Pakistan Journal of Hydrocarbon Research

Vol.14, (June 2004), p.49-59, 4 Figs., 2 Tables

Structural Trends and Focal Mechanism Studies in the Potwar Area with Special Emphasis on Hydrocarbon Exploration

MonaLisa^[1] and Azam Ali Khwaja¹

ABSTRACT

In the present paper an attempt has been made, for the first time, to identify the surface/subsurface structural pattern for the area of Potwar with the help of focal mechanism studies- a technique that is widely known for interpreting structural traps at depth. For this purpose all available earthquake data (during 1964-2002) from international seismological networks and the local seismic observatories have been collected for the compilation of seismicity map of the area. Based upon this seismicity map, the area appears to be very active compared to the adjacent regions of Kohat Plateau and Salt Range with apparently very prominent patterns of seismicity along Jhelum Fault and Main Boundary Thrust (MBT). Focal mechanism solutions of thirteen events for which required parameters were available indicate left-lateral strike-slip faulting for eleven events while two events (4 and 10) represent thrust faulting. Earthquakes number 1, 6, 8 and 11 are located near Qazian anticline and all are representing basement events (as depth indicated by nearest local seismic observatory is > 6 km) except event number 11 whose depth is 5.0 km. The dominance of strike-slip faulting is in agreement with the previous workers. Majority of the interpreted focal mechanism solutions are located in the northeastern (i.e. near Jhelum Fault) and the northern portion (i.e. near MBT) of the area, which are major structures of the Potwar area. P-axes orientations (i.e. the axis of maximum compressions) are in NW-SE and NE-SW directions. Basement is found to be involved in the deformation.

INTRODUCTION

Pakistan possesses the northwestern boundary of the Indian lithospheric plate. The underthrusting of Indo-Pak Plate beneath the Eurasian Plate is producing compressional thin-skinned tectonic features since Eocene time on the northern and northwestern fringes of the Indo-Pak Plate. The continued underthrusting of the Indo-Pak Plate since Cretaceous produced the spectacular mountain ranges of the Himalaya and a chain of foreland fold-and-thrust belts as thick sheets of sediments thrust over the Indian craton (Kemal, 1991).

Foreland Fold-and-Thrust belts are conspicuous features of convergent continental plate boundaries and are very important areas for the recent tectonic movements. NW Himalayan Fold-and-Thrust belt is one of these and is characterized by a number of active faults. In northern Pakistan, thrusting in the Indo-Pak Plate is certainly the main accommodation method of shortening in the Himalayas. Focal mechanism solutions of earthquakes give evidence that these are linked to the thrusts. However, in the NW Himalayan fold-and-thrust belt, complications arise as earthquakes fault planes do not follow the thrusts, which change in orientation, suggesting that other accommodation features besides simple thrusting are occurring in the NW Himalaya (Spencer, 1992).

HYDROCARBON POTENTIAL

The Potwar sub-basin is one of the major hydrocarbon-producing province of the country. Multiple reservoirs of carbonate and clastic sediments of Cambrian through Miocene age occur. In total 11 reservoirs are known to be productive. Out of these fractured carbonates of Sakesar and Chorgali units are the major producing reservoirs (Iqbal and Ali, 2001). Numerous studies have been undertaken to highlight and understand the exploration processes for hydrocarbons in this 9000m thick sediments filled basin but no attempt has yet been made to utilize the Focal Mechanism Studies (FMS), which is a widely used technique for identification of structures at depth, supplemented by conventional techniques for exploration. It is beyond the scope of the present work to correlate FMS with all the exploration techniques, however we tried to carry out FMS with special emphasis on hydrocarbon potential in the area by concentrating the structural trends in the area.

Published literature reveals that Potwar area is characterized by NS compression and transpression related to conversance of Indian and Eurasian plates. However, in the study area presence of evaporites (Eocambrian) has led to the development of duplex type models with the basal decollement in the evaporites above the Precambrian basement. Indications are that levels deeper than the Eocambrian evaporites are also undergoing deformation. Thus, in the present study the nature of fault motions prevailing at depth in the Potwar area is described. Such type of information would lead to incorporation of seismicity data in future structural models of the area.

PHYSIOGRAPHY AND MAIN FEATURES OF THE AREA

The area of Pakistan is bounded by the alluvial covered peninsular shield of India on the east, the great Himalayan arc towards the NE, the Pamir and Hindukush mountains to the north, the central Afghan mountains to the NW, the Zagros folded belt on the west and the Arabian Sea on the south. All the major structures in the area, such as the Chaman Fault, Kirthar Ranges, Sulaiman Ranges, Salt Range, Potwar Plateau, Hazara Kashmir Syntaxis (HKS), Nanga Parbat Haramosh massif, the Main Boundary Thrust (MBT) and the Hazara thrusts owe their origin to the processes of collision that have taken place since Early Tertiary times.

