Geophysical Evidence of the Collisional Suture Zone in the Prydz Bay, East Antarctica

The location and origin of Neoproterozoic‐Cambrian sutures provide keys to understand the formation and evolution of the supercontinent Gondwana. The Larsemann Hills is located near a major Neoproterozoic‐Cambrian suture zone in the Prydz Belt, but has not been examined locally by comprehensive geophysical studies. In this study, we analyzed data collected from a one‐dimensional (1D) joint seismic‐MT array deployed during the 36th Chinese National Antarctic Research Expedition. We found that a sharp Moho discontinuity offset of 6–8 km shows up in the stacked image of teleseismic P‐wave receiver function analysis; coinciding with the abrupt Moho offset, a near‐vertical channel with (a) low resistivity extending to the uppermost mantle depths, and (b) high crustal Poisson's ratio in the crust is identified. These findings provide evidence for the determination of the location and collisional nature of the Prydz belt or a portion of it.

of continental suture zones (Burtman & Molnar, 1993).These investigations can directly detect the presence of high-grade metamorphic sedimentary rocks generated by the subduction of oceanic plates, manifesting as anomalous seismic, resistivity, and magnetic readings (Von Huene & Scholl, 1991).Moreover, seismic discontinuities often serve as indicators of deformation resulting from tectonic events (Katsumata, 2010).Notably, significant deformation, such as large-scale strike-slip faults, can create Moho discontinuities, thus being identifiable through seismic investigations (McBride, 1994).Crustal deformation associated with orogeny and extension induces cracks, compositional changes, and fluid presence, all of which influence the Poisson's ratio (Lowry & Pérez-Gussinyé, 2011).These effects can be detected using converted phases of remote seismic events.
Despite advancements, geophysical investigations of the Prince Charles Mountain area have faced challenges due to sparse observations, hindering a comprehensive understanding of the belt's origins.For instance, previous receiver function investigations in the Lambert Glacier region were conducted at scattered stations across the Prydz Belt, limiting the resolution needed to precisely identify the location of suture zones (Reading, 2006).A recent study constructed a 3D resistivity model using Magnetotellurics (MT) data, which identified low electrical conductivity and magnetic high-intensity between the Vestfold Hills and Rauer Group, interpreted as a continental suture zone (Peacock & Selway, 2016).However, these studies have not presented a unified model that definitively supports either of the Prydz Belt's origination models.Consequently, geophysical signatures related to the origin of the Prydz Belt remain inadequately investigated.This study presents the first comprehensive in situ geophysical observations within the central Prydz Belt zone by deploying a local 1D seismic and MT array in the geologically significant Larsemann Hills during the 36th Chinese National Antarctic Research Expedition (CHINARE36).The Larsemann Hills, being an important site for geologists and geophysicists studying the Prydz Belt, have the potential to preserve structural signatures (e.g., deep crustal compositional anomalies (Tarney & Jones, 1994) or discontinuity anomalies (Dong et al., 2004) related to orogenic development.Therefore, investigating the subsurface structure in this region provides valuable insights into the tectonic environment of the Prydz Belt and the formation of Gondwana.
The analysis of seismic and MT data reveals several key observations.First, a distinct Moho discontinuity is identified, delineating a suture zone that coincides with the previously perceived Proterozoic tectonic belt.Second, the deep crust within the suture zone exhibits higher Poisson's ratio, anomalously low resistivity, and elevated magnetic anomaly.These signatures suggest the presence of a deep crustal compositional anomaly, such as high-grade metamorphic sedimentary materials (e.g., graphite, as suggested by Cost et al. (1968)), likely transported from the surface to the lower crust.These geophysical observations strongly indicate a subduction origin for the Prydz Belt or a portion of it.Collectively, these findings support the hypothesis that the Prydz Belt represents a continental suture zone with a subduction-related origin.

