Deformation of the Girvan succession, Southern Uplands

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Stone, P, McMillan, A A, Floyd, J D, Barnes, R P, and Phillips, E R. 2012. British regional geology: South of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.

Deformation of the Girvan succession[edit]

Stages in the closure of the Iapetus Ocean . P912315.
Syndepositional fault control on Ordovician stratigraphy in the Girvan area. After Williams (1962). P912324.
Stratigraphical columns for the Ordovician (Llanvirn) to Silurian (Wenlock) sedimentary successions in the Girvan area. P912323.

Whilst the Southern Uplands accretionary complex was building up at the subducting margin of the Iapetus Ocean, the Ordovician to Silurian Girvan succession was accumulating on the subsiding margin of the Laurentian continent. The depositional setting was essentially a forearc basin between the Midland Valley volcanic arc, then rising to the north, and the developing accretionary complex to the south (P912315c). However, a note of caution must be added. Though this overall association of geological regimes fits the present-day outcrop pattern, it is highly unlikely that the original spatial relationships are preserved. The arc, forearc basin and accretionary complex all extended for considerable distances along strike, and substantial, sinistral strike-slip movement, post-Ordovician and particularly focused on the Southern Upland Fault, has brought about the juxtaposition currently seen.

For much of its history (and as described in Chapter 2) deposition of the Girvan succession was controlled by a series of major faults, throwing down to the south and sequentially stepping back into the Laurentian hinterland (P912324). There would undoubtedly have been some rotation of the early-deposited beds during subsequent fault growth, but there is only one stratigraphical break preserved in the Girvan succession. This coincides, approximately, with the Ordovician–Silurian boundary where the basal Silurian strata overlie the Ordovician at an unconformity with low-angle structural discordance.

The major structural disruption occurred in the mid to late Silurian, when the faults that had controlled sedimentation were reactivated as north-directed thrusts. The cause of this reversal is uncertain, but was most probably related to the collision, farther south, of Laurentia and Avalonia. The effect was to carry parts of the Girvan succession northward, over other parts of the succession and the underlying Ballantrae Complex. In the process, strata were rotated and folded so that steep dips are now commonly seen at outcrop. The thrust reactivation could have begun quite early in the Wenlock, with the youngest strata affected (and now preserved) being the Straiton Grits Formation, with an age equivalent to the murchisoni Biozone (P912323).

The following are related articles:[edit]

Obduction of the Ballantrae Complex, Southern Uplands
Deformation of the Girvan succession, Southern Uplands
Southern Uplands accretionary complex
Late Caledonian dyke swarms, Southern Uplands
Late Caledonian plutonic rocks, Southern Uplands

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