# Evolution of an oblique volcanic passive margin: The case of Nuussuaq in West Greenland

Huixin Guan, Mansour M Abdelmalak, Min Xu, Laurent Geoffroy

► To cite this version:

Huixin Guan, Mansour M Abdelmalak, Min Xu, Laurent Geoffroy. Evolution of an oblique volcanic passive margin: The case of Nuussuaq in West Greenland. Tectonophysics, 2023, 862, pp.229936. 10.1016/j.tecto.2023.229936. hal-05268985

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# Evolution of an oblique volcanic passive margin: The case of Nuussuaq in West Greenland

Guan Huixin <fnref n="1" />, <fnref n="2" />, *, Abdelmalak Mansour M. <fnref n="3" />, <fnref n="4" />, Xu Min <fnref n="1" />, <fnref n="2" />, *, Geoffroy Laurent <fnref n="5" />

## Abstract :

Most rifted margins involve a certain degree of obliquity. Oblique extension has been well-studied in a magmapoor continental rift setting. However, few studies have addressed the structure and evolution of oblique rifting in a magma-rich continental rift setting. In this study, we used remote sensing and field measurements combined with geophysical data in order to study the tectono-magmatic evolution of seaward-dipping basalt sequences, known also as seaward dipping reflectors (SDRs), along the western Nuussuaq oblique volcanic passive margin. We calculated the directions and dips of lavas within onshore SDRs based on remote sensing and field data and analyzed the offshore SDRs through seismic reflection profiles to obtain a precise 3D structure of the inner SDRs along this margin. Our results show that the development of the inner SDRs can be divided into two stages. During the first stage (the late Paleocene), the oblique extension was partitioned into strike-slip and dip-slip components, and the basalt sequences were mainly bounded by N-S-trending continentward-dipping faults. During the second stage (the early Eocene), a local stress reorientation occurred in the western Nuussuaq and the basalt sequences were mainly bounded by NE-SW-trending continentward-dipping faults. The obliquity of the margin originated from both the reactivation of the inherited Itilli Fault and the magma intrusion into the crust, which weakened the crust and produced a pressure-induced extension orthogonal to the margin.

## Highlights

<footnote n="1">¹ Key Laboratory of Ocean and Marginal Sea Geology, South China Sea Institute of Oceanology, Innovation Academy of South China Sea Ecology and Environmental Engineering, Chinese Academy of Sciences, Guangzhou 511458, China</footnote>

<footnote n="2">² Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou), Guangzhou 511458, China</footnote>

<footnote n="3">³ University of Oslo, Department of Geosciences, P.O. Box 1047, Blindern, NO-0316 Oslo, Norway</footnote>

<footnote n="4">⁴ Volcanic Basin Energy Research (VBER), Høienhald, Blindernveien 5, 0361 Oslo, Norway</footnote>

<footnote n="5">⁵ Université de Bretagne Occidentale, CNRS UMR 6538, Laboratoire Domaines Océaniques, 29280 Plouzané, France</footnote>

<footnote>* Corresponding authors : Huixin Guan, email address : huixinguan@scsio.ac.cn ; Min, Xu, email address : mxu@scsio.ac.cn</footnote>
► The successively emplaced inner seaward dipping reflector wedges (SDRs) exhibit an angular relationship on the western Nuussuaq margin. ► Inner SDR development divided into two stages. 1) Late Paleocene: lavas bounded by N-S faults. 2) Early Eocene: primarily NE-SW faults. ► Oblique extension involved strike-slip and dip-slip components in the 1st stage, followed by a local stress reorientation in the 2nd stage. ► The obliquity of the Nuussuaq margin resulted from reactivation of the Itilli Fault and magma intrusion into the crust.

