Satellite derived geophysical gravity data are the modern powerful tool of regional
tectono-geophysical examination of the Earth’s crust and upper mantle. It is well known that
regional long-term seismological prognosis, strategy of searching economic deposits
and many other important geological-geophysical problems are based mainly on
constructions derived from the combined tectono-geophysical zonation. Some authors’
experience of the tectono-geophysical zonation in the Eastern Mediterranean (both sea
and land) with satellite derived gravity field (Eppelbaum and Katz, 2015a, 2015b)
indicates a high effectiveness of the data employment for delineation of different
tectono-structural units. Therefore, on the basis of the previous successive application,
satellite derived gravity field analysis was applied for a giant (covering > 10 mln.
km2) and complex Arabian-African region (including Zagros Mts.). The gravity
field retracked from the Geosat and ERS-1 altimetry (e.g., Sandwell and Smith,
2009) was processed by the use of different mathematical apparatus employment
enabling to underline these or those tectonic (geodynamic) features of the region under
study.
The main goals of present investigation are following: (1) employment of a new powerful
regional geophysical tool – satellite derived gravity data and its transforms for unmasking
some buried tectonic and geodynamic peculiarities of the study area, (2) finding definite
relationships between the novel tectonic map and the gravity field transformations, (3)
development of a novel tectonic map of this area (on the basis of careful examination of and
generalization of available geological and geophysical (mostly satellite gravity)
data).
The compiled gravity map (for the map compiling more than 4 mln. observations were
utilized) with the main tectonic features shows the intricate gravity pattern of the investigated
area. An initial analysis of the gravity field behavior enabled to separate two main types
of tectonic structures: (1) stable zones of continental and oceanic crust, and (2)
mobile geotectonic belts. First type is characterized by homogeneous character of
gravity field pattern (for instance, East Arabian Craton), whereas second type is
characterized by mosaic and variable behavior of gravity field (especially, active rift
zones).
It should be noted that ’youngest’ mobile structure (Alpine-Himalayan orogenic belt and
active rift systems of the Red Sea – East Africa) significantly differs in the gravity field
pattern from the Mesozoic terrane belt and Neoproterozoic belt.
In this investigation six satellite gravity transforms (SGT) are described: multidimensional
statistical analysis (MSA) by the use of sliding window, low-pass filtering, informational
approach, gradient operator, entropy processing by sliding window of adaptive form, and 3D
inverse methods.
Application of the MSA enabled not only to delineate geodynamical parameters of the
studied region (collision zone at the boundary between the Arabian and Eurasian
Plates, and active rift zones between the Arabian, Nubian and Somalian Plates, etc.),
but also to estimate generalized properties of the Earth’s crust. Results of MSA
employment clearly show zone of development of the oceanic crust of the Easternmost
Mediterranean and zone of oceanic crust of the Gulf of Aden and eastern (oceanic) part of
the Somalian Plate. Besides this, in this map the Arabian and East African active
rift zones and collision zone between the Arabian and Eurasian Plates are visibly
traced.
Applied low-pass gravity field filtering enabled to recognize the most contrast
crust-mantle structures. For example, the Afar triangle zone is clearly detected. Zones of the
Neotethys closing Eastern Mediterranean, Persian Gulf, Zagros Fault Zone and South
Caspian Basin can be easily identified. Subduction zones associated with the plate boundaries
are reflected by elongated gradient pattern. These nonstable zones are conjugated with large
mobile belts: Alpine-Himalayan belt and Mesozoic terrane belt. The zone of active rifting of
the Red Sea, Gulf of Aden and complex structure of Afar triangle as well as East African rift
system are noticeably fixed. The boundary between the continental and crust in the SE part of
the region (where occurs a transfer zone between the Gulf of Aden and Arabian Sea) is
visibly detected.
Application of informational approach (Eppelbaum and Khesin, 2012) enabled to reliably
fix both continental and oceanic cratons and all belts. To south-east of the Horn of Africa the
Arabian Sea Basin with oceanic crust is clearly distinguished. The East Arabian Craton
(platform) as well as its framing are noticeably detected.
