**3. Estimation of tectonic evolution**

On the basis of the seismological evidence mentioned above, a regional tectonic model of LHB was estimated so as to explain formation process of the present upper mantle structure. A history of evolution, including major tectonic events in LHB, was summarized in **Figure 5**(**a, b**). Dedicated geological studies revealed regional metamorphism of the area during the Pan-African age [1, 2]. Metamorphic grade increased progressively from amphibolite facies in the NE to granulite facies in the SW of LHB. The maximum thermal axis runs through the southern LHB with a NNW-SSE striking direction [26]. From geological evidence, the LHB experienced deformation of compression stress oriented perpendicular to the thermal axis, almost parallel to the coast, during the last stage of deformation process within a mobile belt between East and West Gondwana [1, 2]. The high-amplitude magnetic anomalies occurring in LHB compared to those in surrounding terrains [27] indicated that the LHB might be located in one of the major suture zones of the Pan-African mobile belt. These major suture zones appeared to continue from LHB to the Shackleton Range of West Antarctica [1, 28].

along the direction might produce well-defined seismic anisotropy associated with a thermal axis of the progressive metamorphism. Since the direction of paleo-compression was consistent with resultant fast polarization by SKS splitting [17], anisotropy in the upper layer in **Figure 3** could be explained by "lithospheric" deformation during the formation of LHB. **Figure 5b** illustrates the tectonic evolution of Pan-African orogeny in Eastern Dronning Maud Land-Western Enderby Land (modified after [30]). The tectonic evolution process is supposed to be divided into three stages: (1) collision of East Gondwana (Archaean Napier Complex), (2) LHB exhumation by wedge uplift of basement of underlying the Napier Complex, and (3)

**Figure 5.** (a) Tectonic evolution model in LHB estimated by several seismological studies. Pan-African orogeny and mid-Mesozoic breakup could be the major two events affecting to form the present upper mantle structure. (b) Illustration of the collision tectonics around LHB at Pan-African orogeny in Eastern Dronning Maud Land-Western Enderby Land (modified after [30]). East Gondwana block includes the Archaean Napier Complex in the middle part of Enderby Land.

Seismological Implication to the Tectonic Evolution of the Lützow-Holm Bay Region (East…

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7

At the breakup between Antarctica and Australia-India in 150 Ma [31], LHB experienced extensional stress, which caused thinning at the continental margins of Antarctic continent. The flatlying reflectors above the crust-mantle boundary identified in the 2000 seismic profile (**Figure 4**) suggested the presence of an extensional stress regime in NW-SE direction resulting from the breakup. The seismic reflective layers at the crust-mantle boundary and lithospheric mantle might have been enhanced by extensional conditions during the final stages of the breakup. The LHB experienced regional high-grade metamorphism during the Pan-African age [32]; the metamorphic grade increased progressively from the north to the south, and the maximum thermal axis lied in the southernmost part of LHB [9]. The "fossil" anisotropy in the lithospheric mantle could be deformed by the past regional tectonics. A majority of the fast polarization directions in the upper layer, which corresponded to the "lithosphere," were orientated in a NE-SW direction (**Figure 3**). This direction was consistent with that of the paleo-compression

LHB exposure due to surface erosion, respectively.

Lower crust and upper mantle beneath LHB were characterized to have lateral as well as vertical variations as revealed by seismological studies mentioned in this paper. The gently inland dipping Moho discontinuity (38–42 km) beneath the 2000 deep seismic profile was inverted by travel time analysis from refraction and wide-angle reflection surveys [29]. The present structure was characterized to hold the past regional tectonics, in particular metamorphic activities during the Pan-African. Inferred thrust duplicated (similar to the wedgeshaped) lower crust-upper mantle transition structures interpreted on the 2002 seismic profile (**Figure 4**) [19] implied compressive stress regime along the profile oriented in NE-SW direction during the Pan-African events. In spite of these geophysical and lithologic information, LHB was assumed to be formed under convergence, perpendicular to the thermal axis, during the collision between supra terrains of the Gondwana at the last stage of supercontinent formation [15, 30]. When the LHB underwent NE-SW compression, related paleo-mantle flow Seismological Implication to the Tectonic Evolution of the Lützow-Holm Bay Region (East… http://dx.doi.org/10.5772/intechopen.71972 7

supercontinent in mid-Mesozoic could explain the formation of stretched reflection structure