Seismic and MT Data
An ultra-dense short-period nodal seismic array consisting of 100 seismic stations was deployed in the Larsemann Hills, with an average station spacing of 200 m (depicted as brown triangles in Figure 1c).Notably, the seismic profile deviated eastward near station MT08 to circumvent ice cracks.Due to limitations imposed by the nodal station batteries, seismic recording was conducted from 5 December 2019, to 3 January 2020, resulting in a record duration of approximately 30 days.In this study, we employed a recently developed array-based technique called Coherent Receiver Function (CRF) imaging, operating within the Bayesian framework (Wang et al., 2021).This approach effectively capitalizes on the ultra-dense yet short-duration nodal array, enabling the generation of high-resolution subsurface structure images (see Supporting Information S1 for the CRF Method).Additionally, broadband (0.003125-1,000 s) Magnetotelluric (MT) data were collected at 10 sites (indicated by yellow triangles in Figure 1c) within the same seismic deployment.The recording durations for the MT measurements ranged from 3 days to 1 week.The average spacing between MT stations was approximately 2-3 km, resulting in an MT observation aperture of approximately 26 km.

Seismic Structure
Seismic analysis of the Larsemann Hills seismic data yielded three seismic products: the average crustal Poisson's ratio (Zhu & Kanamori, 2000) beneath each seismic station, stacked Coherent Receiver Function (CRF) image, Writing -review & editing: Lei Fu, Weisen Shen, Xiaofei Chen and shallow seismic velocities previously published by Fu et al. (2022), shown in Figures 2a-2c, respectively.The stacked CRF results, generated from four earthquakes with magnitudes greater than 5.8 (indicated by purple circles in Figure S1 in Supporting Information S1), are presented in Figure 2b.A prominent positive P-s conversion is clearly observed at a depth of approximately 30 km, signifying the presence of a sharp Moho discontinuity.At a distance of approximately 5-10 km from the northern end of the seismic array, a sudden Moho offset of approximately 6-8 km is observed, dividing the study area into northern and southern blocks.The average depths of the Moho in the northern and southern blocks are estimated to be around 30-32 km and 34-36 km, respectively.The CRF analysis, accompanied by Bayesian uncertainty assessment, demonstrates a robust observation of the abrupt Moho change, with the Moho signals imaged at a confidence level of 68% or higher (Figure S3 in Supporting Information S1).This observation highlights a significant alteration in geophysical properties within the deep crust, likely resulting from large-scale tectonic activity in the region.Furthermore, the average Poisson's ratio for the Moho-disturbed region is approximately 0.28, whereas the average Poisson's ratios for the northern and southern blocks are approximately 0.25 and 0.26, respectively.The Poisson's ratio in the Moho-disturbed region is notably higher than that in the adjacent blocks.In addition, a low shear wave velocity (vs.) zone between 7 and 12 km, extending within the uppermost 2 km, is observed in Figure 2c.While the low Versus zone appears to be distinct from the deeper crustal deformation zone, its formation is closely associated with the fault system.The fault system in the suture zone region could be highly complex, and it may influence (O'Connell & Budiansky, 1974) or guide the formation (Fu et al., 2022) of the low Versus zone in the adjacent upper crust.

Resistivity Model
We conducted a dimensionality analysis based on phase tensors (Figure S4 in Supporting Information S1), and the results indicate that for most of the stations, especially in the low-frequency range (>100 s), the skew angles are less than 5°, which suggest that performing a two-dimensional (2D) inversion is appropriate (Caldwell et al., 2004).Previous studies propose that TM mode data tend to offer greater reliability for 2D interpretation in the context of 3D effects (Ledo et al., 2002;Wannamaker et al., 1984;Yang et al., 2023).The 2D electrical resistivity model (Figure 2d) presented herein is obtained through inversion of TM mode data using the non-linear conjugate gradient method (Newman & Alumbaugh, 2000).The calculated apparent resistivity and phase derived from the resulting preferred resistivity model exhibit excellent agreement with the observed data (Figure S5 in Supporting Information S1).The final RMS misfit of the inversion is shown in Figure S6 in Supporting Information S1.The preferred model reveals three distinct layers extending from the surface to a depth of approximately 60 km within the uppermost mantle.The first layer corresponds to an upper crust characterized by high resistivity (referred to as R1).The second layer comprises a significant and highly conductive zone spanning depths between 10 and 30 km (referred to as C2).The third layer, located beneath the Moho, exhibits a more intricate electrical structure, consisting of two sub-vertical zones with high resistivity (R2 and R3) and a sub-vertical zone with low resistivity (C3) in the middle.The low-resistivity zone (C3) extends downward into the upper mantle and connects to the conductive zone (C2) in the lower crust, forming a vertically oriented conduit for electrical conductivity.The sensitivity analysis for three anomalies (R3, R2 and C3) below the high conductor C2 indicate that MT data is relatively sensitive to the three anomalies (Figure S7 in Supporting Information S1).

