I also thought that the main difference between the two was that marzipan is cooked. But I haven't found any mention of that in the Larousse Gastronomique.
In The Sweet Kitchen Regan Daley writes:
Almond paste is a finely blended mixture of ground almonds and sugar. In some cases, glucose or glycerine replaces part of the sugar for a smoother texture. Almond extract may be added to intensify the flavour. Almond paste is used extensively in many traditional European pastries, baked goods and confections such as frangipane and macaroons. It should be firm but malleable…
Almond paste should not be confused with marzipan, a similar paste confection with a higher ratio of sugar to almonds. Marzipan appears frequently in the baking and confection of many European cuisines, as a garnish or petit four on its own, incorporated into cake or biscuit doughs or as an ingredient in filling for cakes, pastries and confections It is available from many gourmet and speciality food shops, as well as most good European Deli.
In The Professional Pastry Chef, Bo Friberg gives the definition for almond paste and marzipan:
Almond paste: A moist mixture of finely ground blanched almonds and sugar (generally 50-50 by weight) that has a doughlike consistency. Almond paste has many applications in the pastry kitchen. It is the primary ingredient in macaroons, marzipan…and frangipane filling, and is used to flavor cake and cookie batters and numerous pastry and cookie fillings.
Marzipan: Marzipan is used extensively in European pastry shops, particularly in Germany, Austria and Switzerland, and Scandinavia. It is made of almond paste and powdered sugar, with the addition of a moistening agent such as glucose or corn syrup. Some recipes substitute egg whites or even fondant, but the purpose is the same. Marzipan, plain or tinted, is rolled into thin sheets and used for covering cakes and pastries, It is also sculpted into ornamental figures such as animals and flowers.
Here is the recipe for Marzipan: :
2 pounds almond paste
1/2 cup glucose or light corn syrup
2 pounds sifted powdered sugar
In a stainless steel mixing bowl, use the hook attachment to mix the almond paste with the glucose or corn syrup at low speed until combined.
Start adding the sugar, scraping down the sides of the bowl as necessary. Add enough of the powdered sugar to make a fairly firm yet malleable dough.
Store the marzipan, wrapped in plastic, inside an airtight container in a cold place.