These seismic reflection cross sections were assumed to reflect multi-genetic origins, including igneous intrusions, lithologic/metamorphic layering, mylonite zones, shear zones, seismic anisotropy, and fluid layers [21, 22]. In spite of the multi-genetic origin, metamorphic layering could be principal candidates in the case of LHB. A strong reflectivity in the deeper crust-upper mantle might be expected by layered sequences of mafic and felsic rocks [23]. Moreover, such the reflectivity might be originated when the mafic rocks had been interlayered by a combination of the upper amphibolite and lower granulite facies metapelites [24]. In any continental terrains on the Earth, the primary causes for reflectivity might be enhanced by ductile stretching during a late tectonic extensional process [25]. The reflecting layers near the Moho, moreover, were predominantly identified at the crustal thinning tectonic regimes. In these regards, reflectivity in the lower crust and lithospheric mantle beneath LHB might be

On the basis of the seismological evidence mentioned above, a regional tectonic model of LHB was estimated so as to explain formation process of the present upper mantle structure. A history of evolution, including major tectonic events in LHB, was summarized in **Figure 5**(**a, b**). Dedicated geological studies revealed regional metamorphism of the area during the Pan-African age [1, 2]. Metamorphic grade increased progressively from amphibolite facies in the NE to granulite facies in the SW of LHB. The maximum thermal axis runs through the southern LHB with a NNW-SSE striking direction [26]. From geological evidence, the LHB experienced deformation of compression stress oriented perpendicular to the thermal axis, almost parallel to the coast, during the last stage of deformation process within a mobile belt between East and West Gondwana [1, 2]. The high-amplitude magnetic anomalies occurring in LHB compared to those in surrounding terrains [27] indicated that the LHB might be located in one of the major suture zones of the Pan-African mobile belt. These major suture zones appeared

Lower crust and upper mantle beneath LHB were characterized to have lateral as well as vertical variations as revealed by seismological studies mentioned in this paper. The gently inland dipping Moho discontinuity (38–42 km) beneath the 2000 deep seismic profile was inverted by travel time analysis from refraction and wide-angle reflection surveys [29]. The present structure was characterized to hold the past regional tectonics, in particular metamorphic activities during the Pan-African. Inferred thrust duplicated (similar to the wedgeshaped) lower crust-upper mantle transition structures interpreted on the 2002 seismic profile (**Figure 4**) [19] implied compressive stress regime along the profile oriented in NE-SW direction during the Pan-African events. In spite of these geophysical and lithologic information, LHB was assumed to be formed under convergence, perpendicular to the thermal axis, during the collision between supra terrains of the Gondwana at the last stage of supercontinent formation [15, 30]. When the LHB underwent NE-SW compression, related paleo-mantle flow

above the Moho discontinuity as imaged by the 2000 active source profile.

enhanced under extensional conditions by the last breakup of Gondwana.

to continue from LHB to the Shackleton Range of West Antarctica [1, 28].

**3. Estimation of tectonic evolution**

6 Tectonics - Problems of Regional Settings

**Figure 5.** (a) Tectonic evolution model in LHB estimated by several seismological studies. Pan-African orogeny and mid-Mesozoic breakup could be the major two events affecting to form the present upper mantle structure. (b) Illustration of the collision tectonics around LHB at Pan-African orogeny in Eastern Dronning Maud Land-Western Enderby Land (modified after [30]). East Gondwana block includes the Archaean Napier Complex in the middle part of Enderby Land.

along the direction might produce well-defined seismic anisotropy associated with a thermal axis of the progressive metamorphism. Since the direction of paleo-compression was consistent with resultant fast polarization by SKS splitting [17], anisotropy in the upper layer in **Figure 3** could be explained by "lithospheric" deformation during the formation of LHB. **Figure 5b** illustrates the tectonic evolution of Pan-African orogeny in Eastern Dronning Maud Land-Western Enderby Land (modified after [30]). The tectonic evolution process is supposed to be divided into three stages: (1) collision of East Gondwana (Archaean Napier Complex), (2) LHB exhumation by wedge uplift of basement of underlying the Napier Complex, and (3) LHB exposure due to surface erosion, respectively.

