Open Journal of Geology, 2013, 3, 28-33
doi:10.4236/ojg.2013.32B007 Published Online April 2013 (http://www.scirp.org/journal/ojg)
Gravity Modeling for the Rifted Crust at the Arabian
Shield Margin – Further Insight into Red Sea Spreading
Saad Mogren, Manoj Mukhopadhyay
Department Department of Geology and Geophysics, King Saud University, Riyadh, Kingdom of Saudi Arabia
Received 2013
ABSTRACT
A large variation in elevation and gravity anomaly prevails from the Red Sea coast to the interior of the Arabian Shield
(AS) across the Asir Igneous Province (AIP); The Asir Mountain (AM) is developed on AIP. Here the elevation varies
from 45 - 2700 m, corresponding changes in F.A. are from –30 to + 220 mgal and B.A. from +22 to –175 mgal. Regres-
sion relationships between elevation and gravity anomalies demonstrate significant changes in trend at about 400 m
threshold of elevation across the pediment west of AM, at about 45 km inland of the shoreline, flanking the Hizaz-Asir
Escarpment (HAE). Gravity anomaly variation along a traverse taken across HAE and AIP is interpreted here in terms
of anomalous masses in crust as well as due to deeper crustal configuration. 2D gravity interpretation is, in part, con-
strained by surface geology, available geologic cross-sections for crust, interpretations from the IRIS Deep-Seismic
Refraction Line, and to a lesser extent by the available gross results from shear-wave splitting and receiver function
analysis. The gravity model provides probable solutions for the first time on geometric configuration and geophysical
identification: a) for the seaward margin of the mid-Tertiary Mafic Crust (TMC) below sediment cover of the Asir
pediment that coincides with the 400 m threshold elevation. This signifies an anomalous uplift at the rifting phase.
Moho below TMC extends from 10 - 22 km depth across HAE and west margin of AIP, b). Thinned continental crust
below the Asir margin whose upper layer coincides with a seismic reflector is at about 22 km depth, c). Rift-margin
characteristic detachment fault associated with basaltic flows on top surface of TMC at its inner margin, d). Two geo-
logically mapped low-angle normal faults dipping to the east developed between the basic rocks intruding the AIP and
e). felsic pluton farther east within AS. Large scale igneous activity followed by intense deformation affecting AIP
clearly owes their origin to the rifting architecture of the AS at the Red Sea extensional margin.
Keywords: Shield-margin Mafic Crust; Asir Igneous Province; Seismic Moho; 2D Gravity Crustal Model; Red Sea
Extensional Margin
1. Introduction
Configuration and the overall mass distribution both in
crust as well as in top most part of upper mantle below
the AS and Red Sea coastal plains are of much geody-
namic significance from the viewpoint of effects due to
Red Sea spreading. Saudi Arabia has an area of about
2.25 million km2, out of which, igneous and metamor-
phic rocks cover nearly 1/3 of it. These rocks are exposed
in the west, north-west and south-west. The AS is narrow
in the north (50 - 100 km), widening to about 200 km in
the south; but it is widest at its middle (about 700 km).
The present study area focuses on this middle part corre-
sponding to the AIP [1] (Figure 1). The AS is nearly
2000 km long, it has an average elevation of 1500 m and
a crustal thickness of approximately 42 km (refer below).
Between the AM and the Red Sea coast, the narrow but
elongated coastal plains in southwestern Saudi Arabia is
formally known as “Tihamat-Asir’ (TS). TS is about 30
km wide at about 17 degrees latitude in the present study
area where its average elevation is less than 100 m but
the topography rises sharply eastward to about 600 m at
the foothills across the pediment of the Asir; further
inland the shield margin crust is elevated to 3.5 - 4 km
across the HAE. Average distance of HAE from the Red
Sea axial trough is 250 km, while, HAE locates inland at
about 115 km from the coast. Rift-related extensional
deformation extends for nearly 150 km inland from the
coast between 17 °N and 18 °N encompassed within the
present study area (Figrue 1).
Mainly three contrasting views are advocated in lit-
erature on the evolutionary history of the AS: i) Island
Arcs theory: The AS developed as a result of collision of
more than one Island Arc. These Island Arcs formed in
oceanic crust [2,3]. ii) Continental Crust theory: The AS
developed as a result from tectonic deformation from
magmatic activities [4]. iii) Suture zones and Microplates

S. MOGREN, M. MUKHOPADHYAY 29
Figure 1. Digital elevation map of the Asir Igneous Province
and the Tihamat coastal plains bordering the Red Sea. Ele-
vation contours in m. IRIS seismic shot points SP 4 & 5 and
the gravity traverse location is shown.
theory: The AS formed by microplates that have sutured
to each other. These plates are of oceanic and continental
affinities [5].