**Keywords** : Volcanic passive margin, Oblique extension, Seaward dipping reflectors, Strain partitioning, Western Greenland
# 1. Introduction

The obliquity of a rift or a rift segment is measured by the angle between the regional trend of the extension and the orthogonal to the long axis of the rift (e.g., Withjack and Jamison, 1986; Tron and Brun, 1991; Dewey et al., 1998; Agostini et al., 2009; Autin et al., 2010, 2013; Brune et al., 2018). Both analog and numerical models have demonstrated that oblique rifting can facilitate breakup processes (e.g., Corti et al., 2013b; Brune, 2014). More than 70% of global rifted margins show an obliquity exceeding 20° (Brune et al., 2012, 2018; Philippon and Corti, 2016). At non-volcanic passive margins (NVPMs) where most of the structures can easier be identified and mapped through seismic data and field studies, rift obliquity is often studied based on stress distribution and strain partitioning pattern (e.g., Corti et al., 2001, 2013b; Morley et al., 2004; Philippon and Corti, 2016; Schiffer et al., 2020). The oblique NVPMs are usually strongly influenced by inherited pre-rift structures (e.g., Morley et al., 2004; Corti et al., 2013a; Nonn et al., 2017; Bonini et al., 2019). However, on volcanic passive margins (VPMs), where the crust and associated sedimentary basins are largely intruded by magmatism and the margin is strongly covered by extruded volcanic product. The development and evolution of oblique rifting are still unclear.

The central West Greenland margin is characterized by well-exposed seaward-dipping volcanic wedges that correspond to the seaward-dipping reflectors (SDRs) in seismic reflection data (Geoffroy et al., 1998, 2001; Abdelmalak et al., 2012; Chauvet et al., 2019) (Fig. 1). This margin displays a zig-zag pattern with three distinct segments, namely from north to south, the Svartenhuk Halvø, Nuussuaq, and Disko segments (Geoffroy et al., 2001) (Fig. 1b). The SDRs that cover this region are of “inner SDRs type” (Geoffroy et al., 1998, 2001) that consist of thick wedges of seaward-dipping volcanic flows emplaced in a subaerial environment (Planke et al., 2000; Abdelmalak et al., 2016). Two of those segments, the Disko and Nuussuaq segments, are oblique to the regional trend of syn-magmatic extension (~N070°E) during the Paleogene
(Geoffroy et al., 2001), making this case example a unique one to study an exposed oblique VPM segment.

<figure><img src="image_2.png" /><figcaption>**Fig. 1.** (a) Geological and structural map of the Labrador-Baffin region (modified from Larsen et al., 2016, Abdelmalak et al., 2019, 2023, and Chauvet et al., 2019). (b) Simplified geological map of the central western Greenland area (adapted from Abdelmalak et al., 2012). The purple dashed line represents the plane of an offshore continentward-dipping fault, the Offshore Nuussuaq Fault (ONF), identified from seismic data.</figcaption></figure>

In this contribution, we present results from an integrated interpretation using onshore measurements and observations, remote sensing, and offshore seismic
reflection datasets to investigate the spatio-temporal evolution of the oblique volcanic rift segment exposed in Nuussuaq, central West Greenland. We discuss the 3D onshore and offshore geometry of the inner SDRs, the strain partitioning pattern, the syn-magmatic stress evolution, and finally present a schematic model to describe the tectono-magmatic evolution of this oblique margin segment during the magmatic breakup.

