LATE EOCENE-OLIGOCENE PIGGYBACK TO TRANSTENSIONAL FORLANDSUNDET BASIN, SVALBARD: LINKAGE TO INITIATION OF THE MOLLOY MID-OCEAN RIDGE SEGMENT, NORWEGIAN-GREENLAND SEA

The onshore Paleogene Forlandsundet Basin of Svalbard, Norway, records oblique stretching as the Mid-Atlantic Ridge propagated northward to meet the Gakkel spreading ridge in the Arctic Ocean. The NE-striking (025°) Molloy Ridge segment, whose Late Eocene-Oligocene position lay immediately west of Forlandsundet Basin, helped split Laurasia and now represents an ultra-slow spreading ridge, exposing ultramafics on the modern seafloor. The onshore continental Forlandsundet basin constrains the timing and kinematics associated with initiation of the Molloy Ridge segment.

The eastern margin of the Forlandsundet basin records a three-phase kinematic history starting with Middle to Upper Eocene meandering, coal-bearing fluvio-deltaic strata deposited in a broad piggyback basin during shortening in the Eurekan/Greenland/West Spitsbergen Fold-Thrust belt, with sediment derived from Svalbard or Greenland. This first-phase fluvio-deltaic sequence then experienced oblique deformation prior to lithification during initial rifting under transtension in a Late Eocene second phase. Basin structures include folding and faulting in transtension and indicate initial rifting with NW stretching (308°-320°; oblique to the 340° trend of the margin) driven by the incipient Molloy Ridge.

During the third phase, the basin narrowed as an Early Oligocene graben cut across and was superimposed on the broader, older deformed Forlandsundet piggyback basin. In outcrop, the upper fill of the >1 km-thick, marine graben (trending ~330°-340°) displays undeformed, unlithified alluvial-fan/fan-deltaic strata bearing foraminifera with easterly derived coarse (≤4 m diameter) detritus shed from an exposed intrabasinal high or the eroding fold-thrust belt. This marine graben has implications for the demise of the land bridge between Eurasia and N America. The ages of the deformed versus undeformed onshore Forlandsundet strata limit the age of initial oblique continental rupture associated with the nascent Molloy Ridge. Initial oblique stretching occurred in latest Eocene to Early Oligocene time (37-33 Ma), certainly prior to 33-30 Ma, and at basin temperatures <104 °C based on vitrinite-reflectance values.