Geophysical Interpretation
Based on the Moho discontinuity and the disintegration of the negative seismic phase within the upper crust, we interpret the sudden Moho offset as a pair of crustal-scale high-angle strike-slip faults, which we've designated as F1 and F2, ∼5 km separated from each other.The deep fault system indicates a significant structural interface in this region.The high crustal Poisson's ratio between F1 and F2 is either caused by the existence of fluid or cracks, or from the crustal composition (mafic rocks generally have higher Poisson's ratio).As shown in Figure 2c, the near surface low Versus anomaly, although appears outside of F1 and F2, lies only within 10 km of the fault F2 in the same region, and is sitting above the top of the tilted mid-crustal low resistivity anomaly (Figure 2d), we interpret them (low vs., high Poisson's ratio) as a signature of wide-spread cracks that are associated with the fault system that includes F1 and F2, a common phenomenon of continental suture zone.
The preferred 2D resistivity model shows that electrical structures also vary across the conduit defined by seismic signature.Off the conduit, the average resistivity to the south is higher than that to the north, indicating different compositional properties of the two crustal bodies.The location of near-vertical low resistivity (less than 10 Ω-m) channel in the lithospheric mantle connecting to lower crust basically overlaps with the abrupt Moho.The relatively high conductivity zone (C3) in the uppermost mantle exhibits a near vertical structure, clearly separating the lithospheric mantle into northern and southern parts.These low conductivity bodies in the crust and uppermost mantle (esp.C2 and C3 shown in Figure 2d), provide some of the direct evidence of compositional alternation to the lithosphere.Generally, high conductivity of deep crust and uppermost mantle is either due to existence of fluids (e.g., existence of melt and water) or due to compositional anomaly (e.g., electron conduction due to sulfide and graphite).Given the Neoproterozoic age of the recent tectonism and low geothermal heat flux (Shen et al., 2020), it is unlikely that melt exists in shallow lithosphere (0-60 km depth).The high conductivity in deep crust is not due to water since the pressure would close the cracks and minimize the porosity of the rocks (Selway, 2014).Hence, we interpret the extremely high conductivity body (C2) as a compositional origin, most likely due to graphite and sulfide (Yang et al., 2015), usually found in a convergent plate boundary (Abdul Azeez et al., 2015).

Discussion
Overall, the new geophysical data collected in the Larsemann Hills suggest a continental suture zone at the periphery of the East Antarctica.Evidence such as the deep cutting faults that penetrates through the Moho, as well as the wide-spread cracks within the crust that produces lower Versus and higher Poisson's ratio localized between the faults all support strong crustal deformation, perhaps related to the collapse of an orogeny resulting from suturing.The low-resistivity bodies found in the deep crust and the uppermost mantle are most likely due to the existence of graphite and sulfide films along the grain boundaries, usually found in metamorphism associated with lithospheric suturing (Selway, 2014).Magnetic anomaly map (Golynsky et al., 2018) in the Larsemann Hills, Prydz Bay presents a nearly east-west stripe of high magnetic intensity (Figure S8 in Supporting Information S1) that runs through the southern part of the seismic survey line and corresponds to the high-conductivity anomalies C1-C2 in the resistivity model.The high-magnetic anomalies probably represent the breakdown of magnetite, another suture-related metamorphism signature; and finally, the steep low resistivity channel indicates the channel for the transport of heat that induces the metamorphism.In addition, A WSW-ENE trending suture zone was speculated from MT data (Peacock & Selway, 2016) between Rauer Group and Vestfold Hills, which is about 100 km north-east of our study area, Larsemann Hills.
A subduction-induced orogenic belt (Boger et al., 2001) has been proposed based on a clockwise P-T evolution trajectory (Kelsey et al., 2003;Liu et al., 2009) of compression, and extensional deformation (Carson et al., 1995) accompanied by isothermal decompression.In addition, the age difference of the Greenville terrane on both sides of the orogenic belt and the inconsistency of the paleomagnetic pole shift curves between India and Australia before the Late Neoproterozoic-Early Paleozoic also support this hypothesis (Torsvik, 2001).The lack of direct indicator of subduction-related facies signs in orogenic belts can be used to support an intraplate orogen (Yoshida et al., 2003).Both collisional models predict a continental suture zone at the edge of the East Antarctica near the Larsemann Hills.Additionally, the low resistivity likely due to graphite at depth up to ∼60 km is consistent with the subduction-induced collisional model, since subduction is the major mechanism that brings hydrogen and carbon to such a depth (Dasgupta & Hirschmann, 2010).
Therefore, based on the deep geophysical imaging structures and background geology, it is suggested that the Moho disturbed region (5-10 km along the profile) probably represents an ancient subduction-induced suture zone between the Indo-Antarctica and East Antarctica (Zong et al., 2022).A possible scenario of the Pre-Cambrian-Early Proterozoic tectonism in the region can be proposed as follow: during or before the Pan-Africa, the Indo-Antarctic block collided (probably have subducted) with East Gondwana along the southern Prince Charles Mountain-Prydz Bay (Figure 3a).
During the same (or post) subduction, due to the extension and thinning of the originally thickened crust, perhaps driven by the high gravitational potential energy, the mantle-derived magma rose along the lithospheric suture zone (weak zone) to the crust-mantle transition zone and under-intruded into the lower crust (Figure 3b), forming the A-type magma granite (Figure S9 in Supporting Information S1) near the surface (Zong et al., 2022).The presence of carbon dioxide-rich fluid or reduction of mineral grains caused by local shearing or a combination of both results in the growth of graphite films along the grain boundaries, resulting an increase in electrical conductivity (DeLucia et al., 2019;Frost, 1979).The same extension generated large-scale high angle normal faults that cut through the crust, and caused massive cracks within the crust.These signatures have been preserved as part of the Gondwana, and the weak suture zones allows the later breakup of the supercontinent, and separates India Plate from Antarctica.