## 2. Geological setting

In the NW Atlantic system, seafloor spreading began in the southern part of the Labrador Sea between C31 (~70 Ma) and C27 (68-62 Ma) (Roest and Srivastava, 1989; Chalmers and Laursen, 1995; Dickie et al., 2011; Hosseinpour et al., 2013), then propagated northward across the inherited Ungava Fault Zone (UFZ) into the Baffin Bay (Chauvet et al., 2019) (Fig. 1a). Onshore, the central West Greenland margin documents the transition between a faulted Cretaceous sedimentary basin to a volcanic-type passive margin (e.g., Geoffroy et al., 2001; Abdelmalak et al., 2012; Guan et al., 2019). The different structures such as the border faults along the basement, the Cretaceous-early Paleocene sedimentary strata, and overlapping and sealing of the sedimentary basin by Paleogene volcanism can be observed in an area of about 200 km by 300 km (Abdelmalak et al., 2019). Unlike Svartenhuk and Disko, which are dominated by Paleocene basalts (e.g., Larsen and Pulvertaft, 2000; Skaarup and Pulvertaft, 2007; Larsen et al., 2016; Chauvet et al., 2019), the western part of the Nuussuaq Peninsula (Fig. 1b) exposes more recent basalts of Eocene age (e.g., Chalmers et al., 1993; Storey et al., 1998; Larsen et al., 2016). The whole volcanic succession in western Nuussuaq is composed of three units: the Vaigat Formation (62-61 Ma), the Maligat Formation (61-60 Ma), and the Kanisut Formation (56-54 Ma) (Hald, 1976; Storey et al., 1998; Larsen et al., 2016; Pedersen et al., 2017; Chauvet et al., 2019). The western part of the peninsula is bounded eastward by a conspicuous fault zone, including the Itilli Fault. The Itilli Fault, trending N37°E, is a major discontinuity of uncertain age that was active during the Late Cretaceous (Dam and Sonderholm, 1994) or early Eocene (Chalmers et al., 1999; Wilson et al., 2006). It shows a kilometer-
scale (>2 km) downthrow to the northwest during the Cenozoic. To the east of the Itilli Fault, the basal volcanic formations (belonging to the Vaigat Formation) are mainly composed of hyaloclastites overlain by horizontal plateau basalts (Chalmers, 2000). These volcanic formations cover a Precambrian metamorphic basement overlain by fluvio-deltaic to marine sediments of Early Cretaceous to Danian age (e.g., Hald, 1976; Chalmers et al., 1995; Storey et al., 1998; Larsen and Pulvertaft, 2000; Dam et al., 2009; Larsen et al., 2016).

Oceanward, to the west of the Itilli Fault, all volcanic successions were involved in the development of conspicuous inner SDRs with strong internal deformation of late Paleocene-early Eocene age (e.g., Geoffroy et al., 2001; Chauvet et al., 2019). Many inherited faults, from Cretaceous-Danian basins and the basement, were reactivated during the Paleogene extension, notably the basin border faults (Chalmers et al., 1993; Larsen and Pulvertaft, 2000; Wilson et al., 2006; Abdelmalak, 2012; Peace et al., 2018).

Based on the mapping of the regional principal minimum stress (σ3) inferred from numerous dykes and fault-slip data, Abdelmalak et al. (2012) concluded that the regional direction of σ3 changed from N60-70°E to N115°E in the early Eocene across Western Nuussuaq. They proposed that the Itilli Fault had accordingly been transiently activated as a transtensional sinistral shear zone during the late Paleocene and as a dip-slip normal fault during the early Eocene. According to this view, the Itilli Fault, which possibly formerly controlled the late Cretaceous W-Greenland sedimentary basin geometry, acted as a major disruptive factor of the stress field during the VPM edification.

### 3. Methodology

Onshore field observations and measurements, remote sensing data, and offshore seismic reflection data were integrated in this study. Onshore field measurements were collected during a scientific expedition in the western part of Nuussuaq (west of the Itilli Fault) in 2015 to complete a former structural analysis (Geoffroy et al., 1998, 2001; Abdelmalak, 2010). In addition, we also incorporated published data and data from
existing regional geological maps are also incorporated in this study (Escher et al., 1971; Escher and Larsen, 1971; Hald, 1976; Rosenkrantz et al., 1976; Geoffroy et al., 2001; Skaarup and Pulvertaft, 2007; Abdelmalak, 2010; Larsen et al., 2016; Chauvet et al., 2019). Onshore observations were completed by information extracted from the digital elevation model (DEM, resolution 3 m×3 m, downloaded from open access website https://lta.cr.usgs.gov/get_data) and combined with Spot 6 satellite images (resolution 1 m×1 m) using the Arcgis™ software to determine the dip and strike of lava flow from their structural configuration (Fig. 2).

