO grids, but the original track data-products are of higher resolution and were used in the interpretation.
The GEBCO bathymetric grids were processed into 100 m contour maps and colour gradient maps for use in imaging processing software and geographic information systems (GIS; Geosoft, ArcGIS and qGIS were all used). The 2020 grid benefits from finer resolution (15 arc seconds versus 30 for the 2014) although this finer point resolution means increased 'contour noise' with regards to ship-tracks. Also integrated is 12 kHz multibeam survey area from the Belgian sponsored ISA contractor GSR. The published 12 kHz bathymetric images were georeferenced and included into the GIS projects as needed, using grid data, contract vertices or natural features.
The contour data was analysed visually and after some experimentation with different depth intervals, a set of primary contours at 400 or 500 m depth intervals selected for their ability to elucidate discrete units within the CCZ (both 2014 and 2020 GEBCO grids; Figure 4). The specific contours selected for the final interpretation are: . 4000 m; . 4500 m; . 4900 m; and . 5400 m.
The colour gradient maps helped more with picking boundaries as well as other lineaments. The gradients were also sectioned to support the interpretation further. The area of each unit was measured in GIS using a Goode homolosine projection.
CCZ wide interpretation was then made using the GEBCO grids with occasional reference to the 12 kHz images. Boundaries between features and domains as well as other structural features (lineaments and potential faults) were interpreted directly into the GIS package and the final maps thus are principally qualitative. Interpretation is generally completed at screen-scales 1 of between 1:1,000,000 and 1:2,000,000 and the final map is at 1:6,000,000. Comparison and validation of the main interpreted units was also made using hypsometric curves from the GEBCO, 2020 grid.
The interpretation of this map can be classified within the thousands of km 2 Tectonic Setting

Subcomponent of the Coastal and Marine Ecological
Classification Standard (CMECS (US Federal Geographic Data Committee, 2012)). This allows associated or future work, being done at a variety of scales and on a variety of themes (e.g. towed photo profilebased habitat mapping), to be integrated into the same management system.