Conclusion
Through comprehensive analysis of seismic and Magnetotelluric data within the Larsemann Hills region, we have uncovered compelling evidence supporting a notable Moho discontinuity offset of 6-8 km.This distinctive feature is prominently displayed in the stacked image derived from teleseismic P-wave receiver function analysis.Furthermore, our investigations have unveiled a nearly vertical conduit exhibiting two key characteristics: (a) a region of low resistivity that extends to uppermost mantle depths and (b) a pronounced high Poisson's ratio within the Earth's crust.These geophysical findings are of paramount importance as they contribute significantly to our understanding of the Prydz orogenic belt, shedding light on its collisional nature and precise location.Building upon the contextual framework of geological knowledge, we propose that the region exhibiting disturbance in the Moho is indicative of an ancient subduction-induced suture zone.This geological feature likely represents a historically significant tectonic boundary between the Indo-Antarctica and East Antarctica continental masses.Nonetheless, the current seismic and MT profiles might provide insights into a portion of the Prydz belt.Additional geophysical investigations along the Zhongshan-Kunlun profile will be necessary to verify the precise location and collisional characteristics of the Prydz belt in future research.

Figure 1 .
Figure 1.Geological map for the seismic and MT survey in the Larsemann Hills.(a) Geological sketch map of the Prydz Belt and adjacent areas, the black dashed curve indicates the Indo-Antarctica suture zone proposed by Gupta et al. (2021), (b) showing its location in the reconstruction of Gondwana at circa 500 Ma (Modified after Kelsey et al. (2008)), (c) the short-period seismometers (brown triangles) and MT stations (yellow triangles) deployed in the Larsemann Hills, Prydz Bay, during CHINARE36.

Figure 2 .
Figure 2. Geophysical results.(a) The Moho depth and Poisson's ratio along the profile constrained from seismic events with Ms > 5.0 using H-k method; (b) CRF stacking image of curst structure along profile; (c) shear-wave velocity model for the ice sheet and uppermost curst; (d) resistivity model of curst structure along profile.The two dashed lines marked with "F1" and "F2" in (b) and (d) indicate a pair of crustal-scale high-angle strike-slip faults running through uppermost curst and Moho.

Figure 3 .
Figure 3. Cartoon of crustal structure in the Larsemann Hills, Prydz Bay.The Indo-Antarctic block subducted and collided with East Antarctic Carton, and the low resistivity channel caused by graphite and sulfite films along grain boundaries along the lithospheric suture zone (weak zone) within the crust-mantle transition zone as well the mid-lower crust.