<figure><img src="image_3.png" /><figcaption>**Fig. 2.** (a) Spot6 satellite image of the studied area. The triangles correspond to planes of the identified lava flows. b) Zoom on one of the identified lava flows. Three points were picked on the same lava flow to define its plane (see a development of the methodology in Jaud et al., 2022). c) The directions of the T-shape markers correspond to the lava direction and the figures are lava dips. Note that the directions of the lava flow show a large variation (>50°) along this part of the margin.</figcaption></figure>

Offshore, the shallow sedimentary and volcanic formations were studied from the Nuussuaq Seis 2000 seismic survey acquired by the Geological Survey of Denmark and Greenland in central west Greenland, which has been partly interpreted and published in previous works (Marcussen et al., 2001; Skaarup and Pulvertaft, 2007; Abdelmalak, 2010). The survey was carried out using a relatively short streamer (96-groups, 594 m long) allowing more flexibility to avoid icebergs, the removal of seabed multiples, and the acquisition of high-resolution data (Marcussen et al., 2001). However, the only limitation is the shallow depth of penetration. It is only possible to see the first kilometer
of the basalt section before the seabed multiple obscures deeper reflections (Marcussen et al., 2001; Skaarup and Pulvertaft, 2007).

# 4. Observations and interpretations

## 4.1. Onshore western Nuussuaq Peninsula

Based on remote sensing data, we have calculated the direction and dips of 141 lava flows in all three onshore basaltic formations (Figs. 2 and 3, including 116 in the Kanisut Formation, 11 in the Maligat Formation, and 14 in the Vaigat Formation with uncertainties less than 3°). In the Vaigat Formation, most lavas show an approximately N-S trend (Fig. 3a). In the Maligat Formation, lavas near the Itilli Fault show a NE-SW direction while most of them to the west generally display an N-S direction similar to those in the Vaigat Formation. In the Kanisut Formation, lavas generally display an N-S direction in the northwestern part and bend to an average N45° direction in the southwest (Fig. 3a). West of the Itilli Fault, lavas dip toward the west and the northwest. The strongest dips are observed in the Vaigat Formation and decrease upwards in the Kanisut Formation (Figs. 3b, c, and d). To the east of the Itilli Fault, the lava piles in the Vaigat Formation are generally horizontal. However, to the west of the Itilli Fault, the fault looks to exert a major control on lava dips where an average dip of approximately 50° is measured in the south (Fig. 3c) and 30° in the north (Fig. 3b). In the Maligat and Kanisut formations, lavas manifest a sub-horizontal dip near the Itilli Fault and lava dips augment to approximately 20-25° oceanward. The dipping direction of the basaltic units demonstrates an angular relationship changing over time.
<figure><img src="image_4.png" /><figcaption>**Fig. 3.** (a) Geological map of the western Nuussuaq Peninsula illustrating lava flow directions. The solid T-shape marks represent local lava dips. The orange/black dashed lines are iso-dip lines of upper Paleocene/lower Eocene lavas. The Itilli Fault bifurcates into two branches in its northeastern part. A close-up view of the northern coast (b) and the southern coast (c). The figures represent lava dips. d) Two cross-sections along the northern (1) and southern (2) coasts illustrate the structure of inner SDR formations (adapted from Chauvet et al., 2019). All lava directions and dips are from remote sensing data, field data, and Escher et al. (1971), Hald (1976), Geoffroy et al. (2001), and Abdelmalak (2010).</figcaption></figure>