Computation of entropy map from the satellite derived gravity field was earlier
successfully tested by the authors in the Eastern Mediterranean (Eppelbaum and Katz,
2015a). Application of the adaptive form sliding window enables to receive the most
reliable entropy estimations in conditions of complex field caused by superimposed
influence of targets of different order. Obviously, computation of an entropial map
by the same method for the region under study reproduces mainly deep tectonic
units (elements) of the region. Complex pattern of entropial field in the SE part of
the region reflects transfer from the Somalian Plate to Indian Plate (this area is
characterized by the most mosaic pattern). This map nicely indicates position of the
Mesozoic terrane belt and transition zone between the Victorian and Tanzanian
plates.
On the basis of advanced inverse method employment, the map indicating the most
density contrast surface (discontinuity) in the upper mantle was developed. This
map presents an intricate density-tectonic depth pattern of the region. Here such
important tectonic features as the Afar Triple Junction and collision zone between the
Arabian and Eurasian lithospheric plates are noticeably recognized. Besides this, we
can note increasing of lithospheric thickness in central parts of the Arabian and
Somalian plates. Both these plates are countered by low-thickness lithospheric zones
corresponding to the active rift zones. As it is indicated in the map, the thick lithospheric
zones are associated with collisional zones at boundaries between the cratons and
mobile belts. We suggest that the lowered values in the northern boundaries of the
Arabian Plate correspond to subduction zones. The zones of lowered values in the
middle of western part of the region correspond to the Neoproterozoic belt where
ophiolitic and back-arc complexes with a thinned crust (e.g., Stern et al., 2004) are
developed.
Compiled satellite derived gravity field and a set of SGT were utilized for development of
a novel tectono-geophysical zonation map of the Arabian-African region. Structurally-
geodynamically this region is one of the key Earth’s megastructures where are closely
disposed remain elements of the Tethys Ocean crust (Ben-Avraham et al., 2002; Robertson,
2004), most ancient Early Permian reversly magnetized Kiama zone (Eppelbaum and Katz,
2012b; Eppelbaum et al., 2014), and the youngest modern oceanic crust of the Afar
triangle developed among the continental lumps (Yirgu et al., 2006; Bastow et al.,
2011).
The tectonic zonation was carried out with application of three main principles of tectonic
analysis:
(1) classic basis of space-temporary reflection of structural complexes,
(2) modern structural-geodynamic approach derived from the plate tectonic reconstructions
where essential role plays analysis of rift, tectonic transform and collision forms of Earth’s
development,
(3) revealing of intricate correlation between the mapped tectono-structural elements and
lithospheric-mantle complexes delineated by using both conventional geophysical methods
(seismic, seismological, thermal data, etc.) and comprehensive analysis of satellite derived
gravity data.
Compiled tectonic map of the region (00 − 35.60 north, and 300 − 570 east) indicates that
Precambrian basement and Mesozoic-Cenozoic structures play dominating structural-
geodynamic role in this region. Precambrian generations include two main structural
elements: (1) Archean platforms (Eastern Arabian, Tanzanian and Eastern Saharan cratons),
and (2) Neoproterozoic belt. In the Neoproterozoic belt we distinguish: (a) final
Proterozoic back-arc belts with ophiolites, and (b) more ancient Early/Middle Proterozoic
massifs (detected both in some previous works of various authors and recognized
by the authors of the present investigation using a set of geological-geophysical
indicators).
In the areas of development of sedimentary Phanerozoic cover in the northern part of
Arabian and African (Nubian) Plates, boundaries of Early/Middle Proterozoic massifs
(Tabuk, Haif-Rutfah, Widyan and Nile Cone) and Neoproterozoic belts (Azraq-Sirhan,
Ga’ara and Northern Western Desert) were delineated by analysis of: (1) land and airborne
geophysical data, and (2) satellite derived gravity data.
