Gravity Modeling for the Rifted Crust at the Arabian Shield Margin – Further Insight into Red Sea Spreading

doi:10.4236/ojg.2013.32B007

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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

Email: [email protected]

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

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

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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