1.1. Gravity Anomaly Versus Elevation
Regression Relationships at the Arabian
Shield Margin
The most remarkable feature of the AS is its sudden at-
tainment of surface elevation against HAE bounding the
TS. Here, the eastern edge between the Tihamah and the
Precambrian shield rocks is interpreted as boundary be-
tween continental crust to the east and oceanic crust to
the west that developed due to Red Sea spreading in the
mid-Tertiary [6,7]. However, we differ with details of
this interpretation as we shall discuss below that this
boundary is not actually that sharp, rather it represents a
deformed belt where TMC is developed (that consists of
plutons of gabbroic composition) together with the de-
tachment faults and large normal faults and their associ-
ated Tertiary basalts over the shield crust. Scatter plots
between Free-air, Bouguer and Airy-Heiskanen Isostatic
anomalies vis-à-vis station elevation for the TRS, AIP
and AS are shown on Figure 2. Their corresponding re-
gression equations with standard deviations are also
given in the respective plots. Such regression relations
are known to provide good indications on the prevailing
isostatic compensatory status for physiographic prov-
inces [8]. Figure 2 plots demonstrate a distinct change in
the pattern of anomaly-elevation relationships that roughly
corresponds to a threshold elevation of 400 m spanning
the pediment of the AM. Changes in gravity anomalies
pattern associated with such topographic front are well
known from other regions as well, for instance; 4 km
threshold elevation in the Himalayas corresponds to
enlarged seismicity and crustal masses underneath [9]. In
the present case, we interpret this threshold elevation and
corresponding gravity change as indicative of anomalous
uplift at the rifted margin, following which, there are
changes in composition as well as configuration between
a thinner crust seaward of the pediment as against a
thicker but denser crust below the AIP. Such an inference
is in agreement with the broad conclusions drawn on
crustal configuration between seismic Shot Points 4 and
5 from IRIS seismic profile [10,11], generalized crustal
cross section based on both geophysical [12] and geo-
logic inferences [13] and 2D gravity modeling along
traverse AA’ presented below.
Figure 2. Anomaly-elevation scatter plots for the study area.
Blue: gravity stations located over the coastal plains, Red:
gravity stations located over the Asir Igneous Province and
the Arabian Shield. F.A.: Free Air; B.A.: Bouguer; I.A.:
Isostatic Anomaly. Respective regression equations are
given. Note the change in regression relations for the eleva-
tion threshold 400 m at the base of the Asir Mountain.

S. MOGREN, M. MUKHOPADHYAY
30
Table 1. IRIS crustal models for the Arabian Shield (SP4
and SP 5) [10,11] and results on inferred density values (ρ
in g/cm3) using the seismic velocity to density relationship
[14].
(a). Moho at comparable depths (38.0 km) for Velocity Models 3, 10,
11* and 13 [10,11]; inferred ρ varies from 2.77 - 2.98 g/cm3
(b). Moho at comparable depths (38-40 km) for Velocity Models 5, 7
and 8[10,11]; inferred ρ varies from 2.72 - 2.98 g/cm3
Table 2. Crustal structure and upper mantle stratigraphy
below the Saudi Arabian Shield from Receiver Function
analysis and shear-wave splitting studies [15*, 16**].
1.2. Seismically Determined Crustal
Configuration at the Arabian Shield Margin
Basic results on crustal configuration deduced from IRIS
seismic experiment [10,11] between the Shot Points SP
4-5 (Figure 1) corresponding to the gravity traverse AA’
are summarized in Table 1.
Regional crustal configuration and information on
mantle stratigraphy as deduced from seismological stud-
ies for the Arabian Shield [15,16] are summarized in
Table 2. They provide generalized regional estimates on
crustal thickness in this part of the Western Arabian
Shield. We will incorporate this regional crustal thick-
ness value into the gravity 2D model for Traverse AA’ in
the following section.
1.3. 2D-Gravity Interpretation along Traverse
AA’
For gravity modeling we use the following: Major geo-
logic and geophysical evidences supporting rift related
extension at the shield margin:
i) Although extension is negligible near HAE but it
may reach as much as 10% in western parts of AM,
ii) Rocks in the Asir province display a system of
low-angle normal faults,
iii) There is an abrupt increase in extensional deforma-
tion in the foothills and pediment west of AM,
iv) Tertiary mafic dike swarms and plutons of gabbro
and granophyres are concentrated both in the Asir foot-
hills and pediment [13],
v) Steep Moho slopes imaged by IRIS Line at this
juncture [10, 11] developed during rifting.