The Potwar Plateau belongs to the Himalayan zone of convergence in which two prominent sutures are situated. The northern suture known as the Main Karakoram Thrust (MKT) formed about 100 million years ago (Treloar, 1989) and the southern suture referred to as Main Mantle Thrust (MMT) is believed to have originated about 50 million years ago (Treloar and Rex, 1990). Kohistan Island Arc lies between these two sutures. The deformation is believed to have shifted southwards with time to Main Boundary Thrust (MBT) and in the Himalayan foreland. Kohat Plateau, Potwar Plateau and Salt Range represent a zone of foreland deformation south of the MBT (Figure 1).

The Himalayan mountains, which have a general NW-SE trend, take a sharp bend near the north of Islamabad and west of Kashmir, forming the Hazara mountains with a general NE-SW trend. This sharp bend is known as Hazara Kashmir Syntaxis (HKS). Potwar Plateau lies in the west of Hazara mountains (Figure 1).

GEOLOGY OF THE AREA

Potwar Plateau has undulating topography. It is characterized by a series of parallel ridges and valleys, generally trend in the E-W direction. Geologically, it forms part of the foreland zone of the NW Himalayan Fold-and-Thrust belt. This foreland zone comprising of Salt Range, Potwar Plateau, Kohat Plateau and Hazara ranges is an area bounded by the Salt Range Thrust in the south and the Panjal-Khairabad Fault in the north (Figure 1). At its eastern end is the nearly N–S running left-lateral Jhelum Fault (Kazmi and Jan, 1997).

In this zone of convergence, intense deformation has resulted in the formation of complex structures.The northern part of Potwar Plateau, also referred to as the Northern Potwar Deformed Zone (NPDZ) lies between the Main Boundary Thrust and the Soan Syncline (Figure 2 & 3). It is more intensely deformed than the southern Potwar and the Salt Range. Mostly E-W trending tight and complex folds with their southern limbs overturned with steep angle faults occur in NPDZ.

The area contains a series of thrusts. Lillie et al., (1987) describe the northern Potwar as an imbricate stack of thrust faults on the surface and in the subsurface as blind thrusts. General trend of these thrusts changes from E-W to N-E direction in the eastern part of the NPDZ (Figure 2 and 3). A number of workers have already given the description of these faults (e.g. Lillie et al., 1987; Jadoon et al., 1995, Jadoon and Frisch, 1997 and Jaswal et al., 1997).

Soan Syncline is the major structural feature of Potwar. Its southern limb is less steep than the northern limb. In the study area, it is believed to have evolved between 3.4 to 1.9 Ma with the southern limb forming prior to the northern limb. According to Johnson et al., (1986) the development of the southern limb took place due to thrusting along the Riwat thrust (Figure 2). This thrust trends in the NE-SW direction lies about 20 km south of Rawalpindi. Jadoon et al., (1995) believe that cessation of movement along the Riwat thrust stopped at about 2.7 Ma.

Soan (Dhurnal) backthrust is a distinctive feature of the Northern Potwar Deformed Zone, occurs in the northeastern portion of the NPDZ (Figure 2). The dips along the backthrust are nearly vertical in contrast to near horizontal along axis of the Soan Syncline. The top of Kamlial Formation marks its location. North of the backthrust, highly deformed rocks of Murree Formation with steep to vertical dips occur, where as further north till the Khair-i-Murat Fault steeply dipping Siwalik Group rocks are exposed.

According to Pennock et al., (1989) the basement along the Soan Syncline is at a depth of about 6 km. It increases towards the north and near the MBT is about 8 km (Jaswal et al., 1997). The MBT itself is represented by many high angle thrusts along which Eocene and older rocks have been thrusted over the molasses of the NPDZ. The NPDZ is considered to be a thin-skinned tectonic feature by most workers in which the basal decollement is in the Eocambrian Salt Range Formation. In this interpretation the Soan (Dhurnal) backthrust is a passive backthrust and the area bounded by it and the Khair-i-Murat Fault is a triangle zone of complex geology (e.g. Jadoon et al., 1999).

The models invoking duplex structure have been questioned by Pivnik and Sercombe, (1993); Sercombe et al., (1998). These workers recognize the presence of strike-slip faults at the surface and even in the basement. They relate the structures (high angle strike-slip faults and associated flower structures) to transpressional deformation.