At the breakup between Antarctica and Australia-India in 150 Ma [31], LHB experienced extensional stress, which caused thinning at the continental margins of Antarctic continent. The flatlying reflectors above the crust-mantle boundary identified in the 2000 seismic profile (**Figure 4**) suggested the presence of an extensional stress regime in NW-SE direction resulting from the breakup. The seismic reflective layers at the crust-mantle boundary and lithospheric mantle might have been enhanced by extensional conditions during the final stages of the breakup. The LHB experienced regional high-grade metamorphism during the Pan-African age [32]; the metamorphic grade increased progressively from the north to the south, and the maximum thermal axis lied in the southernmost part of LHB [9]. The "fossil" anisotropy in the lithospheric mantle could be deformed by the past regional tectonics. A majority of the fast polarization directions in the upper layer, which corresponded to the "lithosphere," were orientated in a NE-SW direction (**Figure 3**). This direction was consistent with that of the paleo-compression stress during Pan-African and the conversion stage between East and West Gondwana terrains at the age. In these concerns, it was proposed that the mantle anisotropy had been originated by lithologic orientation of the mantle minerals during amalgamation process of Gondwana rather than the current asthenospheric flow which parallel to the absolute plate motion.

**Author details**

Systems (ROIS), Tokyo, Japan

\* and Vladimir D. Suvorov<sup>2</sup>

1 National Institute of Polar Research, Research (NIPR), Organization of Information and

Seismological Implication to the Tectonic Evolution of the Lützow-Holm Bay Region (East…

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9

2 Trofimuk Institute of Petroleum Geology and Geophysics of Siberian Branch, Russian

[1] Fitzsimons ICW. Grenville-age basement provinces in East Antarctica: Evidence for

[2] Fitzsimons ICW. Proterozoic basement provinces of southern and southwestern Australia, and their correlation with Antarctica. Geological Society of London, Special

[3] Harley S, Hensen BJ. Archean and proterozoic high-grade terranes of East Antarctica (40- 80E): A case study of diversity in granulite facies. In: High Temperature Metamorphism and Crustal Anatexis. Dordrecht, Netherlands: Kluwer Academic Publishers. 1990. pp. 320-370

[4] Shiraishi K, Hokada T, Fanning CM, Misawa K, Motoyoshi Y. Timing of thermal events in eastern Dronning Maud Land, East Antarctica. Polar Geoscience. 2003;**16**:76-99

[5] Lawver LA, Dalziel IWD, Gahagan LM. Intercontinental migration routes for South American land mammals: Paleogeographic constraints. In: Origins and Evolution of Cenozoic South American Mammals. New York: Springer; 2014. 978-3-319-03700-4 [6] Storey BC. The role of mantle plumes in continental breakup: Case histories from Gond-

[7] Goleby BR, Blewett RS, Fomin T, Fishwick S, Reading AM, Henson PA, Kennett BLN, Champion DC, Jones L, Drummond BJ, Nicoll M. An integrated multi-scale 3D seismic

model of the Archaean Yilgarn Craton, Australia. Tectonophysics. 2006;**420**:75-90

[8] Shiraishi K, Dunkley DJ, Hokada T, Fanning MC, Kagami H, Hamamoto T. Geochronological constraints on the late Proterozoic to Cambrian crustal evolution of eastern Dronning Maud Land, East Antarctica: A synthesis of SHRIMP U-Pb age and Nd model age data. Geodynamic Evolution of East Antarctica: A Key to the East–West Gondwana

[9] Motoyoshi Y, Matsubara S, Matsueda H. P-T evolution of the granulite facies rocks of the Lützow-Holm Bay region, East Antarctica. In: Evolution of Metamorphic Belts. Oxford:

three separate collisional orogens. Geology. 2000;**28**:879-882

\*Address all correspondence to: kanao@nipr.ac.jp

Academy of Sciences, Novosibirsk, Russia

Publication. 2003;**206**:93-130

wanaland. Nature. 1995;**377**:301-308

Blackwell; 1989. pp. 325-329

Connection Special Publications. 2008;**308**:21-67

Masaki Kanao1

**References**

The lattice-preferred orientation (LPO) induced mechanical anisotropy developed along the direction of preexisting lithospheric structure during continental rifting [33]. The origin of anisotropy beneath Western Dronning Maud Land was pointed out as the ancient lithospheric structure modified by rifting processes during breakup [34]. Since the spreading direction off the Enderby Land was NW-SE initial stage of breakup [35], a strike of the rift was generally parallel to the continental margin of LHB. The fast polarization directions of the upper layer ("lithosphere") in the SKS analysis were roughly parallel to the continental margin. In this regard, it was plausible that the breakup process affected the formation of anisotropy in the lithosphere. The preexisting lithospheric structure might also influence the formation of anisotropy in the succeeding breakup process.