Figure 3 illustrates the Bouguer anomaly variation
along the traverse AA’ extending from south of the town
Abha to north of Bishah over a distance of 425 km. The
terminal points of the traverse are well constrained by
IRIS Shot Points 4 and 5 respectively [10, 11] (Figure 1
& Table 1). As it can be seen from Figure 3 that there is
a dramatic increase in Bouguer anomalies on approach-
ing the west margin of Asir from the coastal side, where,
a large positive anomaly is developed across the HAE
extending northward. This corresponds to TMC, HAE
and part of the Deformation Belt inland [13]. A second
and third-order gravity high of considerable amplitude is
also developed inside the AP at respective distances of
300 and 375 km from southern terminus of the traverse.
For gravity modeling, we firstly remove a DC shift from
the observed gravity curve. Such an arbitrary DC shift is
often applied to the calculated gravity curve to provide
the lowest misfit to observed Bouguer anomalies [17].
This is felt necessary because the calculated value is an
absolute gravity calculation for the model extending to
30,000 km in the ± X and ± Y directions and to some
arbitrary depth (50 km by default) [18]. DC shift by -50
mgal is found to be best suited for model calculation in
the present case (Figure 3).
Using the seismic velocity and thickness values given
in Tables 1 and 2, gravity anomaly variation along the
traverse AA’ is interpreted in terms of mass anomalies
due to both density as well as thickness changes in crustal
layers. For this, we basically adopt the main results of
IRIS experiment between Shot Points 4 & 5 [10-12],
geologic observations [13], and the results of grav-
ity-elevation regression relations discussed in the fore-
going. Gravity model calculation is done using the po-
lygonal method [18,19]; density values used in modeling
are from Table 1 and published density values for mafic,
basic and felsic rocks [17,20].
The main results of gravity 2D modeling are illustrated
on Figure 3. Most significant features of the model are:
Figure 3. 2D crustal configuration derived from gravity
modeling along the traverse AA’. Lower panel: horizontal
and vertical scales in m and density values in g/cm3. Upper
panel: gravity values in mgal, gravity anomaly curves: ob-
served B.A. in dark, DC shift of -50 mgal in blue colour,
calculated gravity in red colour.

S. MOGREN, M. MUKHOPADHYAY 31
i) There is a massive change in crustal configuration
from the coastal to the interior of the Arabian Shield. A
two-layered crust below the AP extending down to about
38 km depth, segregated between largely metamorphosed
upper crust from a more mafic lower crust, The Asir
crust is assumed to transit across the HAE towards Abha
-- finally meeting the largely mafic crust underlying the
Tihamat coastal plains; the latter is covered by Neo-
gene-Quaternary sediments. We interpret this mafic crust
as the very primary oceanic crust formed at the conti-
nental margin due to Red Sea spreading. The transitional
crust is of average density ρ = 2.8 g/cm3, its lower edge
coincides with the Moho boundary that extends from 12 -
22 km depth. It is having a median width of 115 km. By
definition, this rather-wide transition zone represents the
initial areas of crustal stretching at the Arabian Shield
margin.
ii) There are strong density heterogeneities in the up-
per crust in AP: this is broadly divisible into a denser (ρ
= 2.7 g/cm3) block in western part of Asir as compared to
a less dense top crust (ρ = 2.67 g/cm3) in more eastern
areas. Metamorphosed basalts and altered mafic rocks
are mapped in large terrenes in AP. Two such large bod-
ies, of assumed density ρ = 3.0 g/cm3 are modeled be-
tween 125 - 185 km distance from southern end of the
traverse. Inferred density for the upper crust is likely to
represent rocks of greenschist facies [12]. The denser
upper crust extends to about 22 km depth, where, it cor-
responds to a horizontal velocity discontinuity of 6.4 -
6.5 km/s, mapped below most areas of the Arabian shield
[10,11].
iii) Gravity model infers a dipping but laterally exten-
sive lower rock layer, of density ρ = 2.8 g/cm3, between
22 - 38 km depth below the thinned continental crust of
AP. This inferred density is practically the same as that
derived for TMC. Gravity interpretation however cannot
decide whether the geometric configuration of the layer
or its inferred rock density represents the lower conti-
nental crust or it as an area of thick magmatic under-
plating under a thinned-out continental crust below the
AP. Joint interpretation of seismic reflection and gravity
data are required for the purpose; refer the studies on the
Gabon Margin [21].
iv) The other two prominent gravity highs along the
traverse, at respective distances of 300 and 375 km from
its southern end, located within the AP are interpreted
due to basic rocks intruded in top crust. A felsic pluton,
of density ρ = 2.58 g/cm3, explains a localized gravity
low at 240 - 260 km distance along the traverse.