Lavas are highly tectonized in the Vaigat Formation (including both normal faults which show a few meters to tens of meters downthrow to the southeast, and strike-slip faults), less tectonized in the Maligat Formation and much less tectonized in the Kanisut
Formation (we can rarely identify any faults onshore in the Kanisut formation) (Fig. 3b, c). Numerous continentward-dipping syn-magmatic normal faults are observed showing apparent low angles, which were probably initiated with a dip >60° and then tilted to a low angle by syn-magmatic flexing. The continuity of those faults in the Maligat Formation is uncertain due to the outcrop condition. The contact between the Vaigat and Maligat formations is an angular discordance above an erosional surface (Fig. 3d). The Maligat Formation thins strongly toward the east and is mainly composed of massive flows including an alternation between lava flows (average thickness of less than 2 m) and paleosol (average thickness of 0.5 m) (Hald, 1976). A sedimentary unit, the Ifsorisok Formation, is located between the Maligat and Kanisut formations. It is mainly composed of volcanicastics and tuffs, which indicate a pause of 1-2 million years during volcanic extrusion (Hald, 1976, Larsen et al., 2016; Chauvet et al., 2019). Oceanward, the Kanisut Formation consists of lava flows and/or sill intrusions with clear cooling joints. Near the Itilli Fault, the Kanisut Formation lies directly on top of the Vaigat Formation, showing an angular discordance (Fig. 3d). Within each basaltic formation, lava dips decrease oceanward, in accordance with observations from remote sensing data. Structurally, the normal faults are dipping continentward and offsets of several hectometers are observed in both cross-sections along the northern and southern coasts outcrops (Fig. 3d).

## 4.2. Offshore western Nuussuaq

Offshore, the seismic reflection data show a series of discontinuous high-amplitude reflections within a range of 0.7-1 s two-way traveltime (TWT), displaying a successive northwestward-dipping wedge shape (Fig. 4). These reflectors are interpreted as offshore SDR units. Each SDR wedge (or sub-wedge) abruptly terminates along a more transparent area which could be interpreted as the footwall of a normal fault dipping continentward. The SDR wedges generally show an oceanward rotation larger than 30°. Based on seismic reflection data, we mapped out a major continuous fault trace which is named hereafter the Offshore Nuussuaq Fault (ONF) (Fig. 1b). It presents an average dip of approximately 20°-40° toward the continent. It appears to be
connected to the onshore Arfertuarssuk Fault in Svartenhuk Halvø to the north (see Geoffroy et al., 2001 and Chauvet et al., 2019) and it parallels the Itilli Fault in the south (Fig. 1b). Since the Arfertuarssuk Fault represents the master fault bounding the Svartenhuk SDRs, we suggest that the ONF also represents the major fault that bounds the Kanisut SDRs which is divided into subunits by several satellite faults. The minor offshore faults generally show a NE-SW direction manifesting the same direction as the onshore faults observed within the Kanisut Formation. This can be regarded as an indication of a similar tectonic deformation between the onshore and offshore units. Thus, we suggest that these offshore SDRs were also emplaced in the early Eocene.

<figure><img src="image_5.png" /></figure>

**Fig. 4.** Interpretation of part of Lines 12 and 14 (locations are shown in Fig. 1b). Black/red solid/dashed lines correspond to continentward-dipping faults bordering offshore SDRs. The major fault is colored in red. The orange line corresponds to an offshore strike-slip fault. The pink zone represents offshore basaltic units. The bottom of the basalt units is far down according to drill data. The unit to the west of the major fault also consists of basalt but the nature of the crust remains unclear.

The sediments overlying the offshore SDRs show continuous reflections and are not affected by continentward-dipping faults. These sediments are of glaciogenic origin (Chalmers, 2000) (Fig. 4). The overall structure of the offshore SDRs is comparable to
those of onshore inner SDRs in the western Nuussuaq and Svartenhuk Halvø (Chauvet et al., 2019).

# 5. Discussion

## 5.1. Development of inner SDRs on the western Nuussuaq VPM

Based on our onshore/offshore observations, we divide the evolution of the western Nuussuaq VPM into 2 stages:

**Stage A:** During the late Paleocene, lavas of the Vaigat and Maligat formations showed generally an N-S direction (Fig. 5a, b). The strongly tectonized Maligat and Vaigat formations indicate that continental extension was mainly accommodated by crustal stretching and thinning through faulting. The magmatic intrusions were mainly in the form of dykes that fed the upper sequences. The N-S-trending lavas were bounded by N-S-trending continentward-dipping faults as observed onshore to the west of the northern Itilli Fault (Fig. 3a) and offshore in the Nuussuaq Basin (Chalmers et al., 1999) (Fig. 1b). The Itilli Fault acted as a transtensional fault and accommodated the deformation with both down-dip and possibly sinistral shear motion. At this stage, σ3 showed an N70° direction (Abdelmalak et al., 2012) (Fig. 5a). Strain partitioning occurred in western Nuussuaq, and deformation was accommodated by both strike-slip and normal faults (see Figure 5 in Abdelmalak et al., 2012).

