Initiation of a Proto‐transform Fault Prior to Seafloor Spreading

Abstract

Transform faults are a fundamental tenet of plate tectonics, connecting offset extensional segments of mid‐ocean ridges in ocean basins worldwide. The current consensus is that oceanic transform faults initiate after the onset of seafloor spreading. However, this inference has been difficult to test given the lack of direct observations of transform fault formation. Here we integrate evidence from surface faults, geodetic measurements, local seismicity, and numerical modeling of the subaerial Afar continental rift and show that a proto‐transform fault is initiating during the final stages of continental breakup. This is the first direct observation of proto‐transform fault initiation in a continental rift and sheds unprecedented light on their formation mechanisms. We demonstrate that they can initiate during late‐stage continental rifting, earlier in the rifting cycle than previously thought. Future studies of volcanic rifted margins cannot assume that oceanic transform faults initiated after the onset of seafloor spreading.

Observing Oblique Slip During Rift Linkage in Northern Afar

“…Our relocated seismicity between 2007 and 2008 shows that earthquakes cluster along the main fault system and most of them occur in the area of highest coseismic displacement (Figure S13). Illsley‐Kemp, Bull, et al, , Illsley‐Kemp, Keir, et al, ) observed swarms of seismicity in the same area during 2011–2013. The authors also interpreted focal mechanisms computed using low‐magnitude earthquakes as evidence of oblique right‐lateral faulting along NW striking faults and concluded that these structures accommodate the deformation at AP while the NS striking faults are inactive.…”

“…Eight years of seismicity measurements from a local seismic array have shown continuous low‐to‐moderate seismicity at AP (Belachew et al, ; Ebinger et al, ; Illsley‐Kemp et al, ). The largest event during this time period was the M L 5.1 earthquake on 2 October 2007 (Figure ), with no indication from geodetic data of any associated magma intrusions (Illsley‐Kemp, Bull, et al, ; Pagli et al, ). In contrast, earthquake swarms along the magmatic segments such as EA segment are mainly induced by magma intrusion (Pagli et al, ).…”

Structural Analysis of the Western Afar Margin, East Africa: Evidence for Multiphase Rotational Rifting

“…However, a common challenge to researchers is that once breakup occurs and passive margins are formed, the remnants of the once active continental rift are often situated offshore and mostly covered by thick (postrift) sedimentary sequences (e.g., Argent et al, 2000; Straume et al, 2019). Hence, the Afar region in East Africa represents a unique opportunity to study extensional processes and associated structures (e.g., fault geometries and kinematics); it contains active rifts in various stages of maturity, from initial continental rifting to continental breakup, oceanic spreading, and passive margin formation, all accessible onshore (e.g., Illsley‐Kemp, Bull, et al, 2018; Varet, 2018; Figure 1).…”

“…Another important feature is the presence of ongoing seismic activity along the WAM. Most earthquakes in the region are concentrated along the rift axes of the Red Sea, Gulf of Aden, central Afar and the MER (e.g., Illsley‐Kemp et al, 2017; Illsley‐Kemp, Bull, et al, 2018; Illsley‐Kemp, Keir, et al, 2018; Tesfaye & Ghebreab, 2013, Figure 2a). Some minor activity is registered at the SAM as well, but a significant proportion of the region's seismicity is recorded along the whole of the WAM (e.g., Ayele et al, 2007; Craig et al, 2011; Goitom et al, 2017; Gouin, 1970, 1979; Hofstetter & Beyth, 2003; Illsley‐Kemp, Keir, et al, 2018, Figure 2a).…”

The 2017 Eruption of Erta 'Ale Volcano, Ethiopia: Insights Into the Shallow Axial Plumbing System of an Incipient Mid‐Ocean Ridge

“…The slow‐spreading Erta 'Ale volcanic segment (EAVS) in northern Afar, Ethiopia (Figure a), represents the southern end of the Red Sea Rift (RSR) and has been previously proposed as a subaerial analog for a MOR (e.g., Ebinger, ). Although the spreading rate is low (5‐15 mm/year) (McClusky et al, ), the EAVS exhibits features typical of a fast‐spreading MOR such as a broad axial ridge (Barberi & Varet, ), the presence of a shallow axial magma chambers below the EAVS (e.g., Pagli et al, ), and the formation of a proto‐transform fault between the Erta 'Ale and Tat 'Ale segments (Illsley‐Kemp et al, ). Understanding the dynamics of magma movement at the EAVS can therefore give us insights into magma plumbing systems at fast‐spreading MORs.…”