Meso-Cenozoic structures of the region contain two tectonic complexes of its forming. 1st
complex (from Permian to present) is associated with the Neotethys Ocean evolution. 2nd
complex (from Oligocene to present) is associated with initial phases of spreading in the
Arabian-African segment of Earth’s crust.
1st complex structurally and geodynamically is a multiple generation since the Neotethys
Ocean evolution was accompanied by processes of spreading, movements of some giant
blocks along tectonic transforms, and collisions. These processes have formed structures of
three types: (1) Mesozoic terrane belt, (2) Cenozoic orogenic belt, and (3) remain depressions
of the Neotethys with oceanic crust.
Western (Levantine) part of the Mesozoic terrane belt is characterized by more ancient
(Hauterive) age of consolidation comparing with the eastern part of the belt (Persian-Oman).
Its terranes (from Zagros to Makran) and ophiolites were joined to Arabian platform in
the Middle Cretaceous (Senomanian-Turonian). Many authors note an important
role of Zagros terrane in the region under study and within the Caucasian-Arabian
Sintaxis (e.g., Reilinger et al., 2006; Bordenave, 2008; Agard et al., 2011; Verges
et al., 2011; Sharkov et al., 2015; Tunini et al., 2015). We propose that present
study will unmask some tectono-geodynamic peculiarities of this complex tectonic
unit.
The Mesozoic terrane belt was delineated in the Eastern Mediterranean by the use of
variety of geological and geophysical methods (multilevel gravity and magnetic data
examination, thermal data analysis, seismic and seismological data) application (Ben-
Avraham et al., 2002; Eppelbaum et al., 2012; Eppelbaum, 2015; Eppelbaum and
Katz, 2015a, 2015b, 2016). At the same time, eastern Zagros-Makran part of the
Mesozoic terrane belt never was analyzed as a separately developing structural part
(unit) of the Arabian craton. In all known paleogeographical reconstructions the
Zagros-Makran structure is shown as a part of its northern periphery. However, analysis of
facial, sedimentary and structural data (presented in Bordenave, 2008) indicates
that there is a sharp discordant joining between the Arabian craton and Zagros
belt. Axes of anticline structures of the Arabian craton have a meridional strike,
while axes of the Zagros anticline structures are disposed discordantly to them at
SW 35 − 500. Besides this, paleogeological maps of Paleozoic (Bordenave, 2008)
indicate that Devonian and Carboniferous deposits widely developed within the
Arabian craton, do not presented in the Zagros belt. It testifies an uplift of Zagros
structure and its isolated evaluation in the post-Carboniferous time when the Tethys
Ocean began to form. Geological factors of Zagros structure isolation indicate that
it was possibly a part of terrane belt in the southern part of the Neothetys Ocean
forming.
It is necessary to take into account that Zagros structure most likely occupied different
tectonic positions at different periods of geological time: (1) up to Carboniferous period
Zagros was a part of the Eastern Arabian Craton, (2) in the interval between Permian and
Middle Cretaceous it was a part of the terrane belt within Neotethys, (3) at present it is a
marginal part of the Arabian lithospheric plate. All three aforementioned items find a direct
reflection in the compiled gravity and SGT maps:
(1) Common structural-geophysical properties of Zagros structure and Arabian craton can be
recognized in informational and gradient gravity field transformations;
(2) Examination of initial gravity map, entropial transformation map and deep structure map
testify that Zagros is an independent structural unit within the Mesozoic terrane belt.
Presence of thick Cenozoic sediments in the eastern part of Arabian Plate essentially limits
application of conventional geological methods; therefore, contouring of boundaries between
the Mesozoic terrane belt and Precambrian platform is possible mainly by regional
geophysical data analysis. Sharp changing of gravity pattern in all three afore- mentioned
maps enables to utilize this property as criterion for delineation of southern boundary of the
Mesozoic terrane belt;
(3) Examination of the MSA map unambiguously indicates that Zagros suture is a marginal
part of the Arabian lithospheric plate.
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