**Stage B:** During the early Eocene, faults bounding the Maligat and Vaigat formations became inactive (Geoffroy, 2005). Lavas formed the Kanisut Formation (Fig. 5c, d). The minimum horizontal stress σ3 progressively reoriented perpendicular to the Itilli Fault. NE-SW-trending faults developed both onshore and offshore (Figs. 3a and 4). According to fault-slip data, deformation onshore the western Nuussuaq was mainly accommodated by normal faults (see Figure 5 in Abdelmalak et al., 2012). We propose that both the offshore volcanism and the onshore Kanisut Formation were controlled by the ONF, which is perpendicular to the minimum horizontal stress acting to the west of Nuussuaq. Thus, a pure dip-slip component is expected along this fault as along the Arfertuarssuk Fault further north in Svartenhuk Halvø (Fig. 1b).
<figure><img src="image_6.png" /><figcaption>**Fig. 5.** Simplified schema describing the emplacement of mafic volcanism on the western Nuussuaq margin during the Paleocene-Eocene. Figures (a) and (c) illustrate the trajectories of σ3 regarding the distribution of volcanism. Figures (b) and (d) illustrate the emplacement of the Vaigat, Maligat, and Kanisut formations. The development of the inner-SDRs wedges on the western Nuussuaq margin is consistent with the model proposed by Geoffroy (2005) and further described by Geoffroy et al. (2015), which explains the progressive seaward development of inner-SDRs wedges bounded by continentward-dipping detachment faults at paired VPMs. The old continentward-dipping faults gradually became inactive while new faults formed seaward (Geoffroy, 2005; Gernigon et al., 2020).</figcaption></figure>

**5.2. Comparison with non-volcanic oblique extension**

Oblique extension has been studied in natural cases, such as eastern Africa, the Gulf of California, and western Thailand (Morley et al., 2004; Aragón-Arreola et al., 2005; Clifton and Kattenhorn, 2006; Corti, 2008; Sutherland et al., 2012; Autin et al., 2013; Bonini et al., 2019) and in numerical and analog models in magma-poor continental rifting setting (e.g., Withjack and Jamison, 1986; Tron and Brun, 1991; Corti et al., 2001, 2013a, b; Agostini et al., 2009; Autin et al., 2010; Brune, 2014; Zwaan
et al., 2016; Zwaan and Schreurs, 2017; Duclaux et al., 2020). In these models, the obliquity is produced by the implementation of a pre-existing weak zone, which displays a rheological contrast within the lithosphere. In an oblique extension where little or no volcanism is present, the deformation is usually partitioned into different populations of faults including dip-slip normal faults, and strike-slip faults, and the strain partitioning pattern is strongly controlled by the obliquity (e.g., Corti et al., 2001; Morley et al., 2004; Corti, 2008; Agostini et al., 2009; Brune et al., 2012; Philippon and Corti, 2016). An obliquity of 45° is generally considered a threshold between local stress reorientation and strain partitioning, however, the strain partitioning pattern remains debated in different models (e.g., Dauteuil and Brun, 1996; Corti et al., 2001; Morley et al., 2004; Corti, 2008; Agostini et al., 2009; Brune, 2014; Philippon and Corti, 2016). Based on numerical modeling, Brune et al. (2014) and Philippon and Corti (2016) suggested that oblique extension tends to reorient locally within the weak zone when the obliquity is low, while it tends to partition into strike-slip and dip-slip components when the obliquity is high. However, based on analog models, Agostini et al. (2009) proposed that strain partitioning is facilitated when the obliquity is low. They suggested that when the obliquity is lower than 45°, boundary faults first develop with an average direction oblique to both the rift axis and the orthogonal to extension direction (Fig. 6a). Then internal faults develop showing a direction orthogonal to the extension (Agostini et al., 2009). When the obliquity is higher than 45°, the deformation becomes more concentrated within the rift axis, and the deformation is mainly accommodated by oblique-slip faults (Fig. 6b, Agostini et al., 2009). Both numerical and analog models demonstrate an inward migration of deformation from the boundary of the weak zone toward the rift axis (Dauteuil and Brun, 1996; Agostini et al., 2009; Brune, 2014; Philippon and Corti, 2016; Bonini et al., 2019).
<figure><img src="image_7.png" /><figcaption>**Fig. 6.** (a) Evolution of fault pattern in the nonvolcanic oblique extensional system when the obliquity is low. (b) Evolution of fault pattern in the nonvolcanic oblique extensional system when the obliquity is high. (a) and (b) are modified from Agostini et al., 2009 and Philippon and Corti, 2016. (c) Evolution of fault pattern on western Nuussuaq margin (this study).</figcaption></figure>

