Does Deccan Volcanic Sequence contain more reversals than the three-Chron N–R–N flow magnetostratigraphy?—a palaeomagnetic evidence from the dyke-swarm near Mumbai

Abstract

New palaeomagnetic data from a series of Deccan-age dykes cutting across the basement of lava flows along east and south of Mumbai (18◦–19◦N and 72◦–73.5◦E) have uncovered both normal (N) and reverse (R) magnetizations. Out of 33 dykes investigated, 29 dykes have yielded stable characteristic remanent magnetizations (ChRM) amenable for statistical analysis. Twenty dykes exhibit N-polarity and nine dykes show R-polarity. The tilt-corrected dyke virtual geomagnetic poles reveal two distinct groups of dykes. Each group consists of both N- and R-polarity dykes on the Indian apparent polar wander path (APWP). The mean of N-polarity dykes is antipodal to the mean of R-polarity dykes. Group I (GI) comprises nine N-polarity dykes with their mean at Dm = 337.8◦, Im = −39.4◦ (k = 130.7, α95 = 4.5◦), and three dykes of R-polarity whose mean is at Dm = 150.7◦, Im = 44.1◦ (k = 147.1, α95 = 10.2◦). The corresponding pole positions are at 43.5◦S, 102.6◦E (A95 = 4.6◦) and 36.6◦S, 107.6◦E (A95 = 8.9◦) respectively. These pole positions fall close to 65 Ma old Deccan Superpole (DSP) on the APWP, and may therefore be associated with magnetic Chron 29N for the N-polarity and 29R for the R-polarity of the three-Chron (30N–29R–29N) magnetostratigraphy of the Deccan Volcanic Province (DVP) used in the computation of DSP. In Group II (GII), eleven dykes display N-polarity with mean at Dm = 312.6◦, Im = −45.2◦ (k = 93.3, α95 = 4.8◦) and six dykes show R-polarity with mean at Dm = 130.1◦, Im = 48.0◦ (k = 176.8, α95 = 5.1◦). The corresponding pole positions at 25◦S, 120.1◦E (A95 = 5.3◦) and 22.2◦S, 119.1◦E (A95 = 5.2◦) respectively are significantly displaced (by more than 20◦) from the DSP along the older segment (∼80–90 Ma) of the Indian APWP. Hence, the dykes of GII appear to have been emplaced in an earlier episodic Deccan activity than that represented by the DSP. The palaeolatituides corresponding to GI and GII are 23.2◦S ± 4.0 and 27.6◦S ± 4.5 respectively, indicating a latitudinal drift of ∼4.4◦ of the sampled location between the acquisition of two magnetizations. Six lava flows sampled from the basement of the host Deccan flow sequence nearby sampled dykes yielded reverse polarity ChRM mean at Dm = 128◦, Im = 47◦ (k = 156, α95 = 5.4◦) with corresponding pole positions at 21◦S, 121◦E (A95 = 5.3◦). This pole falls near the GII poles, indicating a probable contemporaneous time of emplacement. In the absence of radiometric dates, no magnetochron can be assigned for GII magnetization. However, on the basis of published analyses of several palaeomagnetic and geochronological data of the flows and dykes within DVP and Deccan related dykes from the neighbouring areas, it seems plausible that GII–R magnetization belongs to either Chron 30R or 31R and GII–N magnetization pertains to 31N. Conclusive evidence in support of the presence of these reversals in the DVP, however, has to wait for new geochronological data from the undated dykes investigated in this study, and fresh updating of the stratigraphy of the lowermost flow sequence in the sampled area which also remains undated. This study, however, does indicate the possible presence of two more reversals revealed by GII magnetization on the older side beyond well-established three-Chron magnetostratigraphy.