Ultramafic rocks in northwestern Oriente Province, Cuba, are in narrow arcuate, parallel bands within a Cretaceous eugeosynclinal area. The structural relations between the ultramafic rocks and the "country rock" are seen best in the Silla Gibara hill mass 5 km southeast of Gibara. Here, 1-m to several hundred-meter-wide slivers of isoclinally folded and subsequently imbricated thrust plates appear to be "engulfed" in ultramafic rocks. These slivers, except for undated marble and Eocene conglomerate, are elements of a Cretaceous geosynclinal prism. Cretaceous radiolarian chert, middle Cretaceous pillow lava and reefal limestone, Campanian graywacke and pelagic limestone, and Maestrichtian forereef limestone are present. The slivers are separated from each other by ultrama ic rocks along consistently northeast-striking and steeply southeastward-dipping contacts.

The petrographic and structural appearances of the ultramafic rocks in the area indicate that several types of these rocks are present: "magmatic," "cold diapir," "tectonic," "basement," and even "sedimentary." This sequence suggests that the following series of events may have resulted in emplacement of the ultramafic rocks into their present site.

A geosynclinal depression originated in Early Cretaceous time as a result of tension along the zone of contact between the Caribbean "oceanic" crust on the south and a "continental" crust on the north. Serpentinization of the subcrustal peridotite of the oceanic segment raised the overlying thin gabbro crust. "Digestion" of the gabbro crust by volcanism began in middle Cretaceous time and a "trench" formed, by crustal failure, north of the volcanic zone. Oversteepening and subsequent collapse of the southern trench wall caused the formation of a series of northward-directed, high-angle "crustal" thrusts in Maestrichtian time. These "thrusts" involved the partly intact gabbro crust and the serpentinized ultramafic rocks below. Later the "crustal" thrust wedges were transformed into low angle, northward-directed gravity thrusts. Increased mobility of the serpentinized ultramafic rocks along the sole of the thrusts facilitated the basinward sliding of the plates, throwing some into a series of recumbent folds. Continued hydration under differential stress increased the mobility of the serpentine fluidlike flow. Subsequently, in middle Eocene time, the serpentines in the cores of the folded thrust plates pierced the confining structures, tearing the strata into slivers, yet stringing them out in structurally coherent order as "flow" thrusts.