2. Discussion
An analysis of gravity anomalies and corresponding ele-
vation values at the margin of the AM suggests that there
is a significant change in their regression relationship at
about 400 m elevation. This threshold elevation is taken
here as an indicator for changes in subsurface mass dis-
tribution and crustal thickness variation between the
coastal plains towards the AM across its foothills and
pediment. This area also roughly corresponds to current
seismicity evidenced from instrumental records. Such an
inference is conceptually similar to what is known from
the Himalayas [8,9]. Gravity anomaly variation along a
425 km long traverse extending from the Red Sea coastal
plains to the AM across the HAE is taken up for gravity
interpretation study with an objective to investigate the
nature of the boundary crust between the Asir Province
and the Tihamat coastal plains. Gravity model presented
here is reasonably well constrained by results from the
IRIS Deep Refraction profile, in particular, by the two
seismic shot points located at the terminal points of the
gravity traverse near the towns Ad-Darb south of Abha
and Bishah on the north. The town Abha locates at the
edge of HAE. Results of gravity model provide some
basic insight into the mass distribution and crustal con-
figuration and their significant changes across this tec-
tonically interesting region. Major findings being: a) the
west margin of the Asir represents the zone of crustal
transition between a typical continental crust below the
Asir vis-à-vis a typical mafic crust below the coastal
plains. The rugged topography on the surface of the tran-
sition is an outcome of extension of the continental crust
and its intense deformation at the Red Sea margin. This
zone of crustal transition is rather wide, which contra-
dicts a popular view that transition is sharp here. Transi-
tional crust is denser than the adjacent continental crust
by about 0.1 g/cm3; thus favoring its mafic composition.
This mafic crust developed in mid-Tertiary due to the
stretching episodes at the Arabian shield margin. b).
There are massive changes in crustal configuration be-
tween the coastal plains to the shield interior across the
HAE. Highly stretched continental crust below western
parts of the Asir is modeled to be somewhat denser than
typical continental crust farther to the east. This denser
crust constitutes large proportions of metamorphosed
basalts and altered mafic rocks. This crust is probably
elevated to greenschist facies during the rifting process.
c). Strong density heterogeneities in top crust below the
AP are inferred by gravity model. Typical extensional
features of the stretched Asir crust are presented in the
gravity model and their geodynamic significance is also
briefly reviewed in the foregoing section. d). An impor-
tant finding from gravity modeling is that the laterally
extensive rock layer below the upper crust of the Shield
is of similar density as that of the TMC at the transition
zone. Gravity model alone cannot distinguish whether
this layer represents the normal continental crust ‘sensu
stricto’ or it developed in consequence to magmatic un-
derplating below the thinned continental crust due to Red

S. MOGREN, M. MUKHOPADHYAY
32
Sea extension. e). The crustal configuration derived from
gravity modeling for the entire traverse supports the
consensus view on Red Sea spreading for the region.
However, further detailing of gravity model is inhibited
by the present coverage of gravity stations, in particular,
for the boundary crust and the deformed belt.
3. Conclusions
An integrated interpretation of surface geologic, seismic
deep refraction and regional gravity data allow us to
draw the following broad conclusions on the boundary
crust developed due to Red Sea spreading in mid-Terti-
ary at the Arabian Shield margin. Signatures of rifting
and crustal thinning are dominant here. Some basic con-
clusions of the study are:
a) Predominantly mafic crust below the Thimaya
coastal plains transits into a thicker continental crust be-
low the Asir province across a rather wide transition zone
and deformed belt. The former consists of a deep- pene-
trative mafic crustal column of interpreted width of 115
km, beside a wider zone of detachment faults and basalts
atop the Asir crust. The Tertiary mafic crustal column is
denser than the shield crust by at least 0.1 g/cm3, it has
vertical side to the south but an inclined contact with the
Asir crust. The dipping Moho under the transitional crust
is quite characteristic. Western edge of the TMC is dis-
tinguished by a clear change in gravity signature corre-
sponding to a threshold elevation of about 450 m across
the pediment of the Asir, while, its eastern flank on the
top surface is intimately involved with the basalts and
detachment fault - typically representing an extensional
margin [22].
b) Zone of deformation in the continental rocks of the
Asir is much wider. Gravity modeling suggests that the
upper crust is denser by about 0.07 g/cm3 as compared to
the top basement further east. Surface geologic features
in top crust of AP like: the Basalt detachment Fault, ba-
salts, large normal faults in the deformed belt are consid-
ered quite diagnostic of a stretched crust at the shield
margin. Interface between the crust and mid-crust proba-
bly represents a low angle fault in the deformed belt that
has been inferred geologically. Local gravity ‘low’ and
‘high’ in the deformed belt are interpreted in gravity
modeling as to represent felsic and mafic plutons respec-
tively, invading the top crust in the Asir. Geometry of
such initial intrusions (plutons and dikes) inferred from
gravity modeling need further geophysical investigations
owing to their intimate relationship to Red Sea extension.
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