In our case study, the Nuussuaq margin shows a strong obliquity of 57° (Abdelmalak et al., 2019). The Itilli Fault is a northward extension of the Ungava Fault Zone which is possibly associated with an inherited crustal structure from a Palaeoproterozoic collision (Gower, 1996; Heron et al., 2019, 2023). A mantle lithosphere suture is also proposed to exist underneath this region which can be considered a rheological weak zone (Heron et al., 2019). During the Paleocene, the Itilli fault developed as a transtensional fault showing both strike-slip and normal motion. The strain was partitioned into strike-slip and dip-slip components (Fig. 6c). The dip-slip component is accommodated by several N-S-trending normal faults bounding syn-tectonic Vaigat volcanics to the west of the northern Itilli Fault. Those faults show a direction oblique to both the rift and the regional extension. The strike-slip component
was accommodated by a sinistral motion along the Itilli fault. This fault pattern presents some similarities to the nonvolcanic oblique extension of low obliquity in the model by Agostini et al. (2009) and the nonvolcanic oblique extension of high obliquity in Philippon and Corti's (2016) model (Fig. 6b).

Later in the early Eocene, the deformation migrated oceanward. A series of NE-SW-trending faults developed both onshore within the Kanisut Formation and offshore. These faults and the Itilli Fault all present an orthogonal direction concerning the σ3 and show pure dip-slip motion (Fig. 6c). The stress reorientation pattern is similar to the oblique extension with low obliquity in Philippon and Corti's model (Fig. 6a, Philippon and Corti, 2016). This local stress reorientation possibly resulted from the reactivation of the Itilli Fault itself (see section 5.3). Another possible explanation is that the mantle thermal anomaly that arrived during the development of VPMs has enhanced the emplacement and intrusion of considerable magma into the crust. These magmatic materials softened and localized the deformation along the margin during extension by forming weak zones, which also led to the local stress reorientation (Gudmundsson et al., 2014).

