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AAPG Bulletin

Abstract


Volume: 69 (1985)

Issue: 4. (April)

First Page: 513

Last Page: 524

Title: Continuous Sea-Floor Spreading in Red Sea: An Alternative Interpretation of Magnetic Anomaly Pattern

Author(s): J. L. Labrecque (2), N. Zitellini (3)

Abstract:

The magnetic anomaly pattern over the Red Sea can be modeled as a continuous system of sea-floor spreading from the early Miocene to the present by using a Previous HittimeNext Hit-Previous HitvaryingNext Hit process filter. The half spreading rate is approximately 1 cm/yr (0.4 in./yr) since initial rifting. The parameters that determine the process filter and development of the transition zone are the intrusion parameter (a measure of the dispersion of feeder dikes or horizontal strain about the rift axis), a flow parameter (a measure of the average flow width), and the effusion parameter (a measure of the volcanic effusion and thickness of layer 2).

Application of a Previous HittimeNext Hit-Previous HitvaryingTop process filter appears to be a powerful tool in analyzing magnetic anomalies over passive continental margins. We estimate the flow parameter to be 2.7 km (1.7 mi) and the intrusion parameter to be 7.5 km (4.7 mi) at early rifting. These values suggest that a wide distribution of axial dikes or horizontal strain is the dominant factor in forming the magnetic anomaly pattern. Reduction in the width of the intrusion parameter and the effusion rate as rifting proceeded resulted in focusing of the strain, thinning of layer 2, and formation of the Red Sea deeps.

The interpretation of the anomaly pattern implies that basement within the main trough of the Red Sea is composed of a thick accumulation of volcanic sills and flows deposited during early rifting. Extrusion of the lavas occurred over a wide (up to 60 km or 37 mi) zone of strain distributed about the rift axis. The development of this thickened zone of igneous crust is the second of three successive stages of extension for the Red Sea basin. These stages are as follows.

(1) An initial stage of crustal stretching or listric faulting in the Oligocene accompanied by probable regional uplift.

(2) A period of high volcanic effusion accompanied by the emplacement of stratoid basalts. The stratoid volcanics form overlapping flows and sills with intercalated sediments within the main trough. The spreading center narrowed from the early Miocene to the present.

(3) Steady-state sea-floor spreading from Pliocene to present south of a transitional zone near 22°S.

The process filter width derived from the magnetic anomaly pattern at earliest rifting can be used to estimate the lateral variation from igneous to continental-type crust in a statistical sense. Our data suggest that the transition from 100% continental crust to 100% igneous or stratoid-type crust should occur within an interval of approximately 32 km (20 mi).

Our modeling suggests that phase 2, or the stratoid phase, began about the time of anomaly 5C or chron C5C approximately 16 Ma. This age is compatible with geologic estimates of the initial rifting at the late Oligocene to early Miocene (Coleman, 1974; Gass, 1977). The opening rate for Africa-Arabia plate motion has remained relatively constant since early rifting although the African margin appears to be accreting faster than the Arabian plate.

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