Results
Seven types of tectonic setting geomorphological units are defined. The terms used are somewhat specific to the setting and scale of the mapped area ( Figure 5), so for example differ from the terms used at the global scale by (Harris et al., 2014): (1) Plainsi.e. of abyssal hills and valleys that comprise most of the seafloor; (2) Transitiona plain with an overall west facing slope that in effect separates the eastern and central parts of the CCZ; (3) Lowsdiffuse to distinct areas of abyssal hills at slightly greater depths, some associated with seamount chains; (4) Rises (Crust)diffuse to distinct areas of variably defined abyssal hills at slightly shallower depths; (5) Rises (Volcanic)areas of volcanic seamounts/ knolls associated with slightly elevated abyssal hills; (6) Stepsslightly shallower blocks aligned along the main fracture zones; (7) Troughnarrow, slightly deeper area aligned along part of one of the main fracture zones.
Each of the above units are often defined by depths relative to each other, or by the presence of a major natural feature such as a fault. This is primarily because the CCZ seafloor as a whole deepens westwards to about 128°W (Figures 1 and 4). The boundary between, for example, a 'low' and a 'plain' in the Eastern CCZ may be some 500 m shallower than the same type of boundary in the western, older and deeper part of the CCZ. The visual interpretation used local contour intervals as a guide as well as lineaments. Knolls and seamounts are typically several hundred to thousand m shallower than their host units.
Newly defined tectonic setting geomorphological units are mostly informally named after the International Radiotelephony Spelling Alphabet, for example, Alpha, Bravo, Charlie, etc. Features with existing names are maintained (e.g. GEBCO, 2020; International Hydrographic Organization, 2008; International Seabed Authority, 2010a). As the interpretation evolved some interim units were changed or removed (i.e. merged with a neighbouring unit), thus there are some gaps in the alphabet-name sequence. Thus all names used are informal per (International Hydrographic Organization, 2008) unless indicated in the text below as being gazetted.
Most of the seabed is composed of the twelve discrete Plains and Transition, which as mentioned above deepen from the eastern end of the CCZ towards the central-western part of the CCZ, but then shallow again in the south towards the Line Islands Rise. The Transition also marks a key change from the eastern to central CCZ. The composition of these units is mostly abyssal hills but also include the above-mentioned smaller features such as volcanic cones, deeper trough grabens, and potholes.
The Plains are crosscut by Rises and Lows, which respectively comprise shallower and deeper seabed regions of abyssal hills. Both rises and lows may have knoll/seamount chains associated with them. Conditions relating to a given unit are often more complex or specific; for example, the Cooper Rise is distinctly different to the other rises, including numerous seamount chains striking at a high angle to the rise itself.
Adjacent to the main fracture zones are Steps and a Trough. These narrow blocks typically have an interpreted smaller bounding fault inside the CCZ, that reflect local vertical displacement related to the dominantly transform fracture zones. The occurrence and form of the steps and the trough are influenced by changes in the strike of the fracture zones and with the contact of rises and lows with the fracture zones.
The accuracy of the interpretation based solely on the GEBCO grid, can be tested by comparing the cases where 12 kHz MBES is also available, i.e. in terms of continuity of units over and across the edge of the 12 kHz coverage. While the interpretation process was iterative between the datasets in some areas, normally this was not needed (the GEBCO grid alone was clear enough). An example of this correlation is provided below where east-west trending ridges formed at the north-western end of the Cooper Rise ( Figure 6) were interpreted from GEBCO grid. When the boundaries are overlain on higher resolution multibeam data, the correlation between the ridges and the interpreted boundaries shows good visual corroboration. Similarly, the central part of Hotel Low is clearly expressed in both data sets. This supports the proposed suitability of the GEBCO grid for interpreting seafloor features and accurately distinguishing between morphotectonic zones at the scales considered here.
In association with the above-mentioned interpreted tectonic setting geomorphological units, the following structural element types were also defined and interpreted ( Figure 5): (1) Major transform fracturesthe Clarion and Clipperton Fracture Zones that bound the plate segment; (2) Fault zonessmaller clear breaks in the seafloor fabric the most significant being the Mahi Mahi Fault zone; (3) Axes of the above defined geomorphological Lows; (4) Axes of the above defined geomorphological Rises; (5) Major volcanic chainsincludes numerous seamounts (>1000 m above the adjacent seafloor) as well as single and composite knolls. Some associated with the Lows; (6) Volcanic chainssingle and composite knolls, and occasional seamounts. Some associated with the Lows and others with the former spreading centre at the Mathematician Seamounts, and; (7) Small rifthigh angle tear-apart like structures.
So far all found near the Clipperton Fracture Zone or subparallel to the Mathematician Seamounts.
Example abyssal hill strikes were also highlighted where possible (i.e. where there was 12 kHz bathymetry or clear ship-track bathymetric data), and this shows a change in abyssal hill strike between the western and central-eastern CCZ (Figure 3).

Conclusions
The CCZ is mapped at a regional scale of 1:6,000,000, based on GEBCO grids, and supported by deep sea drilling programme data, magnetic isochron interpretation and some published 12 kHz MBES coverage. The mapped geomorphological and structural units characterize each part of the CCZ, in terms of age, geomorphology, stratigraphy and structure. This includes definition of geomorphic plains, rises, lows of abyssal hills, and steps and a trough along parts of the bounding fracture zone, as well as seamount chains and internal fault zones and rifts. This interpretation should be of interest to the ISA exploration contractors who work there, as well as their regulators and sponsors.

Software
For the map: GEBCO grids were clipped and contoured in (gdal_contour) in qGIS 3.10.7. Testing via hypsometric curves was done using native installed process in qGIS 3.10.7 and via bathymetric position in index using SAGA GIS Hill climbing clustering algorithm (courtesy Alex Fejer of Fathom Pacific). Interpretation and map compilation was done in Arc-GIS 10.4.1. Terrain profiles for the cross-sections were derived using (Profile Tool 4.1.8) in qGIS 3.10.7, and assembled/interpreted in Inkscape 0.92.3. Map was compiled and set in ArcGIS 10.4.1.
For the figures: Rose diagrams were prepared using GeoRose 0.5.1. Figures were drafted either in ArcGIS 10.4.1, qGIS 3.10.7 or Inkscape 0.92.3. Note 1. All of the digitizing for this map used a 24" 1980 × 1080p display at 1:1,000,000 scale 10 km on the map was about 0.9 cm on the screen.