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The Larsen B Ice Shelf on the Antarctic Peninsula collapsed abruptly in February-March 2002. This study provides a detailed structural glaciological analysis of the changes in surface structures on the ice shelf from a series of visible-channel satellite images acquired between January 2000 and April 2002 and the RAMP mosaic of 1997. Mapped features include the ice-shelf front, rifts (fractures that penetrate the entire ice shelf), crevasses, longitudinal linear surface structures (also called flow stripes or streaklines), medial moraines, and meltwater features (streams, ponds and dolines). Longitudinal surface structures are particularly important because they can be used to define individual tributary-glacier flow units and their contribution to the ice shelf. Using this approach, we define domains on the ice shelf related to glacier source areas and demonstrate that, prior to collapse, the central Larsen B Ice Shelf consisted of four sutured flow units fed by Crane Glacier, Jorum Glacier, Punchbowl Glacier and Hektoria/Green/Evans Glacier. Between these active glacier-fed flow units were 'suture zones' of thinner ice where the feeder glaciers merged to form the shelf. Prior to collapse, large open rift systems (with floating brash ice) were present offshore of Foyn Point and Cape Disappointment. In the years just preceding breakup, these rifts became more pronounced and ice blocks in the rifts rotated because of the strong lateral shear in the zone separating active and less-active flow units. Velocity mapping of the suture regions indicates that the major rifts are a recent feature of the ice shelf, and were not present >~20 years ago. We suggest that the ice shelf was pre-conditioned to collapse by partial rupturing of the sutures between flow units. This we believe was the result of ice-shelf front retreat in 1998-2000, reducing the lateral resistive stress on the upstream parts of the shelf and glacier flow units, shelf thinning, and pre-shelf-break-up glacial acceleration.
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