SEISMICITY

Before discussing the focal mechanism solutions, a brief introduction to the seismicity of the area is presented.

Pakistan lies in a high seismicity area with a history of large earthquakes causing a great loss of life and property. Two major destructive earthquakes in the recent past, which killed thousands of inhabitants, are the Quetta earthquake of 1935, claiming about 30,000 lives: and the Pattan earthquake of 1974, with a death toll of about 6,000. Other large earthquake that caused major destruction is the 1981 Darel Valley earthquake.

A seismicity map of the area, prepared from 1964-2002 using the data obtained from the United States Geological Survey (USGS), International Seismological Centre (ISC), International Seismological Summary (ISS) and local seismic networks is shown on structural map (Figure 2). Historical earthquakes from Oldham (1893) and Quittmeyer and Jacob (1979) are also included. Although the entire region of Pakistan is considered to be seismically active, however, the Potwar Plateau appears to be relatively more active as compared to the adjacent areas of the Kohat Plateau and the Salt Range. The seismicity appears to be scattered in most of the area. Only distinct patterns are seen along the Jhelum Fault i.e. in the northeastern part and along the MBT i.e. in the northern portion. The epicentral distribution appears to be concenterated in nearly NE-SW and NW-SE directions along the MBT.

FOCAL MECHANISM STUDIES

The destruction caused by earthquakes is a matter of great concern for the scientific community and different approaches are being implemented to understand the earthquake phenomena. One such approach, whereby an understanding of earthquake is obtained is the focal mechanism solution. The main purpose of a focal mechanism study is to identify seismic faults from seismological observation. If we can directly observe surface faulting due to an earthquake we need not rely on seismological methods to identify the seismic fault. For oceanic earthquakes also, it is difficult to see the fault traces at the sea bottom, even when the fault rupture appears. Thus seismological approaches such as focal mechanism study are indispensable for studying seismic faults and their rupture processes.

Focal mechanism solutions undertaken by earlier workers (Verma et al., 1980; Verma and ChandraSekhar, 1986; MonaLisa et al., 1997; Khwaja et al., 2003) from immediately adjacent regions have shown the dominance of strike-slip faulting with some thrust faulting in this area of collisional tectonics. In the present case, for the determination of focal mechanism solutions, the earthquakes reported by USGS, ISC and the local networks with magnitudes near 4.0 and above occurring in the area during the period of 1964-2002 were considered. Thirteen focal mechanism solutions (Figure 2 and Table 1 and 2) were determined from the P-wave first motion directions. The parameters for the epicenters and the focal mechanism solutions of all the thirteen earthquakes are listed in tables. 1& 2. One of these (event No.2) had previously been analyzed by Seeber and Armbruster, (1979) and Verma and ChandraSekhar, (1986).

PROCEDURE EMPLOYED IN THE PRESENT STUDY

In this study, thirteen focal mechanism solutions of earthquake events (Mb > 4) that occurred in the Potwar area during the period of 1964 to 2002 have been carried out. The standard lower half hemisphere projections on an equal area net have been used. Visual interpretation of these focal mechanism diagrams generated with the help of a computer program PMAN (Suetsugu, 1996) that required input of geographic coordinates, magnitude, focal depth and P wave polarity was carried out for each event. The other two parameters azimuthal angle and take-off angle are determined by the software AZMTAK (Suetsugu, 1996). From the large number of events shown in figure 2, the above-mentioned parameters were available only for the thirteen events discussed in this study.

FOCAL MECHANISM SOLUTIONS

Event No.1

Event No.1 of magnitude 5.1 Mw is one of the few strongest events (Mw>5.0) to have occurred in the area during the study period and for which the required parameters were available (Table 1). The hypocentral depth of this event as given by local network is 33 km. Prominent structural trend of the area where the event occurred is in the NE-SW direction. Qazian anticline (Figure 3) is one of these NE-SW trending anticline and the epicenter of the event is located along the axis of the Qazian anticline. Another important structural feature located about 20-25 km east of the epicentral location is the NW-SE trending Jhelum Fault.

Different workers suggest that the folded structures in the eastern Salt Range (SR) and Potwar Plateau (PP) are cored by blind thrusts. Qazian anticline is also a pop up bounded by two thrusts. Recent workers (Pennock et al., 1989: Moghal et al., 2003) on the basis of seismic data have also confirmed the presence of a pop up structure. Seismic data shows that these two thrusts originate at a depth of about 1.5 km and extend up to the depth of 4 km in the subsurface. In addition, Pennock et al., (1989) also identified a normal fault in the basement.