**5.3 Origin of the obliquity**

Oblique extension usually has two origins: (1) reactivation of inherited structures trending oblique to the kinematics of extension and (2) rotation of the extension direction during the evolution of rifts (Morley et al., 2004; Agostini et al., 2009; Bonini et al., 2019; Maestrelli et al., 2020; Farangitakis et al., 2020; Sapin et al., 2021). According to Homberg et al. (1997), when a discontinuity trends obliquely to the stretching direction (the angle between the maximum horizontal stress and the discontinuity $0°≤\theta≤90°$), the friction coefficient along the fault is variable which induces a decreasing amount of slip along the fault. This leads to the presence of a contractional zone on one side of the fault and an extensional zone on the opposite side near the northern fault tip (see Figure 6 in Homberg et al., 1997). These contractional and extensional zones trigger the largest deviation of local stress (Homberg et al., 1997; Philippon et al., 2015). When the angle between the maximum horizontal stress and the
discontinuity is larger than 45°, and sliding occurs along the fault, the maximum
horizontal stress tends to become parallel to the discontinuity (Hømberg et al., 1997).
These accords well with our observation along the Itilli Fault. During the emplacement
of the basaltic formations in western Nuussuaq, the regional extension was kept in the
direction of N70° in western Greenland (Abdelmalak et al., 2012, 2019). During the late
Paleocene, the extension triggered the start of the reactivation of the Itilli Fault in a
sinistral motion. During the early Eocene, σ3 became perpendicular to the Itilli Fault.
In addition, when the arrival of considerable magma exerts a major control on the
opening of the axial part of the rift, the intrusion of magma into the crust can produce
a pressure-induced extension orthogonal to the margin (Acocella et al., 2011). Hence,
magma intrusion may also have assisted in the rotation of the local extension direction
during the evolution of the western Greenland margin. We thus propose that the
obliquity of the Nuussuaq margin resulted both from the reactivation of the Itilli fault
and the assistance of magma and that the 3D organization of the successive SDR
wedges, was a direct consequence of the stress field perturbation.

**6. Conclusion**

This study set out to characterize the inner SDR development on the oblique
Nuussuaq VPM. In conclusion, we suggest the following:

(1) On the western Nuussuaq margin, the volcanic formations (the Vaigat, Maligat, and Kanisut formations) constituting the inner SDR wedges were emplaced successively but show an angular relationship. The geometry of each basaltic wedge can be associated with a series of syn-magmatic continentward-dipping faults, which are observed offshore western Nuussuaq but are hampered by younger lavas onshore.

(2) The development of the inner SDRs on the western Nuussuaq margin can be divided into two stages. During the late Paleocene stage (Stage A), lavas in the Vaigat and then Maligat formations were bordered by N-S-trending continentward-dipping faults. The Itilli Fault acted as a transtentional fault. During the early Eocene stage (Stage B), σ3 reoriented clockwise in western Nuussuaq. Lavas in the Kanisut Formation was bordered by a series of continentward-dipping faults among which the
major offshore fault, the ONF, has been identified. The ONF is parallel to the Itilli Fault
to the south and connects to the Arfertuarssuk Fault to the north.

(3) The Nuussuaq margin shows a high obliquity of 57°. Similar to nonvolcanic oblique extension, the Nuussuaq case also shows an inward migration of deformation from the rift boundary toward the rift center. However, the stress distribution and strain partitioning patterns are different. On the western Nuussuaq margin, the oblique extension was first partitioned into strike-slip and dip-slip components during the late Paleocene and then demonstrated a local stress reorientation during the early Eocene.

(4) The obliquity of the Nuussuaq margin originated from both the reactivation of the Itilli Fault and the magma intrusion into the crust. During the late Paleocene, the N37° trending Itilli Fault was oblique to the N70° trending regional extension. This induced a variation in slip amount along both sides of the fault and finally triggered the reactivation of the fault and a local stress deviation in the vicinity of the fault. The magma intrusion into the crust during the development of the Nuussuaq VPM weakened the crust and produced a pressure-induced extension orthogonal to the margin, which also assisted in the rotation of the local extension direction.

**Acknowledgements**

Field data collected in this study was supported by the GRI Marges Volcaniques, a TOTALTM and UBO (Université de Bretagne Occidentale) partnership. We thank François Chauvet and the Vagabond family for the pleasant and efficient assistance, both at sea and in the field. We thank the GEUS for the permission of using seismic data. We are grateful to Editor Gideon Rosenbaum and two reviewers, Alex Peace and Christian Berndt, for their valuable comments and helpful advice that improved the quality of this paper. This work was supported by the National Natural Science Foundation of China (42002222, 41730532, 91858207, 41890813, 91958212), Guangdong Basic and Applied Basic Research Foundation (2021B1515020023), and (SL2022A04J00462).
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